Robert H. Tamarin, dean of sciences at the University of Massachusetts at Lowell and author of a standard college textbook on genetics, Principles of Genetics, tackles some of the more controversial issues in the field of genetics in this question-and-answer series. Are genetically modified foods safe for human consumption? Do genetically engineered plants pose ecological hazards? How reliable is DNA fingerprinting in criminal prosecution? And are there scientific and medical benefits to be gained by cloning human beings?

Questions and Answers About Genetics

Q: How much deoxyribonucleic acid (DNA) is in a typical cell?

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A: Since it is safe to say that there are no “typical” cells, it is best to give a range of quantities of DNA. However, note that most geneticists talk about the quantity of DNA in the genome of an organism, not the quantity in a cell. The genome is the full set of genetic information in an organism.

The cells of most higher organisms have two copies of their genome, one in each of the two copies (one coming from each parent) of each chromosome. Thus the quantity of DNA in a diploid cell (a single cell in which the basic chromosome number is doubled) is twice the quantity in the genome. The human genome has about 3 billion DNA base pairs. The common colon bacterium, E. coli, has 4.2 million base pairs (in both the cell and the genome, since it usually has only one copy of the chromosome in a cell). A common plant used in research, Arabidopsis thaliana (the mouse-ear cress, a small weed related to the mustard plant), has about 117 million base pairs in its genome, and the fruit fly has about 120 million base pairs in its genome.

In some cases, very similar species can have radically different quantities of DNA in their genomes. For example, some amphibians have a genome that is 100 times larger than the genomes of other amphibian species.

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Q: What are antisense molecules?

A: Antisense molecules are strands of ribonucleic acid (RNA) that are complementary to another piece of RNA—one that is directing the production of a protein.

During the process of gene expression, a gene (a length of double-stranded deoxyribonucleic acid, or DNA) opens up, and (in a process called transcription) one of the strands is used as a template to create a messenger ribonucleic acid (RNA) molecule. The messenger RNA molecule then migrates to a ribosome in the cell’s cytoplasm, where its information is used to synthesize proteins in a process called translation. In the case of many disease-causing genes, this protein product is harmful. One way to stop production of that protein (disease-causing or not) is to bind the messenger RNA and prevent it from being read by the ribosome. A relatively straightforward way to do this is to synthesize a small strand of RNA that is complementary to a part of the messenger RNA. It will bind to the messenger to form a double helix and prevent the translation of the messenger.

The term antisense comes from the fact that messenger RNA is synthesized from one of the two strands of the DNA double helix—that strand is called the template, or sense strand. It follows that the complementary strand of DNA is called the antisense strand. The two DNA strands—sense and antisense—are complementary to each other and form a double helix. The two RNA strands produced from these DNA strands are also complementary. When bound into a double helix, the RNA strands cannot function to produce proteins.

Q: What is autosomal recessive gene transmission?

A: The X and Y chromosomes are called the sex chromosomes. All of our other chromosomes are called autosomes—we have 22 pairs of these in each of our cells.

If a gene is located on an autosome, its inheritance pattern is not influenced by the sex of the offspring. So a gene that is recessive (a gene that is hidden in its expression when a dominant form of the gene is also present) follows a pattern of inheritance called autosomal recessive inheritance.

For example, the normal, straight hairline on the forehead is recessive; a dominant gene controls widow’s peak. Two people with widow’s peak can have a child with a normal hairline because the people with widow’s peak can each carry the recessive normal gene.

However, two parents with normal hairlines cannot usually have a child with widow’s peak: individuals carrying only recessive genes cannot have, hide, or express the dominant gene.

Q: What types of baldness are there? What gene controls it?

A: The most common form of baldness among men is called male pattern baldness. It begins with recession of the hairline and the appearance of a bald spot at the top of the head. Perhaps 40 million men in the United States have this condition.

Women have an analogous condition called female pattern baldness that usually shows itself as a thinning of hair. Testosterone, the male sex hormone, is needed for the full expression of male pattern baldness; therefore, with the same genotype, a man will be bald but a woman will have thinning hair.

Although this trait differs between men and women, it is not a sex-linked trait (controlled by a gene located on the X chromosome). Rather, it is called a sex-influenced trait because it is controlled by autosomal genes that are expressed differently in the two sexes.

Q: Can you explain the scientific and medical benefits that cloning of humans may have? Some say cloning humans would aid disease research.

A: Cloning of human beings could have scientific and medical benefits if the technical and ethical hurdles can be overcome. The technical hurdles will doubtlessly be easier to overcome than the ethical ones.

Clones are the equivalent of identical twins, and scientific benefits would arise from having clones to study human traits that are influenced by both genes and the environment. Genetic control of many behavioral traits is unknown because there is an unknown magnitude of complex environmental influences. Thus the inheritance of traits such as intelligence, mental illnesses (schizophrenia, depression), and disease susceptibilities (asthma, heart disease, congenital defects) could be better studied with clones.

From a medical point of view, a person’s clone could provide transplantable tissue such as bone marrow if that person had an illness such as leukemia. Kidneys from a clone would be an identical tissue match, and thus easier to transplant.

Q: Is there a biological or genetic cause for violence?

A: Human behavioral genetics is a relatively new area of biology and one that grapples with problems of definition and analysis of complex traits. Although violence may be relatively easy to define and measure, the underlying factors that cause it are not.

It’s easy to analyze a trait such as hemophilia, a blood-clotting disorder that is caused by the change in one gene. Human behaviors, however, are usually controlled by complex genetic and environmental factors. Traits such as intelligence, athleticism, and criminality are poorly defined, and they have not yet been analyzed in terms of the particular genes that might control them.

Although we know that genes are involved in most human behavioral traits, the genetic analysis simply has not been done and will be difficult to do in the future. However, from time to time scientists do uncover genes that seem to have a direct impact on particular human traits.

For example, in 1999 University of Texas scientists discovered a gene (called DRD2) whose variant form leads to an increased possibility of violence and addiction (alcohol, drugs, gambling, and smoking). I would conclude that there is definitely a genetic influence on violent behaviors but that the issue is complex and not yet well analyzed.

Q. What does it mean to have a familial predisposition for cancer?

A. A familial predisposition for cancer is a fancy way of saying that cancer runs in the family. An individual receives two versions of the same gene, one inherited from the mother and one inherited from the father. In order for cancer to develop, both copies of the gene need to mutate in the same cell.

There are several types of cancer family syndromes, in which close blood relatives develop similar cancers. In these cases, one mutated copy of a particular gene is inherited from one generation to the next within a family. This single mutated gene does not cause cancer by itself. But each family member who has inherited this mutated gene is said to have a predisposition for cancer because if the second copy of the same gene mutates within a cell, it will trigger the progression of cancer.

Types of cancer family syndromes include some forms of breast cancer, ovarian cancer, and colon cancer. Individuals with a family history of cancer can undergo medical tests to learn if they have a mutated gene that would make them predisposed to a particular type of cancer. Individuals with a predisposition for a type of cancer can undergo regular cancer screening tests so that if cancer does develop, it can be caught and treated in its earliest stages, when the chance for a cure is more likely.

Q. What is the difference between a carcinogen and a mutagen?

A. Both a carcinogen and a mutagen are substances in the environment that cause changes in a cell’s genetic makeup. But while a carcinogen causes genetic mutations that trigger the progression of cancer, a mutagen may produce genetic mutations that do not cause cancer to develop.

In the past, testing a substance to learn if it was a carcinogen was a time-consuming and expensive process. A suspected carcinogen is administered to large numbers of laboratory mice that are later examined to see if cancer has developed. This type of testing takes a year or more and costs millions of dollars.

Today scientists first test a substance to determine if it is a mutagen before expending the time and money to learn if it is a carcinogen. Identifying a mutagen is a relatively quick and inexpensive process that uses the Ames test, named for its inventor American molecular biologist Bruce Ames. Using the Ames test, substances are tested for their ability to cause genetic mutations in a common bacterium called Salmonella typhimurium. Bacteria grow and reproduce so quickly that it takes only 24 hours for scientists to determine if the test substance has caused bacteria to mutate, indicating a genetic change. Only if a substance is found to be a mutagen is it then put through the more rigorous and time-consuming test on mice to learn if it is a carcinogen.

Q: How does a cheetah’s genetic makeup contribute to its status as an endangered species?

A: When scientists studied the genetic makeup of cheetahs, they found that these animals have a low genetic variability—that is, randomly chosen individuals have a similar genetic makeup.

The genetic similarities between two cheetahs are much greater than, say, between two people or two blue jays chosen at random. Scientists believe that this low genetic variability came about through a population bottleneck that occurred about 10,000 years ago in which the cheetah population was drastically reduced, probably as a result of a significant climate change in the cheetah’s range. This smaller population meant that the number and type of genes that could be passed from one generation to the next was significantly reduced. This placed cheetahs at risk because they no longer had a variety of genetic traits that would enable them to adapt to any changes in the environment.

In addition, the small population of cheetahs had a limited choice of mates, which inevitably resulted in inbreeding, mating that occurs between close relatives. Inbreeding produces several unfortunate physical effects on a population. In cheetahs, inbreeding has resulted in males that have poor sperm quality. It has also caused a reduction in the general vigor of individuals, resulting in earlier deaths among males and females.

The small population of cheetahs, their reduced vigor, and their low genetic variation all contribute to their status as an endangered species. Despite efforts by conservationists these animals have a poor prognosis for future survival.

Q: What is a chromosomal disorder and how does it occur?

A: A chromosomal disorder occurs when a patient has a mutation that involves a whole chromosome or a significant part of one. For example, cystic fibrosis is due to the mutation of a single gene, a mutation that is invisible even with an electron microscope.

Down syndrome, a chromosomal disorder, is due to an extra copy of chromosome number 21. Another name for Down syndrome is trisomy 21, which indicates there are three copies of the chromosome; a normal adult has only two copies.

Anomalies in chromosome number often occur through errors in chromosomal distribution during meiosis, the nuclear process that takes place during the production of sperm and eggs. Down syndrome (named after Dr. John Down, who first described it in 1866) can also come about by the attachment of part of chromosome 21 to another chromosome, in essence creating three copies of a major part of the chromosome.

Q: My 4-year-old daughter has been diagnosed with an abnormality on chromosome 1. She is healthy—nigh robust—in every way but height (she is in the lower 5 percentile in height for children her age). Any idea what conditions she might develop related to this chromosomal abnormality or how it might relate to her slow growth?

A: In the spring of 2000, the sequence of the human genome was announced. Scientists are now in the process of identifying the 30,000 to 130,000 genes of the human genome. Currently we know of several thousand genes and where they are located on our 23 pairs of chromosomes. Each of these chromosomes contains thousands of genes. So far, several hundred genes have been identified on chromosome 1.

It is thus impossible to know exactly which genes are affected by the “abnormality” mentioned, which could be anything from a portion deleted to a portion duplicated to any of numerous other known anomalies of chromosomes. It would cause you needless worry if I actually enumerated all of the genes known on chromosome 1 since many, when mutated, lead to diseases or increased susceptibility to diseases. However, since each of our chromosomes occurs in two copies (with the exception of the sex chromosomes in males), an abnormality of a single chromosome could have no effect on the health and longevity of the carrier. Also, remember that 5 percent of children are in the lowest 5th percentile by definition—so this may have nothing to do with an anomaly of any particular chromosome. Rather than send you to Web sites that enumerate the known genes (and thus potentially heighten anxiety needlessly), I strongly recommend you consult a genetic counselor who can analyze your daughter’s particular chromosomal anomaly and determine what risks, if any, the child might incur.

Q: How do scientists use genetics to classify organisms?

A: All organisms on Earth are related—all arose from a common ancestor and diverged over time as the process of speciation took place. Life on Earth originated from 3.5 billion to 4 billion years ago. The first land plants arose about 435 million years ago, and early human beings arose about 5 million years ago.

In order to learn how and when all the organisms on Earth diverged from a common ancestor, scientists first studied the fossil record and employed methods that measure the age of rocks. From this information they were able to create evolutionary family trees, known as taxonomies.

Later geneticists tried to identify the genetic differences between two organisms to determine when they diverged from a common ancestor. At first geneticists looked for differences in the sequences of amino acids, the building blocks of proteins. After DNA sequencing was developed in the early 1980s, scientists were able to identify changes in the structure of DNA in organisms and correlated them with changes that occurred in the fossil record. From this process they created an evolutionary clock, which determines how long two organisms have been evolving independently from each other and when they diverged from a common ancestor.

Evolutionary clocks are now used as the best available measure of the divergence of species. After all, evolution results from the gradual genetic change of species over time. So it makes sense that the basis of the clock that measures this process should be genetic changes.

Q: Do all organisms have DNA as their genetic material?

A: In a word, yes. In prokaryotes—organisms without a nucleus that include bacteria, archaea, and blue-green algae—the deoxyribonucleic acid (DNA) is usually a single circular molecule, although many species have variants of this shape, including multiple circles or linear molecules.

All eukaryotes (organisms with a nucleus), which include plants and animals, have DNA that is linear in shape.

Viruses are not considered organisms—they are cellular parasites. But in some viruses, such as the human immunodeficiency virus (HIV) that causes acquired immunodeficiency syndrome (AIDS), the genetic material is ribonucleic acid (RNA). During the life cycle of these RNA viruses, however, the RNA is copied into the DNA of a host cell in order to replicate more of the virus. Thus, even in some viruses with RNA as their genetic material, DNA still plays a vital role.

Q: How does DNA differ from RNA?

A: Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) have similar structures—they are both long strings of nucleotides linked by sugar-phosphate backbones. But these nucleic acids do have some important structural differences. Generally DNA occurs as a double-stranded molecule, whereas RNA is single stranded. DNA has a sugar component that lacks a particular oxygen atom, while the sugar in RNA contains that oxygen atom. Because of the difference of just one oxygen atom, the cell recognizes DNA for one purpose and RNA for another. This enables DNA to be the repository of genetic information in the cell’s nucleus and RNA to be the active agent that transports DNA’s genetic information to the cell’s cytoplasm to be used in protein synthesis.

DNA and RNA have another important structural difference. DNA contains four bases: adenine (A), thymine (T), cytosine (C), and guanine (G). RNA contains three of the same bases (A, C, and G) but uses uracil (U) in place of T.

In addition, each DNA molecule typically carries hundreds or thousands of genes. An RNA molecule usually contains information from only a single gene.

Q: How reliable is DNA fingerprinting in matching evidence found at a crime scene to a criminal suspect?

A: Most people are aware that body tissues such as blood, saliva, or semen found at a crime scene can be mishandled, damaging their reliability as evidence in a court case. However, assuming correct sample gathering, preparation, labeling, and storing procedures, the process of DNA fingerprinting to identify the DNA patterns of a tissue sample is virtually 100 percent reliable. In other words, if a laboratory technician used the correct procedures to prepare and study the same criminal evidence over and over again, the DNA patterns identified would be the same each time.

In the process of DNA fingerprinting a tissue sample, scientists use restriction enzymes, specialized proteins that recognize a specific sequence in the DNA base structure, to break apart the tissue sample’s DNA structure at specific intervals. These DNA fragments are then compared to tissue samples from a criminal suspect to see if the DNA patterns match.

Restriction enzymes are incredibly accurate in performing the molecular task that they are programmed to do, some capable of hundreds of thousands of reactions per minute with error rates in the one per million range. As a result, DNA fingerprinting is as reliable as fingertip fingerprinting and much more reliable than eyewitness identification when used to identify a criminal suspect.

Q: What is the difference between a dominant and a recessive gene?

A: The terms dominant and recessive refer to the expression of genes—the phenotype—when two different forms of a gene are present in an organism. In diploid organisms (those with two copies of each gene), such as human beings and fruit flies, the two copies of a gene can be similar or different. If the two copies are different, the phenotype will reflect one or the other or both copies.

If only one of these two different genes is reflected in the phenotype, that gene is referred to as dominant; the other is considered recessive. If both genes are reflected in the phenotype, we use different terminology, such as codominance or partial dominance.

An example of dominance and recessiveness in human beings is earlobe attachment. Free earlobes are dominant; attached earlobes are recessive. Thus, an individual who has the two different gene forms (a heterozygote) will have free earlobes. An example of codominance is the AB blood type (as compared with types A, B, or O); people with this blood type simultaneously express both the A and B genes that they have.

Q: What does it mean to be a gene 'carrier'?

A: In human beings and other higher organisms, each individual has two copies of each gene. If an individual carries two copies of the same gene, that person is called a homozygote. If the individual carries two different forms of the gene, that person is said to be a heterozygote.

If only one of the genes expresses itself in the heterozygote, that gene is referred to as dominant, and the other is considered recessive. The recessive gene carried by a heterozygote is not expressed but can be passed on to offspring, where it can be expressed if two copies come together.

The heterozygote has a “silent” copy of the gene and is referred to as a carrier. For example, a normal woman can have an albino child if both she and the child’s father carry a copy of the recessive gene for albinism. One child in four will be albino.

Q: What is gene splicing?

A: Gene splicing, a process known also as genetic engineering or gene cloning, refers to processes in which a piece of genetic material is inserted into a cell.

The modern genetic revolution was kicked off when scientists discovered restriction endonucleases, enzymes that cut deoxyribonucleic acid (DNA), leaving specific “sticky” ends. Similar ends could be created on pieces of DNA that were widely divergent, leading to the formation of pieces of “foreign” DNA into small circles of DNA in bacteria called plasmids.

Since bacteria double every twenty minutes, scientists could create millions of copies of this foreign DNA overnight. Having large quantities of a region of DNA allowed scientists to study the piece of DNA, understand its function, and eventually sequence its DNA bases. With these tools in hand, scientists can now sequence entire genomes (all of the chromosomal DNA), isolate all of the genes of an organism, and study and manipulate all of the genetic processes in an organism.

Q: Have any diseases been treated or cured using gene therapy?

A: Despite over 400 gene therapy studies involving more than 4,000 patients and a dozen medical conditions, the United States Food and Drug Administration has yet to approve any form of gene therapy. But scientists are confident that they are getting closer to curing some diseases.

In 1990 scientists performed the first human experiments using gene therapy on a four-year-old girl who lacked the gene that produces the enzyme adenosine deaminase. Without this enzyme, the child suffered from a condition called severe combined immunodeficiency (SCID), in which her immune system did not function properly. Children with this condition suffer from a variety of life-threatening infections.

In the gene therapy experiment, the girl’s cells were infused with viruses containing the gene that produces adenosine deaminase. Although the inserted gene began producing the missing enzyme, strengthening the child’s immune system, the procedure had to be repeated regularly in order to ensure the proper production of the enzyme. In addition, the gene therapy treatment was accompanied by other treatments, including the direct administration of adenosine deaminase. Thus the results were not a clear-cut success for gene therapy. Scientists had better luck ten years later; French researchers reported in April 2000 that they had successfully treated two boys with SCID using gene therapy alone.

Early gene therapy studies of patients with hemophilia, a blood-clotting disorder, have also proved promising. The patients were given the gene that produces factor IX, a protein involved in blood clotting that is missing in hemophilia patients. The inserted gene began producing factor IX, lessening the need for patients to inject themselves with extra doses of this protein.

While there have been some promising results from gene therapy, there have also been some setbacks. In 1999 an 18-year-old Arizona man received gene therapy for a rare genetic metabolic disorder called ornithine transcarbamylase deficiency. He died as a result of the gene therapy procedure itself, when large numbers of gene-carrying viruses elicited a severe immune response. Although gene therapy has enormous potential to cure genetic diseases, much work must be done to lessen any risks and to improve the treatment’s effectiveness before it can be considered a viable medical treatment.

Q: What role do genes play in an organism's development (from egg to adult)?

A: Although environment can certainly influence the development of an organism, the genes of an organism determine its development. The genome of an organism contains all the information needed to create the adult organism from a fertilized egg. That is why human beings produce human beings and corn plants produce corn plants.

As development of a higher organism proceeds, proteins called transcription factors control the genes that a given cell expresses. In one of the best-studied cases, the fruit fly, transcription factors are initially controlled by gradients of substances in the developing embryo, substances called morphogens. The gradients result from diffusion of genetic material (messenger ribonucleic acid, or RNA) or protein through the early embryo.

As cells differentiate from one another, different suites of genes become active in different cells, giving rise to shape and function in these cells. Thus a single-celled zygote can give rise to a complex, multicellular organism.

Q: Can genes be influenced by environmental factors?

A: Gene-controlled traits can be divided into two categories: single-gene traits and quantitative traits. Quantitative traits are traits controlled by many genes. Single-gene traits seem to be influenced less by the environment, whereas quantitative traits (height, intelligence, susceptibility to some mental illnesses, such as schizophrenia) seem to be influenced by the environment to a greater degree.

The simplest measures of environmental influence are indexes called penetrance and expressivity. Penetrance is the degree to which a trait defined by genes shows itself in the phenotype (the way an individual looks). Expressivity is the measure of how severe or intense the trait is once it shows itself.

For example, cleft palate, a quantitative trait, is a developmental disorder that shows both reduced penetrance and variable expressivity. That is, the cleft palate is less likely to appear, but if it does, the degree to which it shows itself in the individual’s appearance can vary. Single-gene traits like attached earlobes or widow’s peak generally show full penetrance with little variation in expressivity.

Q: What is the genetic difference between identical twins and fraternal twins?

A: Identical twins are the product of a single fertilized egg that split in two very early in development. The result is two individuals that are genetically identical and therefore always of the same gender. (Genetic differences can slip in by mutation in the development of one or the other of the twins.)

Fraternal twins are the result of the fertilization of two eggs in the female by two different sperm cells, and therefore they are like any pair of siblings. Fraternal twins can be of the same or different genders. They can be born as two brothers, two sisters, or as brother and sister.

When individuals of any species are cloned, the newly created organism is just like an identical twin to the organism that was cloned.

Q: How does the genetic makeup of viruses differ from that of cells?

A: Cells, both prokaryotic (without nuclei, as in bacteria) and eukaryotic (with nuclei, as in human beings), differ from viruses in that viruses are obligate cellular parasites that are not really alive. Once inside a cell, the virus can take over the cell’s metabolism, replicate itself many times, and then release new viruses with or without killing the cell.

Prokaryotic viruses (called bacteriophages, meaning “eaters of bacteria”) are remarkably simple structures, usually consisting of some genetic material—deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) and a protein coat that both protects the viral genetic material and provides a way for the virus to enter the cell. Some bacteriophages are designed like tiny hypodermic needles and function as such, injecting their genetic material into the host.

Animal and plant viruses (eukaryotic viruses) are generally more complex. Many are enveloped, and have a lipid layer in addition to the outer protein coat. The lipid layer is usually captured from the host cell’s outer membrane as the virus exits the host.

Q: How can genetically engineered plants pose ecological risks?

A: In the United States, about 25 percent of farmland is planted with genetically modified crops. Some scientists are concerned that certain beneficial animal species that feed on these crops may be harmed by the genetic modifications.

For example, one form of genetic engineering is to modify a plant’s genetic makeup, or genome, so that the plant produces toxins that poison the major insect pests of the plant. Usually this involves adding toxin genes from the bacterium Bacillus thuringiensis (often referred to as Bt) that express proteins that kill the larvae of various insect pests. However, these toxins can affect any insect that feeds on the crop plant, placing beneficial insect species at risk.

Recent laboratory studies performed at Cornell University in New York indicated that the larval stage of monarch butterflies die when they feed on the pollen of genetically engineered corn. In addition, genetically engineered modifications to one type of plant could be spread inadvertently to other plant species through wind pollination. However, so far studies performed in farm fields have not demonstrated significant ecological risks by genetically engineered plants.

Q: What is a genetic bottleneck?

A: Imagine water flowing out of a bottle—the narrow neck of the bottle reduces the flow of the water. The same sort of bottleneck effect occurs when the size of an animal or plant population is reduced. The smaller population now has only a limited pool of genes that can be passed from one generation to the next. This reduction in genetic variability may mean that members of the population do not have the genetic traits that would enable them to adapt to changes in the environment. This population is then in danger of dying, possibly leading to extinction of a species.

The bottleneck effect is also used to describe cases where genetic variation is lost quickly but returns slowly. A reduction in genetic variation could, for example, take ten years to bring about and 500 years to rectify.

Q: Why is genetic variation in a plant or animal population important?

A: Genetic variation in a plant or animal population is the raw material that fuels evolution in a changing environment. All the plant and animal species alive today have evolved by adapting to changing environments. When the environment changes, an animal or plant population whose members all have a similar genetic makeup may not have any individuals with the genetic traits that can help them survive the change. As a result, the whole population may die.

For example, climates change all the time—changes may occur over a short period, such as a six-month drought, or over a long period, such as a 10,000-year ice age. If a climate undergoes a dramatic change, a population of organisms with a similar genetic makeup that were well adapted to the initial climate conditions may not be able to tolerate the new climate. However, if individuals within a population have certain genes that produce traits enabling them to cope with the new climatic conditions, then these individuals will survive, reproduce, and pass the genes on to their offspring. Natural selection will then favor the genes that enable survival in the changed environment.

Over time the genetic makeup of the population gradually changes. If the environment changes again, either back to what it was or to a more extreme climate, any genetic variation present within the population will once again enable some individuals to survive and reproduce better than others. Without genetic variation, evolution would not take place and life could not flourish.

Q: What is genomic imprinting? How is it related to the transmission of genetic information?

A: Approximately 20 human genes are known that show genomic imprinting, a phenomenon in which the expression of a particular gene is determined by the parent of origin. That is, if a gene comes from one parent it will be expressed differently than if it had come from the other parent. The phenomenon seems to be controlled by different changes to the DNA of genes in the two parents—a difference in methylation, which is the addition of methyl groups to the DNA.

One example is striking. In human beings, two medical syndromes result in mental retardation. In Prader-Willi syndrome, affected persons are extremely obese. In Angelman syndrome those affected are sometimes referred to as happy puppets, exhibiting erratic, jerky movements. Both syndromes are associated with deletions in the long arm of chromosome 15. If the remaining region is of paternal origin, the child will have Angelman syndrome; if the gene is of maternal origin, the offspring will have Prader-Willi syndrome. Additional examples of imprinting are found in other human diseases such as Huntington disease and several cancers.

Q: What is the difference between genotype and phenotype?

A: Geneticists use the term genotype to refer to the combination of genes that code for a particular trait. The term phenotype describes the physical characteristics produced by that trait.

For example, albinism is an inherited condition in which a person lacks normal skin, hair, and eye pigmentation. Let’s say that the gene for albinism is designated as a. A person must receive two copies of this albinism gene (one from each parent) in order to be born with the disease. Therefore a person with albinism has a genotype designated as aa and a phenotype characterized by physical attributes such as colorless skin, hair, and eyes.

In simple cases like albinism, the phenotype reflects the genotype exactly: All aa individuals have albinism. However, in more complex traits, there can be variable expression of the gene. For example, there are different degrees of severity of cleft palate among individuals with the same genotype. There are even cases in which the genotype is not expressed at all in the phenotype, as in the case of those people who have the genotype for a genetic form of rickets, a nutritional disorder, but do not have the symptoms associated with the condition. This situation is referred to as a failure of the genotype to penetrate into the phenotype.

Q: What is the difference between germ-line gene therapy and somatic-cell gene therapy?

A: Many human illnesses are caused by genetic changes, known as mutations. For example, cystic fibrosis, all cancers, and sickle-cell anemia are genetic diseases caused by one or more mutated genes. Scientists hope to develop effective gene therapies to cure genetic diseases. In such treatments, which are still experimental, a patient with a genetic disease receives normal or genetically altered genes that replace the flawed or missing genes that cause disease.

For example, gene therapy experiments are underway to cure cystic fibrosis, a genetic disease caused by a mutation in a single gene. In the first step of this process, scientists insert a normal version of the cystic fibrosis gene into a virus. The virus is then inserted into the patient. The genetic material of the virus, including the normal gene, becomes incorporated into the genetic material of the patient’s cells. Scientists hope that the normal gene will replace the flawed gene in order to cure the cystic fibrosis. This process is called somatic-cell gene therapy—even though the patient is cured of the disease, he or she can still pass the mutated cystic fibrosis gene on to future children.

In germ-line therapy, scientists introduce the normal gene into a very young human embryo so that all of the embryo’s cells, including its gamete cells (sperm and eggs), carry the normal gene. In this case, a patient will not be able to pass on the mutated gene to future generations.

Germ-line therapy is considered highly controversial. Critics worry that manipulating the genetic makeup of gamete cells could lead to unforeseen and potentially dangerous long-term health consequences. A September 2000 report of the American Association for the Advancement of Science called for a moratorium on germ-line therapy until its consequences are better understood.

Q: How does melanoma start? What is the treatment for it?

A: Melanoma is a cancer of skin cells containing melanin pigment, cells called melanocytes. The major risk factor comes from exposure to the ultraviolet rays of the sun, particularly via sunburn. Melanoma, like all cancers, is genetic in origin—that is, genetic changes trigger the disease. Ultraviolet light, like other forms of radiation, has the ability to mutate (change) normal genes into forms that, in combination, ultimately lead to cancer.

Several genes have been located that appear to play a role in the development of melanoma, including a gene on chromosome 21 that normally suppresses cancerous growth. Treatment usually begins with surgery to remove the lesion followed by other treatments including injection with interferon or interleukin-2, substances that help the immune system to eliminate melanomic cells. Clinical trials are currently underway for various alternative treatments for skin cancer, including vaccines.

Q: If scientists cloned a human, would the clone be identical to the original person?

A: Genetically, the cloned human would be an exact copy of the original person, with the exception of the environmentally induced genetic changes that occur in cells as time goes by. The two individuals would be equivalent to identical twins—two children who are the product of the same fertilized egg and are thus genetically identical to each other.

Also like identical twins, the original person and the cloned human would be two independent human beings. Although they would be quite similar to each other, their physical characteristics, health, and behavior could differ quite a bit due to differences in early childhood experiences, diet, and other environmental factors.

Q: How do scientists locate genes for specific diseases?

A: To locate a disease-causing gene, scientists find exactly where the gene resides on a particular chromosome. They then replicate the gene to study it, determine the sequence of its DNA bases, and then begin the job of finding out how the gene works.

In order to find a gene, the inheritance of the gene has to be correlated with a genetic marker in a particular region of a chromosome. Scientists have now identified thousands of these molecular markers on human chromosomes.

Scientists study the pedigree that traces specific genetic characteristics through three or more generations of a family with a history of a particular genetic disorder. They look for one or more known genetic markers—if a genetic marker follows the same inheritance pattern of the genetic disorder, then it follows that the disease-causing gene is located near the genetic marker on the same chromosome. Narrowing down the area on the chromosome where the disease-causing gene may reside makes it easier to isolate the gene.

In 1990 American geneticist Mary-Claire King and her colleagues, then at the University of California, Berkeley, isolated the breast cancer gene BRCA1. Dr. King had to examine the inheritance pattern of 183 different markers to locate the gene on chromosome 17. The gene was particularly difficult to locate because it accounts for only about 5 percent of all breast cancers. However, women under fifty years old who have an altered version of this gene have an 80 to 90 percent risk of developing early onset breast cancer.

Q: How does the DNA found in the mitochondria of a cell differ from the DNA found in a cell's nucleus?

A: Mitochondria are small structures in cell cytoplasm that provide the cell’s energy needs. Scientists believe that mitochondria were originally free-living bacteria that invaded other cells and over evolutionary time lost their independence and became mitochondria. As a result, mitochondria differ from most other cellular structures in that they contain deoxyribonucleic acid (DNA). Unlike nuclear DNA, which is linear, mitochondrial DNA is circular, resembling the DNA of bacteria. Mitochondrial DNA also carries out protein synthesis using bacterial mechanisms.

Over the course of their evolution, mitochondria lost their independence. As a result, mitochondrial function came to be controlled dually, by their own DNA as well as by the nuclear DNA of the cell.

Unlike the DNA found in a cell’s nucleus, which can be inherited from both the mother and the father, mitochondrial DNA typically has unusual inheritance patterns. Mitochondrial DNA is usually inherited only from the mother, not the father—sperm cells generally do not contribute mitochondrial DNA during fertilization, when egg and sperm fuse to form a zygote. This form of inheritance is called cytoplasmic inheritance.

Q: Which organism has the greatest number of chromosomes?

A: The organism with the greatest number of chromosomes is the Indian fern, which has 1,260 chromosomes in an adult diploid cell (a single cell in which the basic chromosome number is doubled). Note that a diploid cell contains two copies of each chromosome, one copy from each parent. In human beings, the diploid number of chromosomes is 46, or 23 from each parent. In the Indian fern, the 1,260 chromosomes occur in 630 pairs. The diploid condition is normal for all higher organisms. The minimal diploid number of chromosomes is 2—the condition found, for example, in Ascaris, a roundworm.

Q: What is p53?

A: p53 is a protein that is under intense scrutiny by scientists since it is mutated in many forms of cancer. Its name derives from its size—it is a protein of 53,000 daltons. (A dalton is a measure of molecular size; one dalton equals the mass of one hydrogen atom.)

Cancers are genetically controlled. Usually, the change of several genes leads to full-blown cancer that invades various organs and also travels throughout the body, leaving foci of growth (metastasis). In more than 50 percent of all tumors, scientists have found a change (mutation) in both copies of the p53 gene in a cell; p53 is thus called a tumor-suppressor protein.

Study of the p53 gene has shown that the gene has numerous functions in the cell, but its primary function is to determine the general state of a cell’s health. If the cell is healthy, then p53 is broken down, allowing the cell to continue to grow and divide. If the cell has genetic problems, p53 can be activated. Once activated, it can activate about three dozen other genes that can lead the cell to stop growing or to commit suicide, a process that scientists call apoptosis.

p53 induces genes by itself, interacting with the DNA of these genes. This causes the genes to begin expression in the process of transcription, which is making messenger RNA from the DNA. p53 is thus a transcription factor. If the overseer role of p53 is prevented by mutation of its gene, the cell’s growth control may be lost and cancer can follow.

Q: Can personality traits be inherited?

A: Although most human personality and behavioral traits have a genetic component (as well as an environmental component), at present very few genes are known that control particular personality traits. The genetics of complex traits, like personality traits, are studied by determining a genetic component called heritability.

Heritability is the measure of how much genes influence the difference among individuals. It is usually calculated by comparing the appearance of a trait among relatives with the appearance of the trait among nonrelated individuals. Heritability ranges from 0 (no genetic component to the difference among people) to 1.0 (complete genetic control of differences).

From heritability studies, we know that there is a genetic component in personality and behavioral traits such as intelligence, shyness, novelty seeking, cigarette smoking, homosexuality, and divorce. This is not to say that there is a “divorce gene,” but rather that a gene influences behavior that more often leads to divorce.

As time goes on, scientists will be able to isolate more specific genes that influence personality and to discover the way that the genes affect personality.

Q. What is protein, and why is it important in a healthy diet?

A. Protein is a macronutrient with a chemical structure containing carbon, hydrogen, oxygen, and nitrogen. It is the nitrogen that gives protein its unique properties. Protein is made up of smaller units called amino acids, which are connected together like a strand of pearls. If two strands of pearls were wound together and then twisted to double up on each other, they would resemble a protein molecule.

Your body breaks down protein from food into amino acids and reshuffles them into new protein to build and rebuild tissue, including muscle. Protein also keeps your immune system functioning up to par, helps carry nutrients throughout the body, has a hand in forming hormones, and is involved in important enzyme reactions such as digestion.

There are 20 different types of amino acids, and all can be combined to form the proteins necessary to build the body and keep it healthy. Some of these amino acids can be made by the body and are called nonessential amino acids.

Others have to be supplied by the foods you eat. These amino acids are termed essential amino acids.

Animal and plant foods contain all 20 amino acids (but in different amounts depending on the food). Animal proteins and soy protein are of higher quality because they contain all the essential amino acids in larger amounts and better proportions. In plants, amino acids exist in smaller concentrations. For the body to make proteins properly, all 20 amino acids must be present at the same time.

Although amino acids work together to form body proteins, individual amino acids have specific roles to play in the body. Certain amino acids, such as tryptophan and tyrosine, are involved in the formation of chemical messengers called neurotransmitters for the brain and nervous system. Three amino acids (leucine, isoleucine, and valine) are constituents of muscle tissue.

Q: What is the difference between DNA replication and DNA transcription?

A: Deoxyribonucleic acid (DNA) is a cell’s genetic material, found in the nucleus. When a cell divides, DNA directs the synthesis of an exact copy of itself. This process, known as DNA replication, enables the new cells that result from cell division to have the exact same genetic makeup.

Proteins, which are composed of long chains of amino acids, control all the processes that occur in a cell. The sequence of amino acids in a protein determines its function. DNA contains the information that determines the sequence of a protein’s amino acids. However, DNA itself is not involved directly in the creation of proteins. Instead, in a process known as DNA transcription, DNA directs the synthesis of an intermediate substance called ribonucleic acid (RNA). The structure of RNA contains the DNA’s instructions that determine the amino acid sequence for a particular protein. RNA leaves the cell’s nucleus and travels to the cytoplasm, carrying DNA’s coded information to structures within the cell call ribosomes, which are the site of protein synthesis.

DNA replication and DNA transcription have similarities. In both processes, the genetic instructions stored within the structure of DNA are used as a template to make new nucleic acids: DNA in the case of replication and RNA in the case of transcription. The processes differ in that during DNA replication, the entire DNA molecule is copied, whereas during DNA transcription, only a section of the DNA structure is transcribed into RNA at one time.

Q: What is reverse transcriptase?

A: Reverse transcriptase is an enzyme capable of creating deoxyribonucleic acid (DNA) from ribonucleic acid (RNA)—a process that is the reverse of transcription, which is the creation of RNA from DNA.

Normally, DNA acts as a template for the production of RNA using the property of complementarity. This property comes from the fact that DNA is a double helix made of “rungs” that consist of A-T (adenine-thymine) and G-C (guanine-cytosine) base pairs. When A is one base of the pair, T will be the other; likewise, if G is one base of the pair, C will be the other. Thus A-T and G-C are called complementary base pairs.

DNA and RNA are very similar in structure. Because of the shapes of each base pair, only complementary bases can fit together to form a rung in the ladder of either DNA or a DNA-RNA hybrid. In DNA replication, normal transcription, and reverse transcription, complementarity determines which base is added during the growth process, either using DNA or RNA as the template.

When DNA is used as a template for the production of RNA, the process is called transcription. However, some animal viruses that have RNA as their genetic material (the AIDS virus, HIV, for example) have the gene for the enzyme reverse transcriptase. During their life cycles, their genetic material, RNA, can be used as a template to make DNA by this enzyme. This DNA can integrate into the host cell’s chromosomes, thus becoming a part of the host cell. The reverse transcriptase enzyme is a target for some AIDS treatments.

Q: What are restriction fragment length polymorphisms (RFLPs), and how are they used to identify genetic differences in humans?

A: Much of today’s genetic engineering revolution is due to the discovery of restriction enzymes—specialized proteins that cut deoxyribonucleic acid (DNA) at specific sequences along its structure. If a sample of a person’s DNA is cut with one of these restriction enzymes, the pieces can be separated using a process called gel electrophoresis. This technique uses an electric current to spread the DNA segments into a pattern of bands.

Two different sequences of DNA from two different people will produce a different banding pattern on the electrophoresis gel. These differences are called polymorphisms (meaning “many forms”). The term restriction fragment length polymorphism (RFLP) derives from these polymorphisms of restriction enzyme fragments on the electrophoresis gel.

Scientists consider RFLPs as accurate as fingerprints to match, for example, skin or blood evidence found at a crime scene with a specific person. Fingerprints and RFLPs are recognized as reliable evidence in courts of law.

Q: How do the different kinds of RNA differ from one other?

A: There are three kinds of ribonucleic acid (RNA) in a cell—ribosomal RNA, messenger RNA, and transfer RNA. Each plays a different role in protein synthesis, the process by which cells string together amino acids to produce proteins with specific properties.

Protein synthesis occurs at sites called ribosomes, small protein assembly factories shaped like lumpy spheres. Ribosomal RNAs (rRNAs), along with proteins, form ribosomes.

Messenger RNA (mRNA) is created by deoxyribonucleic acid (DNA), the cell’s genetic material that is found in the nucleus. DNA contains the instructions for the creation of proteins. Messenger RNA transports these instructions from DNA out of the nucleus to the cytoplasm, where the mRNA attaches to the cell’s ribosomes.

Transfer RNA (tRNA) carries individual amino acids to the mRNA-ribosome structure. The tRNAs are instructed by mRNA to string together specific amino acids at the mRNA-ribosome structure. In this way, a particular sequence of amino acids forms that produces a protein with a specific function.

Q: Are men or women more vulnerable to sex-linked disorders?

A: Men are more vulnerable to sex-linked disorders because men have only a single X chromosome, whereas women have two. Since most genetic disorders are recessive (normally requiring two copies of the gene to express the trait), a male who has an X chromosome with a recessive gene will express it and thus have the disorder. This is a unique situation for genes on the X chromosome.

Since women have two X chromosomes, one recessive gene is not enough to cause the disorder: The second X chromosome may carry the normal gene that will mask the disease gene. In general, men will get a recessive sex-linked trait in the proportion of the gene in the population (on all the X chromosomes), whereas women will have the disorder as the square of the proportion (two X chromosomes).

Thus if the hemophilia gene were at 1 percent (0.01) in the population (1 percent of X chromosomes carried this mutation), 1 in 100 men will have the disease, whereas only 1 in 10,000 women will have the disease (0.01 x 0.01). This algebra explains why many more men than women are colorblind or have hemophilia.

Q: What is a sex-linked genetic disorder?

A: In a sex-linked genetic disorder, the gene causing the disorder occurs on the X chromosome. In humans, women have two X chromosomes; men have only one, accompanied by a small chromosome called the Y chromosome.

The X chromosome is a large chromosome with many genes, whereas the Y chromosome is small and has very few genes. Normal men produce sperm that contain either the X or the Y chromosome. Women produce eggs that all contain an X chromosome. The egg that is fertilized by a sperm with an X chromosome will be a daughter, and the egg that is fertilized by a sperm with a Y chromosome will be a son.

Thus families whose parents carry mutated genes of the X chromosome will have different numbers of affected sons and daughters. For example, if a man with hemophilia marries a normal woman, all of their children will be normal. However, if a woman with hemophilia marries a normal man, all their daughters will be normal, but their sons will have hemophilia.

Q: My daughter is 15 years old and just had her first pap smear. She has atypical squamous cells and needs further testing. The doctor said it is not cancer, but could it turn into cancer or does she have a greater risk of getting cancer because of this?

A: Although I am not an expert on this subject, I checked the Web, which is full of authoritative articles on this topic by medical experts. In a word, you should not be overly concerned, but your daughter should be tested again and you should keep an eye on the results of future pap smears (and cervical biopsies, if deemed necessary).

In general, atypical squamous cells do not become cancerous; they simply indicate cellular activity of the cervix, usually due to irritation. However, in fewer than 1 percent of cases the condition can lead to a precancerous state. This is a low percentage, but it is higher than for those women with normal pap smears.

Even if there is a progression toward a cancerous condition, it usually takes ten years or more to develop. In a recent study of research from the last 30 years, doctors found that 68 percent of cases with atypical cells regressed to normal on their own and only 7 percent became more atypical over a two-year period.

Q: What substances in the environment (whether natural or man-made) lead to higher incidences of mutations?

A: Mutations are permanent, inherited changes to the genetic material, or DNA. Thus, any substance or process that can change the DNA molecule is a mutagen, a substance capable of causing mutations.

Mutagens and carcinogens are usually one and the same. All cancers are genetic, and mutation is a process that can lead to cancer. The two largest categories of mutagens and carcinogens are ionizing radiation and chemicals. For example, the energy of ultraviolet light causes skin cancer. The numerous chemicals in cigarette smoke cause various types of cancers. All cancers are begun by mutation.

When scientists wish to do experiments that require a large number of mutations, they usually treat the subject organisms with either ionizing radiation (from radioactive sources) or with chemicals known to cause mutations. Since many of these mutagens have been well characterized, scientists can generate specific types of mutations by using specific chemicals.

Q: Do you think that there is enough testing done on genetically manufactured foods, such as its effects on humans?

A: The responsibility for labeling and approving of genetically modified foods falls under three different federal agencies: the U.S. Department of Agriculture (USDA), the Food and Drug Administration (FDA), and the Environmental Protection Agency (EPA). Each of these agencies covers a different aspect of genetically modified foods.

The level of protection of the public is controversial: Many people feel that testing and labeling of these foods has been inadequate and that we are moving ahead too quickly. As an example, Monsanto has a potato on the market that has been genetically modified to produce a pesticide to protect it against common insect pests. The potato has been modified with a gene from the bacterium Bacillus thuringiensis (Bt), which produces a protein that protects against some of the beetle pests that affect potatoes. Studies have been done showing that the bacteria and their pesticidal proteins are harmless; however, activists are not convinced by these studies because they are often done by scientists who work for or are paid by the company marketing the product (in this case, Monsanto).

Many activists currently want all genetically modified foods labeled as such. The companies do not want this labeling—they fear that a safety label will convince consumers that the genetic modification is dangerous. On the world stage, many countries won’t allow the importation of foods made from genetically modified crops.

This is an area of active controversy with no definitive answers at the moment, other than to note that federal agencies are charged with protecting the public from dangerous foods and are presumably doing their job.

Q: What tests are used to identify genetic disorders?

A: Genetic disorders can be diagnosed in two general ways: by gross phenotypic analysis or by biochemical analysis.

Gross phenotypic analysis simply means that individuals are identified as having the disorder by their looks or behavior. For example, people with sickle-cell anemia have a syndrome of physical effects as well as red blood cells that form sickle shapes.

Biochemical analysis uses biochemical techniques to identify a particular disorder and often explain the cause of the disorder. For example, in 1949 Linus Pauling and his colleagues—using a technique called electrophoresis, in which various forms of hemoglobin are separated using an electric current—showed that sickle-cell anemia was caused by a mutated form of the hemoglobin molecule. In an electric field, sickle-cell hemoglobin moves at a different rate than normal hemoglobin does, and thus it is identifiable. This was the first example of identifying a so-called molecular disease (a disease attributed to a molecular change in an individual—in this case, a change in an amino acid). Further analysis of sickle-cell hemoglobin showed that a single amino acid was changed in the protein.

Q: What is a transgenic organism?

A: A transgenic organism is an organism whose genetic material has been altered by genetic engineering. Since all the organism’s cells have been altered genetically, the organism will pass on these changes to its offspring.

Transgenic farm animals now exist that produce pharmaceutical compounds, such as blood clotting factors, in their milk; these compounds can be harvested easily and inexpensively for medicinal purposes. The new commercial field of “pharming” encompasses these experiments and processes.

Transgenic crop plants are called genetically modified (GM) crops. As an example, many crops in the United States now produce their own insecticide, often the protein product of a gene from the bacterium Bacillus thuringiensis that has been artificially inserted into the plant. The protein will kill certain types of insect pests, thereby eliminating the need for most externally applied insecticides. These GM crops are often called Bt crops, from the abbreviation of the name of the bacterium.

Appears in

Gene Therapy; Genetics; Genetic Engineering