Heredity

I

Introduction

Heredity, process of transmitting biological traits from parent to offspring through genes, the basic units of heredity. Heredity also refers to the inherited characteristics of an individual, including traits such as height, eye color, and blood type.

Heredity accounts for why offspring look like their parents: when two dogs mate, for example, they have puppies, not kittens. If the parents are both Chihuahuas, the puppies will also be Chihuahuas, not great Danes or Labrador retrievers. The puppies may be a little taller or shorter, a little lighter or a lot heavier than their parents are. Their faces may look a little different, or they may have different talents and temperaments. In all the important characteristics, however—the number of limbs, arrangement of organs, general size, fur type—they will share the traits of their parents. The principles of heredity hold true not only for a puppy but also for a virus, a roundworm, a pansy, or a human.

Genetics is the study of how heredity works and, in particular, of genes. A gene is a section of a long deoxyribonucleic acid (DNA) molecule, and it carries information for the construction of a protein or part of a protein. Through the diversity of proteins they code for, genes influence or determine such traits as eye color, the ability of a bacterium to eat a certain sugar, or the number of peas in a pod. A virus has as few as a dozen genes. A simple roundworm has 5000 to 8000 genes, while a corn plant has 60,000. The construction of a human requires an estimated 50,000 genes.

If the DNA in a single human cell could be unraveled, it would form a single thread about five feet long and about 50 trillionths of an inch thick. To prevent this fine string of DNA from becoming knotted like a big tangle of yarn, parts of the strand are wrapped around proteins like a thread is wound around spools. These units of wrapped DNA are called nucleosomes, and they coil and fold into structures called chromosomes. Humans have 23 pairs of chromosomes. In each pair, one chromosome comes from the mother and the other from the father. Twenty-two of the pairs are the same in both men and women, and these are called autosomes. The twenty-third pair consists of the sex chromosomes, so called because they are the primary factor in determining the gender of a child. The sex chromosomes are known as the X and Y chromosomes.

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Females have two X chromosomes, and males have one X and one Y chromosome. The Y chromosome is about one-third the size of the X chromosome. A sperm, the reproductive cell produced by the male, can carry either one X or one Y chromosome. An egg, the reproductive cell produced by the female, can carry only the X chromosome. When a sperm with an X chromosome unites with an egg, the result is a child with two X chromosomes—a female. When a sperm with a Y chromosome unites with an egg, however, the result is a child with one X and one Y chromosome—a male. Thus, the father determines the gender of the child.

II

Asexual and Sexual Reproduction

Throughout the entire world of life, evolution has brought about only two types of reproduction—asexual and sexual. Asexual reproduction does not require a mate and is less complicated than sexual reproduction. It is used by simple life forms, such as bacteria; complex one-celled organisms, such as amoebas and diatoms; certain worms, such as flatworms; fungi; and many plants.

In asexual reproduction, one parent transmits all of its genetic information to the offspring, and the offspring is therefore identical to the parent. Asexual reproduction typically is a rapid and reliable method of reproduction. It is limited, however, because the genetic uniformity in the offspring makes them all equally susceptible to a change in the environment. If a new disease, a new predator, or a climate change is lethal to one individual, it is lethal to all genetically identical organisms. Such changes can effectively wipe out entire populations of genetically identical organisms. Sexual reproduction results in offspring with diverse traits, and is the predominant form of reproduction among plants, animals, and most other organisms.

In contrast to asexual reproduction, sexual reproduction requires two parents. Each parent creates sex cells, or gametes that contain half the parent’s genetic information. Human sex cells—sperm and eggs—contain 23 single, unpaired chromosomes rather than the 23 paired chromosomes found in all other body cells, or somatic cells. When egg and sperm unite in the process called fertilization, they form one cell that contains 23 pairs of chromosomes, the normal number for human body cells. The cell develops into a child that has a mixture of genetic information from both parents. As a result, the child is similar to each of the parents but not identical to either of them.

If these same parents have a second child, it is the product of fertilization of a different sperm and a different egg. Therefore the second child is unique, because each sperm and egg contains a unique set of chromosomes (see Meiosis). Scientists estimate that each person is capable of producing 2²³ or 8,388,608 unique sex cells. The total number of unique children possible from one couple is a phenomenal 2²³ × 2²³ or 2⁴⁶. This genetic diversity that results from sexual reproduction enables populations to withstand changing environments through evolution.

With the exception of the X and Y chromosomes, genes come in twos on the paired chromosomes, but the genes are not necessarily identical. The hair color gene from the father may carry information for black hair, but its partner on the chromosome from the mother may specify red hair. These different forms of genes that carry information for specific traits are called alleles. A person’s hair color depends on several alleles interacting in complex ways to determine the actual trait of the offspring.

III

Patterns of Inheritance

A pattern of inheritance describes how alleles work together to produce traits. Understanding inheritance patterns enables geneticists to predict the probability that a child will inherit a certain trait. A variety of inheritance patterns influence the diverse traits found not only in humans, but in other animals, plants, fungi, and bacteria.

A

Dominant-Recessive Inheritance

The dominant-recessive pattern of inheritance, a relatively simple pattern, involves paired alleles that influence one trait. In this pattern, one of the two alleles contains information for a certain characteristic—the lavender color of sweet pea flowers, for example—while the second allele directs the production of an alternate characteristic—the white flower color. In sweet peas, if these two alleles occur together, the allele for lavender flowers is expressed, and the flowers are lavender. The allele for lavender is therefore called the dominant allele. The allele for white is known as the recessive allele. Lavender flowers also occur when two alleles for lavender color are paired. Only when two alleles for the recessive characteristic are paired do white flowers appear. This genetic rule applies regardless of the organism or the trait. In the dominant recessive pattern, the recessive trait shows up only when two recessive alleles are paired.

In humans, several hundred genetic diseases and disorders follow the dominant-recessive pattern. These conditions result when a mutation, or a change in a normal allele, is found in a sperm or egg, and the mutation causes disease when the child inherits a pair of mutated alleles. If a child inherits one dominant allele and one recessive allele he or she typically does not have the disease. Such individuals are termed carriers, since although healthy, they carry the recessive allele. A carrier can pass either the dominant or recessive allele to their child. If both parents are carriers, these alleles can be passed along in four ways. The child can receive a normal allele from each parent, in which case it does not develop the disease. It can receive a mutated allele from the mother and a normal allele from the father, or a normal allele from the mother and a mutated allele from the father. In both of these cases, the child will be a carrier. The child develops the disease only if he or she receives a mutated allele from each parent. When both parents are carriers, there is a 25 percent chance that a child will be disease-free, a 25 percent chance that it will have the disease, and a 50 percent chance that it will be a carrier. Examples of genetic diseases that follow the dominant-recessive pattern include sickle-cell anemia, beta-thalassemia, cystic fibrosis, and severe combined immunodeficiency disease (see Genetic Disorders).