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).

B.

Polygenic Inheritance

A significant number of human traits, such as eye color, skin color, height, weight, and muscle strength are typically regulated by more than one allele in a pattern known as polygenic inheritance. Several thousand alleles, for example, may combine to determine a person’s potential for pole-vaulting, and several hundred may play a role in establishing a person’s normal weight. Certain diseases may result from mutations in one or more alleles involved in polygenic inheritance. Researchers have identified nearly a dozen mutated alleles that are associated with diabetes mellitus, and a similar number are linked to asthma. Heart disease may be linked to two or three times that number. Some types of cancer may be correlated with more than 100 different genes. Polygenic inheritance is quite complex, and the ways in which multiple genes interact to produce traits are not fully understood.

C.

X-Y Linked Inheritance

X-Y linked, or sex-linked, inheritance results from the size differences between the X and Y chromosomes. The longer X chromosome carries an estimated 250 genes, which are responsible for critical biochemical functions such as normal blood clotting. The shorter Y chromosome carries 6 genes, which are responsible for other traits, such as producing significant amounts of testosterone, the male sex hormone.

X-Y linked conditions typically occur in a male when the single X chromosome carries a mutated allele, one that prevents normal blood clotting, for example. A male does not have a second X chromosome with a normal allele to override the mutation. As a result, the male in this case will have hemophilia, a disease in which blood does not clot normally. If one of the female’s X chromosomes carries the mutated allele, however, her second X chromosome is usually normal. The normal allele is the dominant allele, so the female does not have hemophilia. Thus, females are typically carriers of X-Y linked diseases but do not develop them unless they receive a mutated allele from each parent, an unusual event. Among the genetic disorders typically carried by females but inherited by males are hemophilia, color blindness, and Duchenne’s muscular dystrophy.

D.

Mitochondrial Inheritance

In most organisms, the chromosomes located in the cell nucleus contain the vast majority of the DNA. But another structure in the cell, called a mitochondrion, also holds a chromosome. The DNA on this chromosome is referred to as mitochondrial DNA. While both sperm and egg contain mitochondria, only the egg’s mitochondria are transmitted to the offspring. The sperm’s mitochondria are contained in the sperm’s tail, which never penetrates the egg.

Mutations in mitochondrial DNA have been implicated in a number of genetic diseases. These diseases include diabetes mellitus, deafness, heart disease, Alzheimer’s disease, Parkinson disease, and Leber’s hereditary optic neuropathy, a condition of complete or partial blindness resulting from degeneration of the optic nerve. Mitochondrial medicine is a relatively new specialty that seeks to explain the disorders and the patterns of inheritance associated with mitochondrial DNA.

Since mitochondrial DNA is inherited only from the mother—a type of inheritance known as maternal inheritance—scientists can trace these genes from one generation to the next, a simpler task than tracing genes that might come from either the mother or the father. The study of mitochondrial DNA has been employed to study human evolution. Recently scientists extracted mitochondrial DNA from Neandertal bones believed to be between 30,000 and 100,000 years old. They compared these ancient genes with those of hundreds of people around the world. As a result, they determined that Neandertals are a different species than humans and not their ancestors, as was formerly believed.