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IBi figure 12-12

The ABO gene has three alleles (a). Allele IA causes a type A sugar to appear on the surface of red blood cells. Allele IB, causes red blood cells to display a type B sugar. The third allele, i, does not cause the display of any sugars. In (b), a pedigree shows two examples of how blood type can be inherited.

figure 12-13

A person who has a red-green colorblindness might not be able to see the number 5 in the center of the circle in this color-vision test chart.

A person who has a red-green colorblindness might not be able to see the number 5 in the center of the circle in this color-vision test chart.

Pratos Ishiara

Multiple Alleles

Many genes have more than three alleles. Genes with three or more alleles are said to have multiple alleles. For example, in humans, the ABO blood groups (blood types) are controlled by the three alleles IA, IB, and i. The alleles IA and IB are codominant. In codominance, both alleles are expressed in the phenotype of a heterozygote. Both IA and IB are dominant to the recessive i allele. The IA and IB alleles encode variants of an enzyme that cause two different sugar molecules to appear on the surface of red blood cells. The i allele lacks the activity of the enzyme entirely, so neither of the sugars appear on the red blood cell surface. Figure 12-12a shows how combinations of the three different alleles can produce four different blood types—A, B, AB, and O. Notice that a person who inherits two i alleles has type O blood. Figure 12-12b shows how blood type can be inherited.

Incomplete Dominance

Sometimes, an individual displays a trait that is intermediate between the two parents, a condition known as incomplete dominance. For example, in Caucasians, the child of a straight-haired parent and a curly-haired parent would have wavy hair. Straight hair and curly hair are homozygous traits. Wavy hair is heterozygous and is intermediate between straight and curly hair.

X-Linked Traits

Some complex characters are determined by X-linked genes, and a pedigree will usually reveal many affected males and no affected females. A male inherits his X chromosome from his mother. One form of colorblindness is a recessive X-linked disorder in which an individual cannot distinguish certain colors, such as red and green. Several X-linked genes encode proteins that absorb red or green light in the eye. Red-green colorblindness occurs because mutations disrupt these genes, so the eye cannot absorb some colors of light. Eye doctors often test for colorblindness by using a chart similar to the one in Figure 12-13.

Sex-Influenced Traits

Sex-influenced traits are involved in other complex characters. Males and females can show different phenotypes even when they share the same genotype. Sex-influenced traits are usually autosomal. For example, an allele that is dominant in males but recessive in females controls pattern baldness, the type of baldness usually found in men. The difference is due to higher levels of the hormone testosterone in men, which interacts with the genotype to produce pattern baldness.

Single-Allele Traits

A single allele of a gene controls single-allele traits. Geneticists have discovered that more than 200 human traits are governed by single dominant alleles. Huntington's disease (HD) is an autosomal dominant condition characterized by forgetfulness and irritability. It develops as an affected person reaches 30 or 40 years of age and progresses to muscle spasms, severe mental illness, and, finally, death. Because a dominant gene exists in every HD heterozygote, each affected person has at least one affected parent. Unfortunately, many HD patients have already had children by the time their symptoms appear. Direct DNA testing is beginning to allow for earlier diagnosis of the HD allele.

figure 12-14

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