Principles of Genetics

Mendel’s studies have provided scientists with the basis for mathematically predicting the probabilities of genotypes and phenotypes in the offspring of a genetic cross. But not all genetic observations can be explained and predicted based on Mendelian genetics. Other complex and distinct genetic phenomena may also occur. Several complex genetic concepts, described in this section, explain such distinct genetic phenomena as blood types and skin color.

Incomplete dominance

In some allele combinations, dominance does not exist. Instead, the two characteristics are equally expressed. For instance, snapdragon flowers display incomplete dominance in their color. There are two alleles for flower color: one for white and one for red. When two alleles for white are present, the plant displays white flowers. When two alleles for red are present, the plant has red flowers. But when one allele for red is present with one allele for white, the color of the snapdragons is pink.

However, if two pink snapdragons are crossed, the phenotype ratio of the offspring is one red, two pink, and one white. These results show that the genes themselves remain independent; only the expressions of the genes appear to “blend.” If the gene for red and the gene for white actually blended, pure red and pure white snapdragons could not appear in the offspring.

Multiple alleles

In certain cases, more than two alleles exist for a particular characteristic. Even though an individual has only two alleles, additional alleles may be present in the population. This condition is known as multiple alleles.

An example of multiple alleles occurs in blood type. In humans, blood groups are determined by a single gene with three possible alleles: A, B, or O. Red blood cells can contain two antigens, A and B. The presence or absence of these antigens results in four blood types: A, B, AB, and O. If a person’s red blood cells have antigen A, the blood type is A. If a person’s red blood cells have antigen B, the blood type is B. If the red blood cells have both antigen A and antigen B, the blood type is AB. If the red blood cells have neither antigen A nor antigen B, the blood type is O.

The alleles for type A and type B blood are codominant; that is, both alleles are expressed. However, the allele for type O blood is recessive to both type A and type B. Because a person has only two of the three alleles, the blood type varies depending on which two alleles are present. For instance, if a person has the A allele and the B allele, the blood type is AB. If a person has two A alleles, or one A and one O allele, the blood type is A. If a person has two B alleles, or one B and one O allele, the blood type is B. If a person has two O alleles, the blood type is O.

Polygenic inheritance

Although many characteristics are determined by alleles at a single place on the chromosome, some characteristics are determined by an interaction of genes on several chromosomes or at several places on one chromosome. This condition is polygenic inheritance.

An example of polygenic inheritance is human skin color. Genes for skin color are located in many places, and skin color is determined by which genes are present at these multiple locations. A person with many genes for dark skin will have very dark skin color, and a person with multiple genes for light skin will have very light skin color. Many people have some genes for light skin and some for dark skin, which explains why so many variations of skin color exist. Height is another characteristic probably reflecting polygenic characteristics.

Gene linkage

A chromosome has many thousands of genes; there are an estimated 20,000 genes in the human genome. Inheritance involves the transfer of chromosomes from parent to offspring through meiosis and sexual reproduction. It is common for a large number of genes to be inherited together if they are located on the same chromosome. Genes that are inherited together are said to form a linkage group. The concept of transfer of a linkage group is gene linkage.

Gene linkage can show how close two or more genes are to one another on a chromosome. The closer the genes are to each other, the higher the probability that they will be inherited together. Crossing over occurs during meiosis, but genes that are close to each other tend to remain together during crossing over.

Sex linkage

Among the 23 pairs of chromosomes in human cells, one pair is the sex chromosomes. (The remaining 22 pairs of chromosomes are referred to as autosomes.) The sex chromosomes determine the sex of humans. There are two types of sex chromosomes: the X chromosome and the Y chromosome. Females have two X chromosomes; males have one X and one Y chromosome. Typically, the female chromosome pattern is designated XX, while the male chromosome pattern is XY. Thus, the genotype of the human male would be 44 XY, while the genotype of the human female would be 44 XX (where 44 represents the autosomes).

In humans, the Y chromosome is much shorter than the X chromosome. Because of this shortened size, a number of sex-linked conditions occur. When a gene occurs on an X chromosome, the other gene of the pair probably occurs on the other X chromosome. Therefore, a female usually has two genes for a characteristic. In contrast, when a gene occurs on an X chromosome in a male, there is usually no other gene present on the short Y chromosome. Therefore, in the male, whatever gene is present on the X chromosome will be expressed.

An example of a sex-linked trait is colorblindness. The gene for colorblindness is found on the X chromosome. A woman is rarely colorblind because she usually has a dominant gene for normal vision on one of her X chromosomes. However, a male has the shortened Y chromosome; therefore, he has no gene to offset a gene for colorblindness on the X chromosome. As a result, the gene for colorblindness expresses itself in the male.

Another example of sex-linked inheritance is the blood disease hemophilia. In hemophilia, the blood does not clot normally because an important blood-clotting protein is missing. The gene for hemophilia occurs on the X chromosome. As females have two X chromosomes, one X chromosome usually has the gene for normal blood clotting. Therefore, the female may be a carrier of hemophilia but normally does not express hemophilia. Males have no offsetting gene on the Y chromosome, so the gene for hemophilia expresses itself in the male. This is why most cases of hemophilia occur in males.