Introduction To Genetic Principles Pdf
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.
Introduction To Human Genetics Pdf
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3d style pack wondershare serial key. 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.