Some genes exist in more than two allelic forms—that is, there are multiple alleles within the collective genetic makeup, or gene pool, of the population as a whole.

For example, there are over 100 alleles responsible for cystic fibrosis, and there are 3 alleles for ABO blood types.

Two of the ABO blood type alleles are dominant and symbolized with a capital I (for immunoglobulin) and a superscript: A and B.

There is one recessive allele, symbolized with a lowercase i.

Some alleles are equally dominant, or codominant. When both of them are present, both are phenotypically expressed. For example, a person who inherits allele A from one parent and B from the other has blood type AB.

These alleles code for enzymes that produce the surface glycolipids of red blood cells. Type AB means that both A and B glycolipids are present, and type O means that neither of them is present.

Other alleles exhibit incomplete dominance.

When two different alleles are present, the phenotype is intermediate between the traits that each allele would produce alone.

Familial hypercholesterolemia, for example, is a disease in which people who inherit an abnormal allele from both parents have blood cholesterol levels up to six times higher than in the general population, whereas those who inherit it

from only one parent have levels about two or three times normal. If untreated, those homozygous for the defective allele often die of heart attacks in childhood and untreated heterozygous individuals often die as young adults.

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Question	Answer
Some genes exist in more than two allelic forms—that is, there are multiple alleles within the collective genetic makeup, or gene pool, of the population as a whole.	For example, there are over 100 alleles responsible for cystic fibrosis, and there are 3 alleles for ABO blood types.
Two of the ABO blood type alleles are dominant and symbolized with a capital I (for immunoglobulin) and a superscript: A and B.	There is one recessive allele, symbolized with a lowercase i.
Some alleles are equally dominant, or codominant. When both of them are present, both are phenotypically expressed. For example, a person who inherits allele A from one parent and B from the other has blood type AB.	These alleles code for enzymes that produce the surface glycolipids of red blood cells. Type AB means that both A and B glycolipids are present, and type O means that neither of them is present.
Other alleles exhibit incomplete dominance.	When two different alleles are present, the phenotype is intermediate between the traits that each allele would produce alone.
Familial hypercholesterolemia, for example, is a disease in which people who inherit an abnormal allele from both parents have blood cholesterol levels up to six times higher than in the general population, whereas those who inherit it	from only one parent have levels about two or three times normal. If untreated, those homozygous for the defective allele often die of heart attacks in childhood and untreated heterozygous individuals often die as young adults.

Created by: Russells3709

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