Different Dominance Relationships

Most genes exhibit complete dominance or recessive-ness. Interesting exceptions are incomplete dominance and codominance. In incomplete dominance, the heterozygous phenotype is intermediate between that of either homozygote. For example, in familial hypercholes-terolemia (FH), a person with two disease-causing alleles completely lacks LDL (low-density lipoprotein) receptors on liver cells that take up cholesterol from the bloodstream (fig. 24.7). A person with one disease-causing al-lele (a heterozygote) has half the normal number of cholesterol receptors. Someone with two wild-type al-leles has the normal number of receptors. The associated phenotypes parallel the number of receptors—those with two mutant alleles die as children of heart attacks, individuals with one mutant allele die in young or middle adulthood, and people with two wild-type alleles do not develop this type of hereditary heart disease.

Different alleles that are both expressed in a heterozygote are codominant. For example, two of the three alleles of the I gene, which determines ABO blood type, are codominant (see fig. 14.21). People of blood type A have a molecule called antigen A on the surfaces of their red blood cells. Blood type B corresponds to red blood cells with antigen B. A person with type AB has red blood cells with both the A and B antigens, and the red cells of a person with type O blood have neither antigen.

10 Ways To Fight Off Cancer

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