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Many traits are determined by pairs of complementary genes, each inherited from a single parent. Often when these are paired and compared, one gene (the dominant) will be found to effectively shut out the instructions from the other, recessive gene. For example, if a person has one gene for blue eyes and one for brown, that person will always have brown eyes because they are the dominant trait. For a person to have blue eyes, both their genes must be blue (recessive). When a person has two dominant alleles, they are referred to as homozygous dominant. If they have one dominant allele and one recessive allele, they are referred to as heterozygous.
Usually, this masking effect is done by virtue of the fact that the recessive gene has a loss of some function that the dominant gene has. For example, in the case of ABO blood types, the O type is recessive because it does not produce any antigens or antibodies, whereas A and B types (which are codominant) do. Or, in the above case dealing with eye color, there is a complete loss of pigment in blue-eyed people, therefore to express the phenotype, both copies of the gene (after all, humans are diploid) must have that same loss of function.
Huntington's disease is an example of a heritable genetic disease that arises from a dominant allele. Heritable diseases that are dominant are often identified in pedigree analysis by the fact that they do not skip generations like recessively inherited diseases do.
The percentage of people expressing a dominant allele in a population is dependent on the frequency of all alleles for the gene. In the case of two possible alleles, the percentage of the population expressing the dominant allele will be p^2+2pq, where p is the frequency of the dominant allele, and q is the frequency of the recessive allele. The math gets more complex with more genes in the system. The equation p^2+2pq+q^2 is referred to as the Hardy-Weinberg law.
- Klug, William S. Concepts of Genetics. Prentice Hall Eighth Edition.
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