Because genes come in pairs, one on each of the chromosomes that make up a chromosome pair, the two genes of the pair are not always identical. The many different forms that a particular gene can take (and still function, for better or for worse, as a gene within that gene’s job description) are referred to as its alleles. If the alleles are identical, you are homozygous for that gene; if they are not, you are heterozygous. Abnormal alleles cause genetic disease or disability: if one allele is enough to cause abnormality then the gene is dominant (inherited with dominant inheritance) and the abnormality is present in the heterozygous and the homozygous state; if two alleles are needed to cause abnormality, then the gene is recessive (inherited with recessive inheritance) and the abnormality is present in the homozygous state and in a state where the two alleles are different but both are harmful (called compound heterozygosity. In the case of alleles found on X-chromosomes (see sex chromosome) but not on Y-chromosomes, which are smaller, a recessive gene will be unopposed in males (and so will act as a dominant gene), whereas female carriers of the allele will be unaffected except in the extremely unlikely event that they inherit (or gain by mutation) a second abnormal allele; this mode of inheritance is called sex-linked recessive inheritance.
Copyright © 1998 Robert
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