Claim: Huntington's disease is a disorder that may be inherited.

Dominance in genetics is a relationship between alleles of one gene , in which the effect on phenotype of one allele masks the contribution of a second allele at the same locus . The first allele is dominant and the second allele is recessive . For genes on an autosome -LRB- any chromosome other than a sex chromosome -RRB- , the alleles and their associated traits are autosomal dominant or autosomal recessive . Dominance is a key concept in Mendelian inheritance and classical genetics . Often the dominant allele codes for a functional protein whereas the recessive allele does not .   A classic example of dominance is the inheritance of seed shape in peas . Peas may be round , associated with allele R or wrinkled , associated with allele r . In this case , three combinations of alleles -LRB- genotypes -RRB- are possible : RR , Rr , and rr . The RR individuals have round peas and the rr individuals have wrinkled peas . In Rr individuals the R allele masks the presence of the r allele , so these individuals also have round peas . Thus , allele R is dominant to allele r , and allele r is recessive to allele R . This use of upper case letters for dominant alleles and lower case ones for recessive alleles is a widely followed convention .   More generally , where a gene exists in two allelic versions -LRB- designated A and a -RRB- , three combinations of alleles are possible : AA , Aa , and aa . If AA and aa individuals -LRB- homozygotes -RRB- show different forms of some trait -LRB- phenotypes -RRB- , and Aa individuals -LRB- heterozygotes -RRB- show the same phenotype as AA individuals , then allele A is said to dominate or be dominant to or show dominance to allele a , and a is said to be recessive to A.   Dominance is not inherent to either an allele or its phenotype . It is a relationship between two alleles of a gene and their associated phenotypes ; one allele can be dominant over a second allele , recessive to a third allele , and codominant to a fourth . Also , an allele may be dominant for a particular aspect of phenotype but not for other aspects influenced by the same gene . Dominance differs from epistasis , a relationship in which an allele of one gene affects the expression of another allele at a different gene .