Some human disorders are inherited. Polydactyly is an inherited disorder where a person has extra fingers or toes. Polydactyly is caused by a dominant allele.

What is a dominant allele?

An allele is a version of a gene. For each gene, an individual inherits two alleles, one from each parent. A dominant allele is one that will produce its characteristic phenotype (the observable trait) regardless of what the other allele is. For example, if 'D' is the dominant allele for polydactyly and 'd' is the recessive allele for the normal number of digits, an individual with the genotype DD or Dd will have polydactyly. The dominant allele masks the effect of the recessive allele when they are present together in a heterozygote.