From that fertilized egg all your cells developed, and virtually all of them have a full set of genes in their nuclei, determining every physical attribute from the color of your eyes to the type of your blood . Dominant and recessive diseases Genes work in pairs. With few exceptions, each gene from your father has a corresponding gene from your mother, and they interact to determine the traits that are expressed in you. But if they differ in any significant way, the resulting trait may be a blend of the effects of each gene. Or one of the genes, called the dominant gene, may produce a noticeable trait, overcoming the recessive gene.
Most genetic diseases result from abnormal changes, or mutations, in genes.
A mutation may prevent the affected gene from functioning at all or may cause it to produce a harmful trait. The abnormal gene may be dominant - producing an evident genetic defect in each individual who inherits it. Or it may be recessive - having an adverse effect only when both genes in a pair are abnormal. Or its effect may lie somewhere in between these extremes. A dominant genetic disease must be inherited directly from a parent who has the abnormal gene and thus also has the disease.