Recessive Inheritance

Definition

A trait that is inherited in a recessive fashion only manifests phenotypically in homozygous individuals, i.e., when the individual has two copies of the same recessive allele. Humans have two versions of all autosomal genes, called alleles, one from each parent. The recessive trait is hidden in the heterozygous individual (Dd) if the other allele is inherited in a dominant fashion, and so this person is a called a “carrier” of the recessive allele, but does not manifest the disease or trait. A recessive trait can only be passed to the offspring if both parents carry (Dd or dd) and transmit the recessive allele to their offspring. In the scenario where both parents are heterozygous carriers of the recessive trait, the children have 25% chance of inheriting two copies (dd) of the recessive allele and exhibiting the recessive trait (see pedigree figure, Fig. 1). An example of a disease with a recessive inheritance is Tay-Sachs disease that occurs when a child has two...