Recessive – Class 12 | Chapter – 5 | Biology Short Notes Series PDF

Recessive: Recessive, as related to genetics, refers to the relationship between an observed trait and the two inherited versions of a gene related to that trait. Individuals inherit two versions of each gene, known as alleles, from each parent. In the case of a recessive trait, the alleles of the trait-causing gene are the same, and both (recessive) alleles must be present to express the trait. A recessive allele does not produce a trait at all when only one copy is present. This contrasts to a dominant trait, which requires that only one of the two alleles be present to express the trait.

Recessive Inheritance

When a trait is recessive, an individual must have two copies of a recessive allele to express the trait. Recessive alleles are denoted by a lowercase letter (a versus A). Only individuals with an aa genotype will express a recessive trait; therefore, offspring must receive one recessive allele from each parent to exhibit a recessive trait.

One example of a recessive inherited trait is a smooth chin, as opposed to a dominant cleft chin. Let (S) represent the dominant allele, and (s) represent the recessive allele. Only (ss) individuals will express a smooth chin. To determine the probability of inheritance of a smooth chin (or any other recessive trait), the genotypes of the parents must be considered. If one parent is heterozygous (Ss) and the other is homozygous recessive (ss), then half of their offspring will have a smooth chin.

Patterns for Autosomal Recessive Inheritance

  • Males and females have the same chance of expressing the trait
  • You can only express the trait if you are homozygous
  • If both parents express the trait, then all their offspring should also express the trait
  • If the offspring express the trait but their parents don’t, then both parents are heterozygous
  • If one parent expresses the trait, then their offspring who don’t express it are heterozygous
  • The trait can skip generations

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By Team Learning Mantras