Pedigree of an X-Linked Recessive Trait - Colorblindness

This three generation pedigree demonstrates some of the distinctive characteristics of an X-linked recessive trait. These include:
  1. more affected males than affected females
  2. affected grandfather to affected grandson transmission through a carrier female intermediate
  3. no male to male transmission

If the affected male in the first generation is designated genotypically as cb Y and the unaffected female is designated as Cb Cb where cb is the colorblindness allele and Cb is the normal allele, the three generations can be diagrammed as follows:
Generation Male
Genotype / Phenotype		 Female
Genotype / Phenotype
First cb Y / affected Cb Cb / unaffected
Second Cb Y / unaffected Cb cb / carrier
Third 50% Cb Y / normal
50% cb Y / affected		 50% Cb Cb / normal
50% Cb cb / carrier



This document maintained by Robert J. Huskey Last updated on July 6, 2000.