Sex-Linked Traits, Codominance + Incomplete dominance

Sex-Linked Traits

• Humans have 22 pair of autosomal (non-sex) chromosomes and 1 pair of sex chromosomes

• Females : XX / Males : XY

• Only 1 chromosome from each parent can be passed (Chromosome from father determines gender)

• Traits controlled by genes on X chromosome are called ‘Sex-Linked Traits’

• For males, a gene on the X chromosome will always be expressed regardless of whether or not it is recessive or dominant because there is no matching allele on the Y chromosome. (Y chromosome does not carry an allele, and ‘only’ determines gender if present)

• Recessive sex-linked traits more common in males

• Only females can be carriers for sex-linked traits (Heterozygous)

Incomplete Dominance

• Creates intermediate phenotype (Blend)

• Some traits will be shown as an intermediate phenotype if the individual is heterozygous

• Upper / Lower case not used because neither is dominant

Codominance

• 2 Alleles expressed at the same time

• Both alleles are expressed

• If 1 parent is homozygous red and the other homozygous white, offspring will be a blend of red and white. Each individual hair of the animal is either completely red or white

Heterozygous Advantage

A survival benefit for individuals who inherit two different alleles for the same trait

• Sickle Cell Anemia : Sickle red blood cells (C-shaped), do not transport oxygen effectively because they cannot pass through enough small blood vessels. This leads to blockages and tissue damage (Codominance)

• Resists Malaria