AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

SEX INFLUENCED INHERITANCE

• A trait which is influenced by th sex of the individual is called sex-influenced trait.
• Sex-influenced traits are those that are dominant in one sex but recessive in the other. This is due to the different cellular environments in males and females provided by sex hormones.
• Different hormonal environments affect expression of heterozygote of a trait. However, both homozygotes are unaffected and express the trait irrelevant of the hormones produced .
  • Example
    • Baldness in man
    • Mahogany and Red colour  in Ayrshire Cattle
    • Horns in Sheep

Certain features of sex-influenced traits

• The genes governing sex influenced traits are carried on autosomes and not on sex chromosomes.
• No difference between reciprocal crosses in F₁ and F₂.
• Dominance in the heterozygous condition depends on the sex of the individual or the dominance of the allele may differ in the heterozygote of the two sexes.

Example: Horned condition in sheep

• Dorset horn – both males and females horned
• Suffolk – both males and females hornless (polled)

• So F1 and F2 ratios must be read for males and females separately. The gene for horns is dominant in males and recessive in females while its recessive allele for hornless condition is dominant in females and recessive in males.