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

INTER - ALLELIC INTERACTION

A classical example of two genes influencing the same character,

• Each variety of poultry possesses a characteristic comb type.
  • Wyandotte – Rose comb
  • Brahmas – Pea comb
  • Leghorns – Single comb
• Each of these varieties breed true.
• Crosses between Rose combed and single combed variety showed that Rose was dominant over single and a 3:1 ratio appeared in the F2.
• Crosses between Pea combed and single combed variety showed that Pea was dominant over single and a 3:1 ratio appeared in the F2.
• When Rose was crossed with Pea, all the offspring showed a new comb form known as “Walnut”
• When the F1 Walnut combed birds were inbred, in the F2 generation Walnut, Rose, Pea and Single combed ones also appeared.
• Because the four comb shapes appeared in a 9:3:3:1 ratio (i.e., nine Walnut chickens per every three Rose chickens per every three Pea chickens per every one Single-comb chicken), it seemed that two different genes must play a role in comb shape.
• This ratio is expected in F2 from a cross of parents differing in two genes.
• Differences from normal dihybrid inheritance are
  • The F1 resembles neither the parent (Walnut comb)
  • Apparently novel characters appear in F2 (Single comb)
  • Walnut character results from an interaction between two independently inherited dominant Rose and Pea genes.

• Single comb results from interaction of their two recessive alleles.

To view "Typical Dihybrid Ratio for a Single Trait Animation"