Epistasis

There are also instances when two or more gene pairs interact with one another. One common example is the inclusion of the scurred condition along with polled vs. horned in cattle. Scurs are horn tissue on the skin, but not fastened to the skull. Inheritance of horns is at a different locus (gene location) than are scurs. The gene action is outlined here:

Genotype

Gender

Poll horn

Scur

Male

Female

PP, Pp

SS

Scurs

Scurs

PP, Pp

Ss

Scurs

Polled

PP, Pp

ss

Polled

Polled

pp

SS, Ss, ss

Horns

Horns

The scur locus is expressed only in an animal that is polled. There is an interaction between two loci such that one locus is expressed only when the other locus is arranged in a specific manner. This is also an example of sex-influenced inheritance. Scurs are dominant in males but recessive in females.

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