agouti The wild type agouti phenotype is caused by yellow pigment deposited in a band on a black hair shaft.

The AY mutation is a deletion in the yellow gene and extending into an adjacent gene (Merc), which is critical to embryonic development.

The "loss of function" in a Merc mutant causes AY/AY homozygotes to die before birth.

yellow Heterozygotes deposit yellow pignment along the entire length of hair shafts.