User:Steffikwok/sandbox

Chickens
Chickens exhibit various traits affected by pleiotropic genes. Some chickens exhibit frizzle feather trait, where their feathers all curl outward and upward rather than lying flat against the body. Frizzle feather was found to stem from a deletion in the genomic region coding for α-Keratin. This gene seems to pleiotropically lead to other abnormalities like increased metabolism, higher food consumption, accelerated heart rate, and delayed sexual maturity.[22]

Domesticated chickens underwent a rapid selection process that led to unrelated phenotypes having high correlations, suggesting pleiotropic, or at least close linkage, effects between comb mass and physiological structures related to reproductive abilities. Both males and females with larger combs have higher bone density and strength, which females then use to deposit more calcium into eggshells. This linkage is further evidenced by the fact that 2 genes, HAO1 and BMP2 affecting medullary bone (the part of the bone that transfers calcium into developing eggshells) are located at the same locus as a gene affecting comb mass. HAO1 and BMP2 also display pleiotropic effects with commonly desired domestic chicken behavior, with chickens who express higher levels of these two genes in bone tissue producing more eggs and displaying less egg incubation behavior.[23]