Genetically controlled quantitative variation of ornithine transcarbamylase in the chick kidney

Sex-linkage of the chicken ornithine transcarbamylase gene

The chicken ornithine transcarbamylase gene has been mapped to the short arm of the Z chromosome, near the centromere. Radioactive in situ hybridization was performed using a human cDNA clone, and chromosomes were scored for silver grain accumulation. This represents the first documented case of a mammalian X-linked gene also being sex-linked in an avian species. The significance of this finding in the context of sex chromosome evolution is briefly discussed. Further, differential sex-related OTC expression, whereby females (ZW) consistently produce higher levels of the enzyme than males (ZZ), must imply a peculiar mode of OTC transcription regulation.

Genetic control of ornithine transcarbamylase induction in chick kidney

After ornithine transcarbamylase (OTC) induction by egg-yolk feeding, OTC activity increases rapidly in chicks bearing an Ocb gene. This response to an egg yolk diet does not appear in chicks having no Ocb gene (showing low OTC activity). The chicks showing intermediate OTC activity also respond to the diet, but moderately. Crossing experiments revealed that OTC induction by egg yolk-diet feeding is inherited as a simple autosomal dominant trait. Since a chick develops during embryonic life by utilizing egg yolk from the yolk sac, the variation of OTC activity among chicken breeds and within a breed in 2-day-old chicks seems to depend on a genetically controlled difference of inducibility by egg yolk. The Ocb is an autosomal gene which controls the induction of OTC activity, but it is difficult to explain the consistent difference in OTC activity between sexes by involving this gene or this locus alone.