Ay and other mutations at the a locus conferring a completely yellow coat color are dominant to all a alleles that produce a darker coat. Hair pigment of Ay heterozygotes is yellow, but eyes are black. Heterozygotes usually become obese and infertile within a few months after birth. Increased adipose tissue mass is due to fat cell hypertrophy, and it has been hypothesized that the obesity results from the observed reduction in hypothalamic norepinephrine and dopamine. Heterozygotes are more susceptible to several kinds of tumors than are normal mice, possibly due, at least in part, to a general increase in cell proliferation that also manifests as a slight increase in lean body mass and skeletal length. Further spleen cells from heterozygotes cause a significantly lower graft vs. host reaction. Mice homozygous for the yellow spontaneous mutation (Ay) die before implantation, or shortly thereafter. The time of death and type of abnormality is, in part, determined by the genetic background on which the mutation is placed. Mice of the KK strain develop diabetes of polygenic origin, and mice of other strains heterozygous for Ay become obese and mildly diabetic. KK.Cg-Ay/J heterozygotes develop hyperglycemia, hyperinsulinemia, glucose intolerance and obesity by eight weeks of age. Studies using isolated adipocytes indicate that tissue responsiveness to insulin is decreased. Histo- and immunocytochemical studies show that the pancreatic islets are hypertrophied and the &beta;-cells are degranulated. These findings suggest that the principal cause of diabetes in these mice is insulin resistance. Body composition analysis shows that both the fat and lean tissue mass are increased compared to non-obese mice, with fat accounting for 30-35% of total body weight. The pleiotropic mutant phenotype of Ay/a mice is attributed to ectopic expression of the agouti protein, while the early embryonic lethality of the Ay mutation in the homozygous state is assumed to result from lack of expression of the Raly gene product (Bultman et al. 1992, Miller et al. 1993, Duhl et al. 1994, Michaud et al. 1993, 1994). That ectopic expression of the agouti protein is probably responsible for the non-lethal aspects of this mutation has been demonstrated by transgenic expression of the protein from a ubiquitous promoter (Klebig et al. 1995, Perry et al. 1995).
For further information, see Mouse Genome Informatics entries for a and for Raly and Michael F.W. Festing's description of <a href="http://www.informatics.jax.org/external/festing/mouse/docs/KK.shtml">KK</a>.

Mice homozygous for the yellow spontaneous mutation (Ay) die before implantation or shortly thereafter. Heterozygotes usually become obese and infertile within a few months after birth. Heterozygotes are more susceptible to several kinds of tumors than are normal mice. Further, spleen cells from heterozygotes cause a significantly lower graft vs. host reaction. Mice of the KK strain develop diabetes of polygenic origin, and KK.Cg-Ay/J heterozygotes develop hyperglycemia, hyperinsulinemia, glucose intolerance and obesity by eight weeks of age. Pancreatic islets are hypertrophied and the &beta;-cells are degranulated. Both the fat and lean tissue mass are increased compared to non-obese mice, with fat accounting for 30-35% of total body weight.

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