Coat colour phenotype, leucopenia, and insulin-dependent diabetes mellitus (IDDM) in BB rats

Susceptibility to IDDM in BB rats is linked to the MHC and to one or two non-MHC genes. It is postulated that one of the non-MHC genes is leucopenia inherited as autosomal recessive trait which is, at least in part, due to the absence of RT6.1 T lymphocyte subsets. Because the RT6 alloantigenic system is located near the coat colour gene c, we analyzed the coat colour genes of BB rats by production of F1 and F2 hybrids by crossing of three diabetic and leucopenic BB/OK females (RT1u) with one diabetes-resistant and non-leucopenia DA male (RT1av1) genetically defined by the coat colour genotype AABBCCHH. The coat colour phenotype and the RT1.A haplotype were determined in all 144 F2 hybrids. Furthermore, PMNL, body weight gain and plasma glucose were monitored up to an age of 30 weeks of life. At an age of 30 weeks the pancreatic insulin content was determined. The results let us assume that, (a) the coat colour genotype of BB/OK rats is defined as aaBBcchh, (b) the RT6 locus is not responsible for the leucopenia in BB/OK rats and (c) there is a third gene in the diabetes development of BB/OK rats which is probably not linked to one of the coat colour genes.