Hypothesis: The Frequencies of Juvenile Diabetes in American Blacks and Caucasians Are Consistent with Dominant Inheritance

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Abstract
A test is described that can be used to distinguish whether a trait (in this case juvenile diabetes) is inherited in an autosomal dominant or a recessive manner. The frequencies of the trait are compared in two populations in which there has been a unidirectional influx of genes from the gene pool of one of the populations into that of the other. Before the racial admixture, the trait should be primarily a characteristic of the first population. If the trait is dominant, its frequency in the second population will be equal to the number of individuals carrying at least one gene causing the trait and thus directly proportional to the overall percentage of its genes received from the first population. If the trait is recessive, its frequency in the second population, relative to that in the first, will be far lower than the overall percentage of its genes received from the first population, because the rate of homozygotes is equal to the square of the gene frequency. The American Caucasian and black populations fulfill the criteria for the test. Racial admixture between the two groups has been entirely from the Caucasian to the black population, and juvenile diabetes was very likely nonexistent in black people before their arrival in North America. About 20% of the American black gene pool is composed of Caucasian genes and the frequency of juvenile diabetes in the American black population is 20%–30% that in American Caucasians. Because juvenile diabetes in American blacks is associated with the same HLA genes as in Caucasians, and these genes are also about 20%–30% as common in American blacks as in Caucasians, the occurrence of juvenile diabetes in American blacks is very likely due to Caucasian genes. If susceptibility to juvenile diabetes is inherited in a recessive manner, its frequency in black Americans should be about (1/5)2 or 1/25 the frequency in Caucasians. However, since the ratio of the frequency of juvenile diabetes in American blacks to that in Caucasians is roughly equal to, or certainly not lower than, the fraction of Caucasian genes in the American black gene pool, the data fit the criterion for dominant better than for recessive inheritance. Because population data are used in the test, factors that confound genetic studies in families, such as incomplete penetrance and recombination should not confound this test.