90 Monohybrids 



the theory that the gene producing minor streblomicrodactyly 

 is a dominant gene. 



If a recessive gene caused minor streblomicrodactyly, the 

 normal individuals who produced some offspring with crooked 

 little fingers were heterozygous for this recessive gene. Although 

 this is possible, it is improbable that at least three out of four 

 normal people apparently selected at random would have the 

 gene for crooked little fingers in a heterozygous condition, for if 

 this were true, we should expect to find many more people with 

 crooked little fingers. If we add to these a fourth individual 

 who also produced a child with minor streblomicrodactyly, the 

 theory of a recessive gene as the causative agent becomes even 

 more improbable. It is much more probable that a dominant 

 gene is involved (although it is also explainable on an assump- 

 tion of low penetrance). In dealing with such problems we must 

 first rule out all explanations that are impossible. Then, of the 

 possible explanations, we should select the most probable at least 

 as a tentative hypothesis, realizing, of course, that high prob- 

 abihty is not the equivalent of certainty. 



Inheritance of Multiple Alleles in Autosomes 



It was shown in Chapter 2 that more than one allele may 

 be present at the same locus of several homologous chromosomes 

 although, of course, there is only one in each chromosome and 

 therefore there would be only two in any one diploid individual. 

 In sheep, three such alleles have been found and are designated 

 H, H' , and h. In the female, gene H is dominant to the other 

 two, and gene H' is recessive to H and dominant to h. In the 

 males, incomplete dominance is found and leads to more compli- 

 cated results. 



Such breeds as the Shropshire, Southdown, Cotswold, and Suf- 

 folk are hornless in both sexes, and, in the place of horns, small 

 depressions are found in the skull (Fig. 30). These strains have 

 been bred for a long time and are homozygous for H. Gene H' is 

 found in the Dorset Horn breed from Dorsetshire. In this breed 

 the male has very large horns and the female also has horns, 

 although hers are smaller than the male's. Purebred strains of 

 this type are homozygous for H' . The Merino group, including 

 the Rambouillet, is the recessive; the males have smaller horns 

 than the males of the Dorset Horn breed and the females are 



