60 INBBEEDING AND OUTBREEDING 



A large series of results on independent Mendelian 

 heredity obtained during the past eighteen years by 

 numerous biologists can be interpreted in the same man- 

 ner. At first glance many of their results appeared to be 

 either very complex or very irregular, but one by one 

 they were shown to be just as simple as the cases given. 

 Considering only instances which may be interpreted by 

 two factors, for example, we have F 2 ratios of 12:3:1, 

 9:7, 9:3:4 and 13 : 3. But these are not difficult to ana- 

 lyze. They are simply the ordinary 9:3:3:1 ratios in 

 which part of the terms are combined in various ways. 

 For example, the F 2 ratio, when certain black beans, are 

 crossed with certain white beans, is 12 black : 3 yellow : 1 

 white. Clearly this is because black + yellow (AB) is in 

 appearance not different from black (A) alone. A purple 

 sweet pea crossed with a certain white variety segregates 

 in a ratio of 9 purple : 7 white. This result is easily ex- 

 plained by assuming that the purple color only appears 

 when a color base factor, A, is present in connection with 

 a color-producing factor, B. The last three terms of the 

 dihybrid ratio, 3AB : 3aB : lab, therefore, are alike in 

 appearance. This assumption has been proved to be cor- 

 rect in other ways. Again if a black variety of rat, mouse, 

 guinea-pig or rabbit be crossed with a white variety car- 

 rying a factor for ticking the hair with yellow, known as 

 agouti, the segregating ratio is 9 agouti : 3 black : 4 white. 

 The reason for the combination of classes Ab and ab is 

 because the agouti factor does not show except in the pres- 

 ence of color (B). Finally, a factor A which inhibits the 

 action of B and therefore makes AB and Ab resemble ab, 

 gives the peculiar ration 13 : 3. Crossing a certain race of 



