Full text: Problems in eugenics

V. G. Ruggeri. Biology and Eugenics.43 
(since if only one had been so, the other, i.e., the homozygote, would have 
exercised its complete dominance, as in the preceding case), that is to say 
both sprung from crossings with the blond type : such is the case of the 
grandson who resembles the grandfather, and the rule is always that of 25 
per cent., which we know already. 
Perhaps the colour of the skin and that of the hair do not sufficiently 
lend themselves to such assertions, there being intermediate tints. But the 
same facts are verified by the colour of the iris. If two men with blue eyes 
marry two women with dark eyes, they may have children with dark 
heterozygote eyes, whose germ cells or gametes have, therefore, only 50 per 
cent, of dark determinants for the iris. If these individuals marry amongst 
each other, one child in four has the blue eyes of the grandparents. 
From this example it is seen that the blue colour of the iris acts with 
regard to the dark colour as the albino does with the grey, and the blond 
with the brunette, i.e., as a subject or recessive character. In fact, if the 
irises are all dark in the first generation, it is because the blue character 
has remained latent, while in the second generation the separation of the 
determinants becomes evident, as our scheme shows, in which, according 
to the recognised convention, d is the dominant character (dark iris), and r 
the recessive character (blue iris). 
The dominant character is completely excluded once in four, i.e., in 
25 per cent.; in fact, the male determinants ¿) cannot unite with the female 
9 except according to the four arrows marked in the scheme, which I have 
supposed, which, therefore, gives a very clear result. 
It follows equally if, instead of blue, the irises are grey; therefore, 
it being the order of dominance, according to the researches of the Daven­ 
ports^), that dark is dominant for grey, grey for blue, it follows that grey 
can be heterozygote, can have blue latent; while blue (as we have seen for 
the albino or the blond character) can have no other latent character; it is a 
pure character. Parents with blue eyes can only have children with blue 
eyes.cLcL (1) C. B. Davenport, Heredity of Eye-Colour in Man. Science, New Series XXVI. 
(Nov., 1907), p. 592.

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