HETEROZYGOTE FORMS 193 
Both these forms, then, the black and the splashed 
white, are clearly pure homozygotes. On mating a 
black and a splashed white together, black-bearing 
gametes and white-bearing gametes will meet together 
in fertilization. In every case in which this form of 
mating was carried out the resulting chicks were 
invariably blue. 
The gametes of the blue type of Andalusians, then, 
according to our supposition, do not bear the blue 
character at all. Half of them contain the black and 
half of them the splashed white allelomorph. Such 
gametes, meeting by chance when a pair of blue An- 
dalusians are mated together, give rise to the zygotes 
—one black-black, two black-white, one white-white— 
the black-whites being, of course, blue in appearance 
as before. 
Now, we may put this explanation to the test by a 
very simple experiment—namely, by mating the sup- 
posed heterozygote blues with the black and with 
the splashed white types respectively. Both these 
forms of mating were examined by Bateson and 
Punnett, and the results were as follows : It was found 
that blues crossed with blacks gave rise to equal 
numbers of blue and of black offspring, whilst when 
blues were crossed with splashed whites there ap- 
peared blue and splashed white chicks in equal numbers. 
And by a repetition of the process it could be shown 
that the blues so obtained were heterozygotes as before. 
Here, then, we have clear evidence that equal numbers 
of the germ-cells produced by the blue birds bear the 
pure black allelomorph and the pure splashed white 
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