CHROMOSOMES, HEREDITY AND SEX. 
509 
characters only to her sons, while the male transmits the 
same characters to both sons aud daughters. Later it was 
found that in Diptera (Drosophila) and in Mammals (cat, 
man) the male transmits characters only to his daughters, 
while the female transmits them impartially. The facts 
show that in one case the female, in the second the male, is 
constantly heterozygous for certain features, and that in 
inheritance the factors for these features are closely coupled 
with a sex-determining factor. The case of Drosophila will 
be the most convenient to deal with first (38-41). Morgan 
finds that the male transmits certain of the normal features 
of the species (red eye, long wing, brown colour) only by his 
female-producing gametes, so that when mated with a female 
lacking any of these characters, the male offspring are without 
them. Now the male Drosophila has been shown by Miss 
Stevens (55) to have a pair of unequal chromosomes, and 
Morgan makes the not unnatural assumption that the larger 
of these chromosomes bears not only the sex-factor, but also 
the factors for the sex-limited characters. Since half the 
spermatozoa contain the larger chromosome, and half lack it, 
and since all the eggs contain it because it is equally ])aired 
in the female, half the zygotes have it in duplicate and 
become females, half receive it only from the mother and 
become males. And since, by hypothesis, the larger idio- 
chromosome of the male bears both the sex-factor and the 
factors for the sex-limited characters, the latter are trans- 
mitted by the male to his female offspring only. Morgan 
has carried this conception further in connection with the 
phenomena of gametic coupling. When a male having two 
sex-limited characters is crossed with a female lacking both, the 
female offspring have both, the males neither. In the game- 
togenesis of the heterozygous female so produced, the two 
factors are, in some cases, not distributed evenly among the 
gametes, but tend to be associated, so that the gametes which 
bear either both or neither are much more numerous than 
those which bear one only. A diagram will make this clearer, 
in which the two characters concerned are represented as A 
