192 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
(b) The Principle of Segregation. The appearance of white mice, as 
just described, in the second hybrid generation, follows from the prin- 
ciple of segregation. The primitive germ-cells of the primary hybrid 
contain both parental characters, D (dominant) and & (recessive), but 
in the maturation of the germ-cells the two are separated, so that the 
ripe germ-cell (or gamete) contains either D or R, but not both. This 
is demonstrably true in both sexes. Accordingly there are ova, D and &, 
and spermatozoa, D and &. If dominants and recessives are produced 
by each parent in equal abundance, and they unite at random, the sorts 
of zygotes resulting and their relative segucnicles of occurrence will be 
expressed by the product, — 
D+ £& (ova) 
D + R# (spermatozoa) 
DD +- 2 D (k)* + KER (zygotes). 
One individual in four will be a pure dominant, DD (gray in the 
case of mice) ; likewise one in four will be a pure recessive, A (white 
in mice) ; while two in four will be hybrids, D (A), like their parents, 
the primary hybrids, though indistinguishable in appearance from the 
pure dominant, DD. 
2. Mosaic INHERITANCE. 
An important exception to the two principles just stated needs to be 
noted. In cases otherwise conforming to Mendel’s law, there sometimes 
occur exceptional hybrid individuals in which the normal dominance of 
one character is not realized, but the two alternative characters coexist 
in a patchwork or mosaic arrangement. Such a condition is illustrated 
in the case of piebald, or spotted, mice. 
Segregation of characters does not commonly occur in the formation of 
the gametes produced by mosaic individuals. The gametes, as well as the 
parents, are mosaic, DR. For when two mosaic individuals are mated, 
they commonly produce only mosaic offspring ; and when a mosaic is 
mated with a pure recessive, RA, no recessive offspring are as a rule 
produced. These facts show clearly that the ordinary mosaic individual 
forms no pure recessive gametes; in other words, that segregation does 
* The parenthesis is used to indicate that the recessive character, though 
present, is not visible. Whenever the recessive character alone is present in an 
individual [as in (RA)], it will of course be visible; but whenever the recessive 
character is present together with the dominant [as in the two individuals D(A) ], 
the recessive character will not be visible. 
“4 aetna 
