pie A. H. STURTEVANT 
resent the factor for normal size, carried by the heterochromo- 
somes, by N, then in ordinary guinea-pigs this is present in both 
gametes of each sex. But in a certain strain used by Miss Sollas, 
from which all her dwarfs descended, it had dropped out of the 
odd chromosome. ‘This strain would go on breeding true, then, 
for some time, according to this scheme: 
NEF NF normal @ 
NF nf normal 
NF NF normal 9 
NF nf normal 
But now suppose crossing over sometimes occurred, and we 
should have: 
NF NF normal 9? 
NF nf (nF Nf) normal o& (gametes) 
NF NF normal ¢? 
NF nf normal & 
(NF nF normal ¢) 
(NF Nf normal o) 
We should still have no dwarfs, but if the heterozygous female 
were mated to a male of her own race we should get dwarfs, thus: 
NF nF normal 9 
NF nf normal “ 
NF NF normal 9? 
NF nF normal 9? 
NF nf normal @ 
nF nf dwarf of 
This explains why Miss Sollas should obtain a great majority 
of male dwarfs, and how, as seems to have occurred, the pecu- 
liarity may descend through the female. But she does get some 
dwarf females from normal parents. These are to be expected, 
since they would appear in the last mating above if crossing 
over again occurred in the male, thus: 
