542 GENETICS IN RELATION TO AGRICULTURE 
up of two classes having different chromatin contents might display dif- 
ferential mortality under the stress of slightly unfavorable conditions. It 
may easily be seen, therefore, that slight or conceivably wide variations 
in the sex-ratio may be due to the operation of factors other than those 
which actually determine sex. 
Causes of Unusual Sex-ratios——In this discussion we desire to 
treat only of causes of sex-ratio disturbance which may operate in the 
higher domestic animals. The curious and interesting conditions which 
are found in some insects and other lower animal forms will not be men- 
tioned save in so far as they may throw light upon the problem of sex- 
ratio disturbance in higher animals. 
In some cases hybridity appears to favor a disproportionate produc- 
tion of one sex or the other. Thus in the bison-cattle crosses, the 
production of males is rel- 
atively rare. This, how- 
ever, appears to be due to 
Taste LX XI.—Ratio or Sexes In Hysrip 
Guinea-pias (After Detlefsen) 
Number of f i 
Generation Males | Females | Total inales to 100 the fact that physical 
females difficulties make it prac- 
tically impossible for a 
F,, slate Deck a 23 37 60.9 cow to bear to maturity a 
= 2 
PF, 74 wild....) 31 52 83 aun bison-cattle fetus and to 
Fy dei wilds cn| 401 196) |) 217 cre) eS Lo 
Fs, 46 wild...| 159 | 153 | 312 | 103.9 give birth to it. There are 
Fs, 2 wild...) 173 | 171 | 344 | 101.2 two reasons for this, the 
Fy, 164 wild...| 58 64 122 90.6 increased size of the hybrid 
F, {28 wild..| 16 21 37 | 76.2 fetus, and the development 
Sane Cae ies aera [a2 con of a large hump which 
| | cannot be accommodated 
by the normal pelvic con- 
formation in the cow. Consequently the fact that practically all 
the animals born of this species cross are females, may simply 
be due to abortion and death of male fetuses. The amount of 
trouble is sufficient in this case to give room for a potential equality 
of sexes in such crosses. Detlefsen reports similar results from a cavy 
species cross, which gave a disproportionately low ratio of males. The 
data are given in Table LXXI. The earlier generations show a con- 
siderable deviation from the normal equality of the sexes. With suc- 
cessive back-crosses, however, the ratio soon becomes one of practical 
equality. 
When races are more closely related a different result is produced. 
Thus Miss King found in crosses between wild and albino rats a sex-ratio 
of 119.1 males to 100 females among 425 hybrids representing the 
first three hybrid generations of such a cross. Among guinea-pigs 
crossed inter se, Minot found among 410 individuals a sex-ratio almost 
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