30 The Journal 
utilize standard males and dark females. 
While this is the natural explanation of 
the facts, and gives the desired results, 
it would be far better for breeders to 
recognize that there is a true sex differ- 
ence, and to make the standard for 
females proportionately darker then 
the standard for males. 
HIGH EGG LAYING POWER 
Pearl (1912) has rendered a great 
service from an econcmic standpoint 
through his discovery that high laying 
is inherited as a sex-linked factor. By 
high laying is meant the ability to 
produce more than thirty eggs during 
the winter laying period of the pullet 
year. This is simply a standard which 
by careful mathematical methods has 
been shown to be very closely correlated 
with high laying throughout life. It 
does not mean that a hen which does 
this and then stops laying is a high 
layer, although such a hen is probably 
potentially a high producer, her failure 
being due to some extraneous cause. 
By selecting the males and females on 
the old idea of breeding the best to the 
best (males from two-hundred-egg hens 
to females laying above 150 eggs), 
during a period of ten years, there was 
no significant change in the mean pro- 
duction of the flock. As soon as the 
system of selection was based on the 
knowledge that the high laying was due 
to the inheritance of a_ sex-linked 
factor (Pearl, 1915), the average began to 
rise and in the first year, November, 1913, 
to July, 1914, the flock produced more 
eggs per hen than even in an artificial 
year made up of the best months during 
the ten years of mass selection. 
POSSIBILITIES IN DAIRY CATTLE 
Cole mentions the fact that dairy 
cattle men have believed that the male 
influences the milk flow of the daughters 
more than the female does, ard quotes 
one or two authors on the subject. 
Definite evidence on the point is difficult 
to obtain, although there have been 
suggestions at various times that sex- 
linkage may be involved. Ina study of 
5,691 Holstein cows with their daugh- 
ters, two of the writer’s former students, 
Hills and Boland, discovered a corre- 
of Heredity 
lation of 0.29 between the maternal 
records and those of the offspring. 
If the cow is homozygous for the 
sex factor, as seems probable, and if 
the animals studied were perfectly 
heterogeneous, this might indicate a 
sex-linkage of certain of the production 
factors, as a correlation of less than .25 
would normally exist. An alternative 
explanation is possible, however, as the 
material studied was selected on an 
advanced registry (high production) 
basis, and the daughters which failed to 
attain the standard were not included 
among the advanced registry indivi- 
duals. This would tend to raise the 
correlation coefficient perhaps suffici- 
ently to account for the amount noted. 
SEX-LIMITED INHERITANCE 
What was designated in the first 
paragraph as the third type of inheri- 
tance has frequently been assumed to be 
a form of sex-linked inheritance. Arkell 
and Davenport have devised a scheme 
which bridges all of the voids between 
the true sex-linked inheritance and the 
characters whose expression is affected 
by the sex gland, but it is doubtful 
whether such an explanation is necessary 
since that assumed by Professor Wood, 
the discoverer of the phenomenon, is 
both simple and reasonable. 
Wood crossed Suffolk sheep, polled in 
both sexes, with Dorset sheep, horned 
in both sexes, and obtained horned 
males and polled females. When such 
crossbred horned rams and polled ewes 
were mated, the male offspring showed 
three horned individuals to one polled, 
while the female offspring showed one 
horned individual to three polled. Dom- 
inance was reversed in the two sexes, 
the horned character being dominant 
in the male and the polled character 
being dominant in the female. 
A few years ago the writer discovered 
a character in swine which showed a 
similar mechanism of inheritance. On 
the lower part of the scrotum of the 
male, and well to the rear of the in- 
guinal pair of mamme in the female, 
there frequently occurs a small rudi- 
mentary pair of mamme. This char- 
acter has been shown to be dominant 
in the male and recessive in the female, 
