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NATURE 
[FEBRUARY 15, 1917 
“*Sex-Linked Inheritance in Drosophila,” by Prof. 
T. H. Morgan and Dr. C. B. Bridges, has now been 
issued by the Carnegie Institution of Washington 
(Publication 237, 1916). These flies are excellent sub- 
jects for investigation, as they can be reared in very 
large numbers, and they show an extensive series of 
characters in eye-colour, body-pattern, wing-nervura- 
tion, etc., which are definitely sex-linked, being trans- 
mitted by the male to his daughters only, who show a 
character if dominant, and conceal it if recessive, while 
the female transmits such characters to her offspring of 
both sexes impartially. Many of these characters have 
appeared as *‘mutants” in the course of the experi- 
ments, which have thus furnished proof of the segre- 
gation of new forms. But the most striking feature 
of the researches on Drosophila is the apparently cer- 
tain connection between the observed inheritance of 
the sex-linked characters and the behaviour of the 
sex-determining (x) chromosomes. ‘Over a hundred 
characters that have been investigated as to their link- 
age relations are found to fall into four groups, the 
members of each group being linked in the sense 
that they tend to be transmitted to the gametes in 
the same combinations in which they entered from the 
parents. .. . A most significant fact in regard to the 
linkage shown by the Drosophila mutants is that the 
number of linked groups corresponds to the number 
-of pairs of the chromosomes.” 
The authors claim that there is conclusive evidence 
-of the x-chromosome’s part as sex-determinant, and 
they believe that they can locate the position in this 
x-chromosome of many of the sex-linked factors. In 
a small proportion of individuals of the F, generation 
it sometimes happens that the sex-linked characters are 
not distributed according to expectation. In such 
cases “‘an interchange has taken place between the 
two x-chromosomes in the female in such a way that a 
piece of one chromosome has been exchanged for the 
homologous piece of the other.’’ This “‘ crossing over”’ 
of pieces of paired chromosomes is one of the most 
remarkable hypotheses founded on sex-limited inherit- 
ance, and would help to explain various anomalies in 
Mendelian results. ‘There are,’ the authors remark, 
“certain facts familiar to the cytologist that furnish a 
clue as to how such an interchange might take place.” 
Those who wish to follow the subject further may 
consult with profit Dr. H. J. Muller’s series of papers 
on “‘The Mechanism of Crossing Over,” the last of 
which, with a summary, appeared in the American 
Naturalist of July, 1916 (vol. 1., No. 595), as well as 
Dr. A. H. Trow’s “Criticism of the Hypothesis of 
Linkage and Crossing Over” (Journ. of Genetics, v., 
No. 4), enforcing the “extraordinary difficulties” 
which prevent some students of heredity from accept- 
ing a theory ‘‘simple enough at first sight.” 
A short paper on ‘‘Sex-Limited Colour in Ayrshire 
Cattle," by Prof. E. N. Wentworth, has been pub- 
lished in the Journal of Agric. Research (vi., No. 4). 
The author concludes that black-and-white—a com- 
bination long known in the breed—is a simple allelo- 
morph to the more favoured red-and-white, black-and- 
white being dominant in bulls and red-and-white in 
cows. 
Miss R. Haig Thomas describes studies of ‘Colour 
and Pattern Transference in Pheasant-Crosses" 
(Journ. of Genetics, v., No. 4); her paper is illus- 
trated by a good series of coloured plates and photo- 
graphs. The Swinhoe, Silver, Formosan, and Reeves 
species formed the subjects of the experiments, which 
afford interesting examples of sex-limited inheritance 
analogous to those shown in other organisms. “The 
male parent always transmits the female characters of 
his species to his female offspring, and the female 
NO. 2468, VoL. 98] 
parent transmits to her male offspring many of the 
male characters of her species. . . . The phenomenon 
of pattern- and colour-transference is present in all 
the experiments made in pheaSant-crosses up to date. 
These consist sometimes of transference from one area . 
to a different area in the same sex, or from one area 
to a different area in the opposite sex, or from an area 
in One sex to the same area in the opposite sex... . 
In the fertile hybrids, plumage, dimension, leg colour 
and structure, habit, call, are all correlated, but 
moult is independent and liable to great disturbance 
in hybridisation.” ‘ 
Sex-linked factors in domestic animals may deter- 
mine characters of much economic importance. For 
example, the work of Dr. Raymond Pearl with barred 
Plymouth Rock fowls has shown that high winter 
egg-production depends on two Mendelian factors, one 
of which is sex-linked. In the American Naturalist 
(xlix., 1915, No. 586) Dr. Pearl gives an account of 
the results of selection-breeding for this character over 
a period of seventeen years. Fron: 1898 to 1907 
““mass selection’ for breeding of high producers was 
carried on without any test of the progeny from par- 
ticular matings, and this was found to be ineffective in 
improving the strain. Since 1907 the fight thrown on 
the problem by research on Mendelian lines has enabled 
selection to be based on the genetic nature of the birds 
as shown by the performances of their progeny, and 
such selection ‘“‘was extremely and quickly effective,” 
so that “if one selects genetically high producers . . . 
he succeeds very rapidly in fixing a high-producing 
Strain.” 
To many students the facts of sex-linked inherit- 
ance, together with the difference indicated by the 
presence of one or two x-chromosomes in the nuclei 
of one or the other sex, have strongly suggested the 
conclusion that maleness and femaleness are them- 
selves to be regarded as Mendelian alternative char- 
acters (allelomorphs), and that sex is irrevocably 
determined in the fertilised egg. As mentioned above, 
this conclusion is strongly urged in Morgan and 
Bridges’s memoir on Drosophila. Yet such facts as 
the development of female characters in male crabs 
parasitised by Sacculina, demonstrated by the late Dr. 
Geoffrey Smith’s well-known researches, forbid sweep- 
ing generalisations as to the determination of sex 
throughout the animal kingdom solely by the nuclear 
constitution of the germ-cells. On this fascinating sub- 
ject Dr. O. Riddle has lately published a contribution 
(American Naturalist, 1., No. 595) on ‘‘Sex Control 
and Known Correlations in Pigeons.” Acknowledgin 
that ‘‘ when one nowadays states that he has obtaine 
a real control—a reversal—of the development of sex, 
he can feel assured that his biological audience de- 
mands a very large volume of rigid proofs,” the 
author believes that such proofs are forthcoming from 
the work on pigéons of the late Prof. Whitman, 
supplemented by researches of his own. He states 
that ‘‘ width of cross” in pairing leads to a high pro- 
portion of males in the offspring, and that in the 
ordinary reproduction of pigeons of the same species 
males predominate among chicks hatched from the 
early, small-yolked eggs, and females among those 
from the later, large-yolked eggs. By appropriate 
treatment it was found possible to “begin the produc- 
tion of females at earlier and earlier stages of the 
season.”’ It is likely that Dr. Riddle will not succeed 
in convincing those biologists who have faith in the 
absolute determination of sex from the nuclear struc- 
ture of the fertilised egg, but his paper may be effec- 
tive in checking the tendency to too positive state. 
ments on this fascinating subject. 
G. Hi -C: 
