K. — BOTANY. 179 



offspring. And conversely, why the dominant male, which is XY, when 

 bred to a recessive female, produces offspring which are either female 

 and dominant or male and recessive. 



Tracing the chromosomes into the next (Fj) generation we see also 

 the reason for the different result obtained from the reciprocal matings 

 if the Fj individuals are inbred. When the female parent has the 

 dominant sex-Unked character half the eggs of the daughters and half 

 the sperms of the sons receive this character. As the sperms receive 

 it along with the X chromosome fertilisation of either kind of egg by 

 these X sperms will cause the character to descend to each grand- 

 daughter. The grandsons, on the other hand, since they arise from 

 fertilisation by the sperms lacking the dominant character — i.e., by the 

 Y sperms — will be dominant or recessive according as these sperms 

 unite with the one type of egg or with the other. Thus we get the 

 Mendelian F^ ratio 3D to IE (fig. 1), but so linked with sex that the 

 dominant class comprises half the males and all the females, while 

 the remaining half of the males make up the recessive class. Where 

 it is the male parent that carries the dominant, and where therefore 

 the dominant character passes along with the X chromosome only 

 to the daughters in F^, their eggs, as in the reciprocal cross, are of 

 two kinds, hut the sons' spei'ms all carry the recessive allelomorphs. 

 Both kinds of eggs being fertilised with both X sperms and Y sperms, 

 the dominant and recessive characters will occur equally in both sexes 

 among the grandchildren, and we get the Mendehan ratio of ID to 

 IE (fig. 2). Muller ^^ puts the number of factors already located in the 

 X chromosome of Drosophila at not less than 500, and in those that 

 have so far been investigated this form of inheritance has been found 

 to hold. 



Instances of sex-linked inheritance are now known in many animals, 

 some of which are strictly comparable with Drosophila, others follow 

 the same general principle, but have the relations of the sexes reversed, 

 as exemplified by the moth Abraxas, which has been worked out by 

 Doncaster,^^ whose sudden death we have so recently to deplore. Here 

 the female is the heterozygous sex, and contains the dummy mate of 

 the sex-chromosome. 



The behaviour of the sex-cKromosomes as here outlined suffices to 

 account for the occurrence of sex-linked inheritance, but the relations 

 found to hold between one sex-linked character and another need 

 further explanation. If a cross is made involving two sex-linked 

 characters, the Fi females when tested hy a double recessive male are 

 found to produce the expected four classes of gametes, but not in equal 

 proportions, nor in the same proportions in the case of different pairs of 

 sex-hnked characters. Partial linkage (coupling) occurs of the kind 

 which has already been described for the Stock and the Sweet Pea. 

 The parental combinations predominate, the recombinations (' cross- 

 overs ') comprise the smaller categories. The strength of the linkage 

 vai-ies, however, for different characters, but is found to be constant 

 for any given pair. Siiice the sex-linked factors are by hypothesis 



i> Aw. A'Cit., vol. liv.. 1920. 



'- Rep. Evolution Committee, iv., 1908. 



N 2 



