GENES 701 



transferred from one part of a chromosome to another, the 

 other when a gene is introduced by crossing into a new genie 

 environment. The total pattern of genes, in which any individual 

 gene is working, is called the gene complex. 



DETERMINATION OF SEX 



Most animals which have been carefully examined have an 

 important cytological distinction between the sexes ; when the 

 chromosomes are arranged in pairs at meiosis there is one pair 

 which consists in one sex of two chromosomes which are not 

 exactly alike. The odd one may be slightly different in shape, 

 as in Drosophila where it is hooked, or it may be very much 

 smaller, as in man ; in some species there may be a single chromo- 

 some left over with no fellow to pair with. It appears in fact 

 as if one sex is cytologically heterozygous, and the normal 

 inheritance of sex shows that this is so. In most animals, including 

 mammals and flies, the males are heterozygous for sex, or hetero- 

 gametic as it is called, but in birds and Lepidoptera the females 

 are. The fully pairable sex chromosomes of the homogametic 

 sex are called X, the odd one of the heterogametic sex is called Y. 

 A cross between a female XX and a male XY should then give 

 females XX and males XY in approximately equal numbers, 

 and this is found to be generally true. Deviations from the 

 fifty per cent, sex ratio can usually be explained in some other 

 way, as for instance in mammals by a differential mortality in 

 the uterus. It has been shown that the other chromosomes, or 

 autosomes, have some influence in determining sex, but the 

 main action is by the sex chromosomes. 



SEX LINKAGE 



If the sex chromosomes carry genes for any other characters 

 one would expect their inheritance to be peculiar. In fact in 

 many animals, as in Drosophila, the X chromosome carries a 

 number of genes all of which are sex-linked. In the heterogametic 

 sex they can never be homozygous, for they cannot exist in the 

 Y. Fig. 543 shows the inheritance of the black/3'ellow colour 

 difference in cats. The reciprocal crosses are different (this is 

 a characteristic of sex-linked inheritance) and all the hetero- 

 zygotes, which are tortoiseshell in colour, are females. (The very 

 rare tortoiseshell males which are on record appear to be sterile 



