96 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS 



of having a simple chromosome complex (only four pairs). 

 For instance, in Drosophila, the characters for black body- 

 colour, as opposed to the grey wild type, and for vestigial 

 or badly developed wings, both recessives, are linked 

 together and are thus known to lie on the same chromosome; 

 one never occurs without the other. Further analysis 

 shows that all the variable characters of Drosophila can be 

 arranged in four linkage groups, corresponding to the four 

 chromosome pairs. There is a special interest in the 

 cases, long known in man though not understood, where 

 characters lie upon the " X " or sex-chromosome, and 

 are said to be sex-linked. Thus if a man suffers from 

 haemophilia, a congenital tendency to excessive bleeding, 

 his spermatozoa will be of two kinds : half carry the 

 X-chromosome and with it the gene for haemophilia 

 (which is a recessive character), and may give rise to females 

 who themselves do not show the condition but pass it on 

 to half of their male offspring, and half the spermatozoa 

 carry the Y-chromosome, on which there are few or no 

 genes ; these may give rise to males, who will be free from 

 the haemophilia altogether, so that it cannot appear in 

 their offspring. 



The studies on Drosophila have demonstrated that 

 exceptions to the law of linkage may occur. Before the 

 reducing division in the maturation of the ova the chromo- 

 some-pairs are tightly interlaced, and an exchange of 

 material or " crossing-over " may take place between 

 them. In this way a gene may pass from the chromosome 

 on which it originally lay to the partner chromosome, and 

 two linked characters may be separated. As the genes do 

 not pass over singly but in groups, the frequency of separa- 

 tion of linked characters is a measure of the distance 

 between their genes on the chromosome, and in this way 

 Morgan and his colleagues have been able to prepare 

 chromosome maps for Drosophila^ showing the position 

 on the chromosomes of the genes of over three hundred 

 mutant characters and the relative distance between them. 

 The chromosome maps of related species are closely 

 similar. 



Conclusion. — Heredity may be defined as the relation 

 of genetic continuity between successive generations, and 



