162 



PHYSICAL BASIS OF HEREDITY 



having different chromosomes, were made by Federley. 

 The hybrids showed intermixed characters of both 

 parents, and their chromosome number was the sum of 

 the haploid numbers of their parents (Fig. 66). 



No reduction in number of the chromosomes takes 

 place in the hybrid at the synaptic stage (except perhaps 

 for one or two small ones), so that the 1st spermatocytes 

 contain nearly the sum of the haploid number of the 



IV 

 KXP 



V 

 F.XP. 



VI 

 RXR 



Fia. 67. — Scheme illustrating the history of the chromosomes, and the back-cross between a 

 hybrid male and one or the other parent; also between two such hybrid Fi individuals. 



parents {A and B) after division of each chromosome 

 (Fig. 67). A second maturation division follows in which 

 each chromosome again divides. As a result each sperm 

 contains the full number of chromosomes, half paternal, 

 half maternal {A and B). The hybrid female is sterile, 

 but the male is fertile. If he is back-crossed to a female 

 of the A race his sperm, carrying both sets of chromo- 

 somes, will produce a 3N individual, A + B -\- A. It will 

 have two sets of the A genes to one set of B. In appear- 

 ance the moth is practically the same as the F^ hybrid, 

 because both contain both sets of chromosomes — the 



