296 THE CONTINUITY OF THE RACE 



Morgan adopted this type of crossing in order to study the assortment 

 that took place between various genes in Drosophila, and the four follow- 

 ing backcrosses show the actual results obtained for two of the pairs of 

 allelic genes in that animal. Gray body color (BB) is dominant to black 

 body color (bb), and long or normal wings (VV) are dominant to vestigial 

 wings (vv). Note that in the "first cross," diagramed below, a homozygous 

 gray-long individual is crossed with a homozygous black-vestigial indi- 

 vidual and that their gray-long Fi progeny are used for two contrasting 

 backcrosses: a female F\ (Bb Vv) is crossed with a pure recessive male 

 (bb vv), and then a male Fi (Bb Vv) is crossed with a pure recessive female 

 (bb vv). In the "second cross" a homozygous gray- vestigial individual was 

 crossed with a black-long individual, and their (again) gray-long Fi prog- 

 eny were used for the same two contrasting (or reciprocal) backcrosses. 



First Cross 



The homozygous parents Gray-long X black-vestigial 



BB VV bb vv 



Gametes formed by parents BV bv 



The resulting i*\ Gray-long (Bb Vv) 



1. Backcross made by crossing female F\ (Bb Vv) with male recessive (bb vv): 41.5 per 

 cent gray-long; 8.5 per cent gray-vestigial; 8.5 per cent black-long; 41.5 per cent 



black-vestigial. 



2. Backcross made by mating male F\ (Bb Vv) with female recessive (bb vv) : 50 per 

 cent gray-long; per cent gray-vestigial; per cent black-long; 50 per cent black- 

 vestigial. 



Second Cross 



The homozygous parents Gray-vestigial X black-long 



BB vv bb VV 



Gametes formed by parents Bv bV 



The resulting Fi Gray-long (Bb Vv) 



3. Backcross made by mating female F\ (Bb Vv) with male recessive (bb vv): 8.5 per 

 cent gray-long; 41.5 per cent gray-vestigial; 41.5 per cent black-long; 8.5 per cent 

 black-vestigial. 



4. Backcross made by mating male F\ (Bb Vv) with female recessive (bb vv): per cent 

 gray-long; 50 per cent gray-vestigial; 50 per cent black-long; per cent black- 

 vestigial. 



These results, including the same percentage values, were found to be 

 duplicated whenever the same crosses were made in these same ways. 

 They thus appear to be regular and predictable (under the conditions 

 given), but how are they to be explained? Note that in both the "first" 

 and "second" crosses, the female F x must form four types of germ cells, 

 since when she is crossed to a pure recessive male, four types of progeny 

 are produced, 83 per cent (41.5 + 41.5) like the combinations shown by 

 her parents and 17 per cent (8.5 + 8.5) of the new combinations. When 

 the reciprocal backcrosses are made, however, a male F\ being crossed 

 with a pure recessive female, only two kinds of progeny appear, half of 



