HEREDITY AND VARIATION 



all the offspring will be gray in both the h\ and F2 generations; the same 

 situation would prevail if both original gray mice had been homozygous. 

 However, if any white mice appeared in a cross between two grays, their 

 presence would prove both parents to be heterozygous. 



The breeding results are explained adequately in these cases of mono- 



Gametes 

 ofFi 



WEE 

 long 

 gray 



WEe 



long 



gray 



VvEE 

 long 

 gray 



VvEe 

 long 

 gray 



WEe 

 long 

 gray 



Wee 

 long 

 ebony 



VvEe 

 long 

 gray 



Vvee 

 long 

 ebony 



VvEE 



long 



gray 



VvEe 

 long 

 gray 



vvEE 



vestigial 



gray 



vvEe 



vestigial 



gray 



VvEe 

 long 

 gray 



Vvee 

 long 

 ebony 



vvEe 



vestigial 



gray 



vvee 



vestigial 



ebony 



Zygotes from 

 which Pi 

 developed 



Gametes 

 of Pi 



Zygotes of F-^ 

 that give rise 

 to long -winged, 

 gray -bodied flies 



Zygotes 

 ofFg 



9 long -winged, 

 gray -bodied 



3 long -winged, 

 fbony- bodied 



3 vestigial -winged, 

 gray - bodied 



1 vestigial -winged, 

 ebony -bodied 



Fig. 6.15. A case of dihybridization between a vestigial-winged, gray-bodied fly (Drosophila) 

 and a long-winged, ebony-bodied fly (cf. Fig. 6.6). The diagram illustrates disjunction and 

 independent assortment of two pairs of homologous chromosomes which carry two pairs of genes; 

 notice the possible combinations that can occur between the gametes formed. E, gene for gray 

 body; e, gene for ebony body; F, gene for long wing; v, gene for vestigial wing. 



189 



