HEREDITY AND VARIATION 



Non-cross-over progeny 99% 



Cross-over progeny 1% 



Fig. 6.24. The effect of crossing over in a case of sex-linked inheritance. 

 A heterozygous female Drosophiia with red eyes and gray body is mated 

 with a male which has white eyes and a yellow body like her father. 

 Ninety-nine per cent of the offspring (shown at A') are of the expected 

 or non-cross-over types that result when the X-chromosomes at A pass 

 into ova and combine with sperm. One per cent of the offspring (shown 

 at B') have unexpected combinations of eye and body colors; these 

 develop from zygotes that have received chromosomes in which crossing 

 over has occurred (shown at B). Chromosomes from the female are 

 represented as empty, and those from the male are dotted. T and y are 

 the genes for gray and yellow body color; W and w for red and white 

 eye color (cf. Fig. 6.23 Cand D). 



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