GENERAL ZOOLOGY 



Gray mouse 



White mouse 



Gray 

 mice 



Intercrossed 



1 homozygous 

 gray mouse 



Intercrossed 



Homozygous gray mice 



2 heterozygous gray mice 



1 homozygous 

 white mouse 



Intercrossed 



Intercrossed 



r 



Homozygous white mice 

 \ 



1 homozygous 

 gray mouse 



2 heterozygous gray mice 



1 homozygous 

 white mouse 



Fig. 6.4. Results of monohybridization of mice which differ in color of coat; gray coat color 

 is dominant to white coat color (cf. Fig. 6.11). 



some are found to breed true for grayness, whereas others produce both gray 

 and white offspring. Complete analysis by breeding reveals that 25 per cent 

 of the ^2 generation are gray and will breed true for gray coat color, 50 per 

 cent appear gray but will not breed true, and 25 per cent are white and re- 

 produce only white-coated individuals when interbred. 



Up to this point the examples used have involved inheritance with dominance. 

 Dominance and recessiveness do not, however, characterize all cases of in- 

 heritance. If red and white four-o'clocks {Mirabilis jalapa) are crossed, the 

 hybrids of the F2 generation have pink flowers, not red or white. When these 

 pink-flowered hybrids are interbred, offspring occur in the ratio of 1 red : 2 

 pink : 1 white. The individuals with red and white flowers breed true for 

 these characteristic colors, whereas those with pink flowers always give three 

 kinds of offspring in the typical 1:2:1 ratio. Another case of inheritance with- 

 out dominance is the Blue Andalusian fowl (Fig. 6.5). Blue Andalusian fowls 



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