588 



COLLEGE ZOOLOGY 



Gametes of F, 



OJI 



Ratio: 1 white-splashed: 2 blue: 1 black 



Figure 415. Lack of dominance. Cross between a white splashed fowl and a black Andalusian 

 fowl. The hybrid (heterozygote) is called a blue Andalusian. 



penetrance. If, however, a dominant gene 

 does not cause the development of the trait 

 in every case, the gene is said to have re- 

 duced penetrance. In other words, a domi- 

 nant gene may possess a somewhat hmited 

 power to "penetrate" the developmental 

 mechanism and cause a given phenotype. In 

 all of Mendel's experiments with mono- 

 hybrids, 100 per cent penetrance was pres- 

 ent, hence the 3:1 ratio; but this ratio is not 

 realized if the penetrance is reduced. Re- 

 duced penetrance of a dominant gene is 

 sometimes called irregular dominance. The 

 gene sometimes comes to expression in the 



heterozygous condition, and sometimes it 

 does not. The causes of reduced penetrance 

 are not very well understood, but modifying 

 genes or external environmental factors, or 

 both, are known to be responsible in some 

 cases. 



Some striking effects of environment on 

 the penetrance of genes are produced by 

 differences in temperature. On Siamese cats 

 the dark pigment is produced only in the 

 cooler parts of the body. The color of 

 Himalayan rabbits corresponds to that of 

 Siamese cats; the fur is white except on the 

 feet, nose, and tail, where it is black. Ex- 



