516 



THE PRINCIPLES OF HEREDITY 



T T T T 



abed 

 abed 



Genes for tall stature 

 Genes for short stature 



T T T T 



abed 

 abed 



T T T T 



abed 



T . T T ,T 



abed 



From Genetics, Winchester, Houghton Mifflin 



Fig. 32.8. Medium-sized parents can have a tall child when the genes for tall stature 

 become segregated into the reproductive cells from both parents during meiosis. 



which influence skin coloration, but these two genes seem to be the ones 

 involved in the differences between the white and Negro races. 



Most inherited characteristics are probably influenced by a number 

 of different genes working together. Since we often find variations in 

 only one of the gene pairs involved, however, it may appear as if this is 

 the only gene involved. Also, we know that one gene may influence 

 more than one characteristic. As an example, there is one gene which, 

 when homozygous in man, causes some defect in the metabolism of one 

 of the amino acids (phenylalanine). As a result, there is an accumula- 

 tion of an acid (phenylpyruvic acid) in the system which is excreted 



