332 INTRODUCTION TO EVOLUTION 



from each other; each germ cell receives one member only of each pair of 

 chromosomes (Fig. 15.1). Since genes are contained in the chromosomes, 

 genes also occur in pairs, and each germ cell receives but one member of 

 each pair (Fig. 15.1 ). The genes composing any pair may be alike, or they 

 may differ. Thus, to use a familiar example which somewhat oversimplifies 

 actual conditions, a person may inherit a gene for brown eyes from each of 



HETEROZYGOUS 

 BROWN- EYED MOTHER 



HETEROZYGOUS 

 BROWN- EYED FATHER 



Genes in 

 chromosomes: 



Ova 

 (two kinds) 



*■ Sperms 

 (two kinds) 



(Possible 

 combinations) 



Children 

 possible: 



<t^ 



— V 



Brown 



Blue 



FIG. 15.1. Mendelian inheritance of eye color in a family in which both parents are 

 brown-eyed but heterozygous. Brown color is shown as dependent upon a single gene 

 which is dominant to the gene for blue color — somewhat of an oversimplification. 



his parents. He will, of course, be brown-eyed. Another person may inherit 

 a gene for blue eyes from each of his parents; he will have blue eyes. A 

 third person may inherit a gene for blue eyes from one parent, a gene for 

 brown eyes from his other parent. What will be the color of his eyes? Ex- 

 perience demonstrates that it will be brown. But it can be shown that the 

 gene for blue eyes in such an individual is not destroyed or altered in any 

 way by its contact with the gene for brown eyes. The gene for blue eyes 

 simply does not produce any visible characteristic when the gene for brown 



