334 INTRODUCTION TO EVOLUTION 



pair of genes, carried in a different pair of chromosomes, has to do with 

 the color of the hair, determining whether it shall be red or some one of the 

 other shades characteristic of human hair (we shall group them together 

 under the term "non-red"). Accordingly, if parents are heterozygous for 

 both these pairs of genes they can have children who have ( 1 ) blue eyes 

 and red hair, (2) brown eyes and red hair, (3) blue eyes and non-red hair, 

 (4) brown eyes and non-red hair. Other pairs of genes in other chromo- 

 somes are concerned with curliness of the hair. Thus heterozygous parents 

 may have children who have (1) curly red hair and blue eyes, (2) curly 

 non-red hair and blue eyes, (3) straight red hair and blue eyes, (4) curly 

 non-red hair and brown eyes, (5) straight red hair and brown eyes — and 

 so on; the reader can complete the series for himself. If such an amount of 

 diversity is possible on the basis of just three pairs of genes, what a vast 

 amount must be possible on the basis of the approximately 24,000 pairs of 

 genes which each human being is estimated to possess! 



One source of diversity in a population, then, lies in the mechanism of 

 inheritance by which thousands of pairs of dominant and recessive genes 

 are reassorted and reassembled generation after generation. The process is 

 analogous to the shuffling of playing cards. How many different "hands" is 

 it possible to obtain from one pack of fifty-two cards? How many different 

 kinds of individuals can arise by the shuffling and "dealing" of many-times- 

 52 pairs of differing genes? 



A little thought will convince one that the diversity arising from the proc- 

 ess just described is primarily a matter of new combinations — new combina- 

 tions of characteristics already in existence. Shuffling and dealing cards re- 

 sults in new combinations of cards, not in new cards; the latter are still the 

 familiar aces, kings, jacks, and so on. To a considerable extent the "shuffling 

 and dealing" of genes occurring in the production of each new generation 

 is primarily a matter of producing new combinations of old characteristics. 

 For the most part the combinations are new, the individual characteristics 

 entering into the combinations are not. Yet the analogy to playing cards is 

 not perfect, since genes do influence each other. The presence of one gene 

 causes another gene to produce a different result from that which it would 

 produce if the first gene were not there. Because of this interaction of one 

 gene with another the diversity actually produced by the shuffling de- 

 scribed above is greater than it would be if the genes were entirely inde- 

 pendent in their activities. Nevertheless, much of the diversity produced by 

 the mechanism of heredity is a diversity of new combinations of old charac- 

 teristics. 



In our discussion of blood groups (pp. 121-125) we noted that the red 



