Chapter 26 



THE GENE POOL IN 

 CROSS-FERTILIZING POPULATIONS 



t: 



|he study of cross-fertilizing in- 

 dividuals has led to our under- 

 standing of the recombina- 

 tional and mutational properties of genes. 

 We have studied the nature of these genetic 

 units and their phenotypic consequences in 

 terms of the traits produced within individuals 

 and their relatives. However, cross-fertiliz- 

 ing individuals exist not only as individuals 

 or as parts of families, but also as members 

 of a general population. In such a general 

 population each individual usually has an op- 

 portunity to mate with numerous other indi- 

 viduals. The gametes of all individuals mat- 

 ing furnish a pool of genes, or gene pool, 

 from which the genes of the next generation 

 will be drawn. What we are concerned with, 

 on this occasion, is the fate of a particular 

 gene in the gene pool of successive genera- 

 tions. Let us construct a gene pool and see 

 what will happen to a gene in it in successive 

 generations. 



Suppose, in the not too distant future, it is 

 decided to colonize Mars with human beings 

 from the Earth. Suppose that the popula- 

 tion arriving there is sufficiently large, and 

 that with respect to eye color genes, only the 



B (brown) allele and the completely reces- 

 sive b (blue) allele are present, in such fre- 

 quencies that the gene pool from the fathers 

 and mothers is comprised of gametes that are 

 2B and M. On the presumption that the 

 marriages which occur will be uninfluenced 

 by eye color phenotype, what will be the 

 genotypes of the Fi generation? What will 

 be their phenotypes? Of what will the Fi 

 gene pool consist? 



The answer to the first question may be 

 obtained from Figure 26-1. As the result of 

 the random union of gametes, .04, or 4%, of 

 children will be SB, .32, or 32%, will be Bb, 

 and .64, or 64%, bb. Phenotypically, the Fi 

 population will be composed of 36% brown- 

 and 64% blue-eyed people. (Since this eye 

 color gene is located autosomally, the same 

 proportions occur among boys and girls.) 

 When, barring mutation, the Fi marry, what 



FEMALE GAMETES 

 .2 B .8 b 



The F, Population 



.04 Brown (BB) + .32 Brown (Bb) + .64 Blue (bb) 



FIGURE 26-1. fi genotypes and 

 tlie gene pool these produce. 

 224 



The F Gene Pool 



.04 

 .16 



.16 

 .64 



