Chapter 15 



THE GENE POOL; 

 EQUILIBRIUM FACTORS 



T! 



|he recombinational and muta- 

 tional properties of the genetic 

 material have been studied in 

 cross-fertilizing individuals and the nature 

 and phenotypic consequences of various ge- 

 netic units have been described in terms of 

 the traits found in such individuals and their 

 relatives. Cross-fertilizing individuals are 

 members of a general population. In a 

 general population, each individual usually 

 has an opportunity to choose a mate from 

 a large number of the other members. The 

 gametes of all mating individuals furnish a 

 pool of genes, or gene pool, from which the 

 genes of the next generation are drawn. 

 Over successive generations what happens to 

 the frequency of a particular gene in the 

 gene pool? Let us construct a gene pool 

 and investigate this question. 



Suppose that Mars is colonized by human 

 beings, that the population sent there is suffi- 

 ciently large, and that — with respect to eye 

 color genes — only the B (brown) allele and 

 the completely recessive b (blue) allele are 

 present in the gene pool in the frequencies 

 .2 B and .8 b. Presuming that marriages are 

 not influenced by eye color phenotype, what 

 genotypes and phenotypes will the F, have? 

 The answer can be seen in Figure 15-1. 

 As the result of the random union of gam- 

 etes, 4% of these children are BB; 32% 

 are Bb; and 64%, bb. Phenotypically, the 

 Fi population is composed of 36% brown- 

 and 64% blue-eyed people. 



In the absence of mutation, what is the 



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gene pool in the gametes of the F,? The 

 4% of F, BB individuals furnish 4% of all 

 gametes, and these carry B. The 32% of 

 F, Bb individuals furnish 32% of all gametes 

 of which half (16% ) carry B and the other 

 half b. Therefore, the total gene pool con- 

 tains 20% gametes with B. The b gametes 

 comprise 80% of the gene pool (16% from 

 the 32% of Bb heterozygotes and 64% from 

 the 64% of bb individuals). Note that the 

 gene pool of the F, is identical to that of the 

 P,. Therefore, in the F 2 and all subsequent 

 generations, the same genotypic and pheno- 

 typic ratios are found, because the fre- 

 quencies of B and b in the gene pool remain 

 constant. 



What would be the consequence if, in- 

 stead of starting the Martian colony with a 

 gene pool of 20% B and 80% b, some other 

 proportion were used? We can generalize 



The F, Population 



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



The F, Gene Pool 



B = .04 + .16 = .2 

 b = .16 + .64 = .8 



figure 15-1. F, genotypes and the gene pool 

 these produce. 



