The Gene Pool

Population geneticists use the term gene pool to describe the total genetic information available in a population. It is easy to imagine genes for the next generation as existing in an imaginary pool. If you could inventory this pool and know all of the alleles that are present, then you could apply a simple set of rules based on probability theory to predict expected genotypes and their frequencies for the next generation.

Suppose, for example, that there are two alleles of a hypothetical gene, A and a, in a set of 10 gametes. If half the gametes in the set (5 gametes) carry the allele A, we would say that the allele frequency of the A allele is 0.5, or 50 percent. Allele frequency is determined by dividing the number of a certain allele (five instances of the A allele) by the total number of alleles of all types in the population (10 gametes, each with either an A or an a allele). Remember that a gamete is haploid and therefore carries only one allele for each gene.

Predicting Phenotype

The population of four o'clock flowers, shown in Figure 16-3, illustrates how phenotype can change from generation to generation. Homozygous RR flowers are red. Homozygous rr flowers are white. Heterozygous Rr flowers are pink rather than red, as you might expect. These flowers show incomplete dominance for color, meaning heterozygotes show a trait that falls between the dominant trait and the recessive trait. Thus, homozygotes and heterozygotes can be easily identified by observing the phenotype.

Compare the parent generation with the offspring generation of the four o'clock flowers shown in Figure 16-3. There are equal numbers of plants with the RR genotype and the Rr genotype in the first generation. You can compute the phenotype frequencies from the figure. A phenotype frequency is equal to the number of individuals with a particular phenotype divided by the total number of individuals in the population. Phenotype frequencies in the first generation are 0.5 pink (4 pink plants out of a total of 8 plants), 0.5 red (4 red plants out of a total of 8 plants), and 0.0 white. Recall that allele frequencies are computed using the same principle: the allele frequencies in the first-generation plants are 0.75 R (12 R alleles out of a total of 16 alleles) and 0.25 r (4 r alleles out of a total of 16 alleles).

We now can predict the genotypes and phenotypes of the second generation. If a male gamete encounters a female gamete, they will produce a new four o'clock plant whose genotype is the combination of both parental gametes. Thus, an R male gamete combined with an R female gamete will produce a plant with the RR genotype, which has red flowers. According to the laws of probability, the chance of an R gamete (a single allele) meeting with another R gamete is the arithmetic product of their allele frequencies in the gene pool:

frequency of R X frequency of R = frequency of RR pair 0.75 X 0.75 = 0.5625

The expected frequency of the rr genotype is then frequency of r X frequency of r = frequency of rr pair 0.25 X 0.25 = 0.0625

figure 16-3

Although the four o'clock flowers differ phenotypically from generation to generation, the allele frequencies tend to remain the same.

figure 16-3

Although the four o'clock flowers differ phenotypically from generation to generation, the allele frequencies tend to remain the same.

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