BIOLOGY Volume 3 - A Guide to General Biology - 2004

27. MECHANISMS OF SPECIATION

27.1. Population Genetics

27.1.2. Allele Frequencies

Any physical trait of an Organism, such as coat color in mice, is determined by one or more genes. Each Gene can exist in several different forms known as alleles (see Table 24.2). The proportion of organisms in a given population carrying a specific allele determines its allele frequency (sometimes less accurately referred to as gene frequency). For example, in humans, the frequency of the dominant allele responsible for normal Skin, Hair, and eye pigmentation is 99%. The recessive allele that causes a lack of pigmentation—known as albinism—has a frequency of 1%. This means that out of the total pool of alleles controlling the synthesis of this pigment, 1% are unable to produce it, while 99% do. In Population Genetics, allele or gene frequencies are conventionally expressed not as percentages or fractions, but as decimals. Consequently, in this case, the frequency of the dominant allele is 0.99, and the frequency of the recessive allele is 0.01. The total frequency of all alleles in a population is 100%, or 1, therefore

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As is customary in classical genetics, alleles can be represented by letters; for instance, the dominant allele (normal pigmentation) by the letter N, and the recessive allele (albinism) by the letter n. For the example given above, the frequency of N = 0.99, and the frequency of n = 0.01.

Population genetics has borrowed two symbols from mathematical probability theory, p and q, to express the frequencies with which two alleles—dominant and recessive—occur in the gene pool of a given population. Thus,

p + q = 1,

where p is the frequency of the dominant allele and q is the frequency of the recessive allele.

In the example of human skin pigmentation, p = 0.99 and q = 0.01;

p + q = 1

0.99 + 0.01 = 1

The Significance of this equation is that knowing the frequency of one allele allows one to determine the frequency of the other. Suppose, for example, the frequency of the recessive allele is 25%, or 0.25. Then, since

p + q = 1

p + 0.25 = 1

p = 1 - 0.25

p = 0.75

Thus, the frequency of the dominant allele is 0.75, or 75%.