A Diploid Gene Pool
A diploid gene pool that comprises N individuals
has 2N alleles at each gene locus.
The gene pool shown above has N = 10
individuals with 2 alleles @, therefore 20
alleles (gene copies) total. Let A and
a
symbolize the green and brown alleles, respectively. Allele
frequencies are fA =
7/20 = 0.35, and fa =
13/20 = 0.65. Genotype frequencies are fAA
= 1/10 = 0.1, fAa = 5/10 =
0.5, and faa = 4/10 = 0.4. If
allele frequencies are written fA
= p and fa =
q, then p = 0.35 and q = 0.65.
Note that q = (1 - p) = 0.65.
Important note: Population geneticists by convention talk about "gene frequencies" when they actually mean "allele frequencies". In this course, we use only the latter term. Talking about "gene frequencies" has several unfortunate implications. These include mathematical confusion as to "gene frequencies" when 10 "genes" have 20 "alleles". More importantly, it is often said (inaccurately) that a person with a particular genetic disease has the "gene" for that condition, whereas unaffected people "lack the gene".
In fact, in humans almost everyone has the same set of 20,050 "genes" or gene loci in the standard exome set. At some loci, some individuals have an allele (or two alleles) that predispose to medical or other conditions, whereas most individuals have one or a pair of 'standard' alleles that do not. Sometimes there may be partial or complete deletion or duplication of a chromosome, as is the case in species with chromosomal sex-determination, sex-linked genes (in mammals, two alleles on two X chromosomes in females, one allele on the single X in males). The implications of talking about "gene frequencies" should be recognized but avoided.
Important note: Population geneticists by convention talk about "gene frequencies" when they actually mean "allele frequencies". In this course, we use only the latter term. Talking about "gene frequencies" has several unfortunate implications. These include mathematical confusion as to "gene frequencies" when 10 "genes" have 20 "alleles". More importantly, it is often said (inaccurately) that a person with a particular genetic disease has the "gene" for that condition, whereas unaffected people "lack the gene".
In fact, in humans almost everyone has the same set of 20,050 "genes" or gene loci in the standard exome set. At some loci, some individuals have an allele (or two alleles) that predispose to medical or other conditions, whereas most individuals have one or a pair of 'standard' alleles that do not. Sometimes there may be partial or complete deletion or duplication of a chromosome, as is the case in species with chromosomal sex-determination, sex-linked genes (in mammals, two alleles on two X chromosomes in females, one allele on the single X in males). The implications of talking about "gene frequencies" should be recognized but avoided.
Figure © 2013 Sinauer Associates; Text material © 2022 by Steven M. Carr