KEY CONCEPTS What is Population Genetics?
▪ Mutation, migration, natural selection, and • Population~ a group of individuals that shares a
random genetic drift can change allele frequency common set of genes, lives in the same
in a population geographic area and actually or potentially
▪ Migration can have an affect on genotype interbreeds.
frequencies • Population genetics~ the branch of genetics that
▪ What is the difference between the founder effect studies the genetic makeup of groups of
and population bottleneck? individuals and how a group’s genetic
▪ What is Hardy-Weinberg law? composition changes with time.
▪ Use Hardy-Weinberg law to calculate • Gene pool~ population geneticists usually focus
allele/genotype frequencies in a population (both their attention on a Mendelian population, which
for autosomal and sex-linked genes) is a group of interbreeding, sexually reproducing
▪ Use Hardy-Weinberg law to figure out if a individuals that have a common set of genes.
population is in Hardy-Weinberg equilibrium • A population evolves through changes in its gene
pool; therefore, population genetics is also the
study of evolution.
Genotypic and Allelic Frequencies are Used to Describe the Gene Pool of a Population
Genetic Variation Calculating Genotypic Frequencies
→ Almost all organisms exhibit variation in • A frequency is simply a proportion or a
phenotype. percentage, usually expressed as a decimal
fraction.
→ Much of this phenotypic variation is
• For large populations, for which a determination
hereditary. of the genes of all individual members is
→ Recognition of the extent of phenotypic impractical, a sample of the population is
variation led Charles Darwin to the idea of usually taken and the genotypic and allelic
evolution through natural selection frequencies are calculated for this sample
→ Genetic variation is the basis of all • The genotypic and allelic frequencies of the
sample are then used to represent the gene
evolution, and the extent of genetic
• pool of the population.
variation within a population affects its • To calculate a genotypic frequency, we simply
potential to adapt to environmental change add up the number of individuals possessing
→ More genetic variation exists in populations the genotype and divide by the total number of
than is visible in the phenotype. individuals in the sample (N)
→ Much variation exists at the molecular level • The sum of all the genotypic frequencies always
equals 1.
owing, in part, to the redundancy of the
• For a locus with three genotypes AA, Aa, and
genetic code, which allows different aa, the frequency (f) of each genotype is:
codons to specify the same amino acid
→ DNA sequences between the genes and
introns within genes do not encode
proteins
→ Much of the variation in these sequences
also has little effect on the phenotype.
→ Genetic structure is described by
enumerating the types and frequencies of
genotypes and alleles in a population.
, Calculating Allelic Frequencies
• There are always fewer alleles than genotypes; so the gene pool of a population can be described in fewer
terms when the allelic frequencies are used.
• In a sexually reproducing population, the genotypes are only temporary assemblages of the alleles
• The genotypes break down each generation when individual alleles are passed to the next generation
through the gametes, and so the types and numbers of alleles, rather than genotypes, have real continuity
from one generation to the next and make up the gene pool of a population.
• Allelic frequencies can be calculated from:
1. The numbers
2. The frequencies of the genotypes.
• To calculate the allelic frequency from the numbers of genotypes, we count the number of copies of a
particular allele present in a sample and divide by the total number of all alleles in the sample:
• For a locus with only two alleles (A and a), the frequencies of the alleles are usually represented by the
symbols p and q, and can be calculated as follows:
• where nAA , nAa , and naa represent the numbers of AA, Aa, and aa individuals, and N represents the total
number of individuals in the sample.
• divide by 2N because each diploid individual has two alleles at a locus.
• The sum of the allelic frequencies always equals 1 (p + q = 1); so, after p has been obtained, q can be
determined by subtraction: q = 1 − p.
• To calculate an allelic frequency from genotypic frequencies, we add the frequency of the homozygote for
each allele to half the frequency of the heterozygote (because half of the heterozygote’s alleles are of
each type):
• We obtain the same values of p and q whether we calculate the allelic frequencies from the numbers of
genotypes or from the genotypic frequencies
Loci with multiple alleles
→ Use the same principles to determine the frequencies of alleles for loci with more than two alleles.
→ To calculate the allelic frequencies from the numbers of genotypes, we count up the number of copies of
an allele by adding twice the number of homozygotes to the number of heterozygotes that possess the
allele & divide this sum by twice the number of individuals in the sample.
→ For a locus with three alleles (A1 , A2 , and A3 ) and six genotypes (A1 A1 , A1 A2 , A2 A2 , A1 A3 , A2 A3 , and
A3 A3 ), the frequencies (p, q, and r) of the alleles are:
→ Can calculate the frequencies of multiple alleles from the genotypic frequencies by extending the allelic
frequency.
→ we add the frequency of the homozygote to half the frequency of each heterozygous genotype that
possesses the allele.
