Evolutionary Change in Populations

Chapter 18

Frequencies

• Gene pool – all the alleles in a population• Genotype frequency – total = 1.0 (100%)

– homozygous dominant– heterozygous– homozygous recessive

• Phenotype frequency – total – 1.0 (100%)– dominant– Recessive

• Allele frequency– A or a– Each individual has 2: AA, Aa, or aa

Hardy-Weinberg Principle

A way of measuring if a population is evolving Non-evolving populations ‘fit’ the principle – they

are in genetic equilibriumFive conditions must be met:

1. random mating2. no net mutations3. large population size4. no migration5. no natural selection

In other words: no evolutionary change

Hardy-Weinberg Principle…

• Shows that the process of inheritance by itself does not cause changes in allele frequencies

• Explains why dominant alleles are not necessarily more common than recessive ones

• Seldom occurs in the natural world but instead provides us with a model for understanding evolution in sexually reproducing populations

Hardy-Weinberg Principle…

p = frequency of dominant allele

q = frequency of recessive allele

Therefore:

p + q = 1

From this we know that:

p = 1 – q (frequency of dominant allele)

q = 1 – p (frequency of recessive allele)

Hardy-Weinberg Principle…

Each individual has 2 alleles, therefore:

p 2 + 2 pq + q 2 = 1{AA + 2Aa + aa = 1} remember, there are 2 different ways an individual can inherit

Aa

What the heck does this mean??

Problem #1

• You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that, calculate the following:

A. The frequency of the aa genotypeB. The frequency of the a alleleC. The frequency of the A alleleD. The frequencies of the genotypes of AA and AaE. The frequencies of the two possible phenotypes if A

is completely dominant over a

Problem #2

• Sickle-cell anemia is an interesting genetic disease. Normal homozygous individuals (SS) are more susceptible to malarial infection. Those with the sickle-cell trait (ss) often die due to the condition. Heterozygous individuals (Ss) tend to survive better than either of the homozygous conditions. If 9% of an African population is born with a severe form of sickle-cell anemia (ss), what percentage of the population will be more resistant to malaria because they are heterozygous (Ss) for the sickle-cell gene?

Problem #3

• There are 100 students in a class. Ninety-six did well in the course whereas four blew it totally and received a grade of F. In the highly unlikely event that these traits are genetic rather than environmental, if these traits involve dominant and recessive alleles, and if the 4% represent the frequency of the homozygous recessive condition, calculate the following:

A. The frequency of the recessive alleleB. The frequency of the dominant alleleC. The frequency of heterozygous individuals

Problem #4

• Within a population of butterflies, the color brown (B) is dominant over the color white. And 40% of the butterflies are white. Given this simple information, calculate the following:

A. The percentage of butterflies in the population that are heterozygous

B. The frequency of homozygous dominant individuals

Problem #5

• A rather large population of Biology instructors have 396 red-sided individuals and 557 tan-sided individuals. Assume that red is totally recessive. Calculate the following:

A. The allele frequencies of each alleleB. The expected genotype frequenciesC. The number of heterozygous individuals that you would predict

in this populationD. The expected phenotype frequenciesE. Conditions happen to be really good this year for breeding and

next year there are 1,245 young Biology instructors. Assuming all of the H-W conditions are met, how many of these would you expect to be red-sided and how many tan-sided?