Ch 6: Mendelian Genetics

What is Darwin’s 4 Postulates (restated in genetic term)?

1. Allelic variation exists among individuals

2. Alleles are passed down from parents to offspring by meiosis and fertilization

3. More young are born that can survive

4. Some allelic combinations are fitter than others and can reproduce more often

What is Evolution

The change in frequency of alleles in a population over generations

Hardy-Weinberg Equilibrium Principle

Specifies what will happen to the frequencies of alleles and genotypes

(applies to diploid organisms)

Ex. Imagine that mice have a particular locus A w/ 2 alleles: A and a. And they choose their mates randomly from the gene pool

What is population

A group of interbreeding individuals and their offspring

Solve: pt1

60% of eggs and sperm recieved allele A and 40% recieved allele a. When egg and aperm meet, what proportion of genotypes will be AA?

Freq. “A” = 0.6; Freq. “a”= 0.4

60% egg will be “A” and 60% sperm will be A

(0.6)(0.6) = 0.36 × 100% = 36% zygotes will have genotype AA

Solve: pt2

How many would be “aa”?

(0.4)(0.4) = 0.16 × 100%= 16%

Solve: pt.3

How many would be “Aa”?

(0.6)(0.4)(2)=0.48 × 100% = 48%

If a population is in Hardy-Weinberg equilibrium, will it evolve?

NO! It will never evolve because the allele frequencies are in equilibrium, so they are the same as in the 1st generation.

“p” = frequency of allele “A”

“q” = frequency of allele “B”

Why do we use Hardy-Weinberg Equilibrium Principle?

• to show when evolution does not happen

• gives a specific set of testable assumptions

• Serves as a null model to test evolution against

Assumptions of Hardy-Weinberg

1. There is no selection

2. There is no mutation

3. There is no migration

4. There is an infinitely large population size (no random events = no genetic drift)

5. Mates are chosen randomly (called Panmixia)

The equations allow calculation of allele frequencies (assuming that the population is in Hardy-Weinberg Equilibrium)

By having explicit assumptions….

it can be used to determine which forces are causing disequilibrium

What happens if HWE assumptions are violated?

Evolution!! It occurs when allele frequencies in H-W have changed overtime.

(Remember: Allele frequencies usually should not be able to change in H-W equilibrium)

used to test if Hardy-Weinberg Equilibrium holds or is broken: Chi-squared test

X² Table

Testing HWE Assumptions: Selection

• individuals with particular phenotypes survive to reproduce more than others.

• Must be heritable

• The population evolved in response to selection

• Requires many generations to change allele frequencies

• RARELY is selection so strong

Chi-square test example

Dominance and allele frequency interaction

• If recessive is common = rapid evolution

• If recessive is rare = very slow evolution and hidden from selection

Selection coefficient: S = 1 - W

“W” = fitness of an allele (ranges from 0-1)

“S” = strength of selection on an allele

Negative Selection on RECESSIVE phenotypes

w₊₊=1 ; w₊₁=1 ; w_II=1-S

“S” gives strength of selection on the homozygous recessive phenotype

• (+)S = in favor of phenotype

• (-)S = against phenotype

negative selection on DOMINANT phenotypes

w₊₊=1-S ; w₊₁=1-S ; w_II=1

“S” is the amount of selection against phenotype

Heterozygote fitness

when one allele is dominant and one is recessive, the fitness is (=) to that one kind of homozygote

• fitness is intermediate to 2 homozygotes

• fitness is superior or inferior to either homozygote

Treat chromosomes as alleles: C(2)C(2) and N(2)N(2) genotypes

“C(2)” = compound chromosomes

“N(2)” = normal chromosomes

• Populations w/ high C(2) frequency = C(2) rose to fixation

• Populations w/ low C(2)” frequency = C(2) was lost

Ex. C(2)C(2) and N(2)N(2) genotypes in Fruit flies

Mean fitness as a function of P: OVER vs. UNDER dominance

HWE assumptions: Mutation

• How effective is mutation as a force of evolution?

• A mutation introduces new alleles into a population

• Mutation alone is not a potent evolutionary force

New frequency of A (p) is…

old frequency MINUS fraction LOSTT to mutation

p = 0.9 - (0.0001)(0.9)

New frequency of a (q) is…

old frequency PLUS fraction GAINED by mutation

q= 0.1 + (0.0001)(0.9)

Particular mutation rate?

Can mutation alone cause great changes in allele frequencies?

No, but it is still important in evolution. Mutation can be a potent evolutionary force

Lenski’s E. coli study

After growing billions of cells, they removed approximately 5 million cells from each and transferred them to a new medium.

• Fitness and cell size increased in response to natural selection

• Fitness increases occurred in jumps

• Beneficial mutations swept through the population to fixation

• Mutations caused bacteria to divide faster and increase in size

• Mutation is ultimate source of genetic variation

Mutation Selection Balance

Mutation is the ULTIMATE source of genetic variation

• Most mutations are deleterious

• Selection eliminates these mutations

• Mutations are created anew

Deleterious Recessive allele at equilibrium

• If selection is SMALL and mutation is HIGH, the equilibrium frequency of the allele will be high

• If selection is HIGH and mutations are LOW, equilibrium frequency will be low