Population Genetics

Question 1

Population def

Answer 1

Any group of members of the same species in a given geographical area who are potentially capable of mating and producing fertile offspring.

Question 2

Population genetics def

Answer 2

Branch of genetics that studies allele frequencies in groups of organisms of the same species in same area. Usually focuses of a Mendelian population

Question 3

What is a Mendelian population

Answer 3

A group of interbreeding, sexually reproducing individuals that have a summon set of genes- the gene pool.

Question 4

Gene pool definition

Answer 4

Refers to the combination of all the genes, alleles, present in a reproducing population or species. Large gene pool has extensive diversity and is better to withstand environmental challenges.

Question 5

Genetic variation def

Answer 5

Basis of all evolution and the extent of genetic variation within a population affects its potential to adapt to environmental change. Much variation exists at the molecular level and cannot be seen.

Question 6

Genotype def

Answer 6

Genetic makeup of an organism (broad). The alleles or variant forms of a gene carried by an organism (narrow).

Question 7

What is a Genotypic frequency

Answer 7

Indicates which genotypes are the most or least common in a sample population.

Question 8

Frequency def

Answer 8

A proportion or percentage, expressed as a decimal or fraction.

Question 9

How to calculate the genotypic frequency

Answer 9

Add the number of individuals possessing the genotype and divide by total number of individuals in the same sample. (N)
f(Aa) = number of Aa/N
Sum of all the frequencies always one.

Question 10

What can alleles frequencies be calculated from?

Answer 10

1. The number of genotypes
2. The frequencies of genotypes

Question 11

Calculating the alleles frequency from the numbers of genotypes

Answer 11

Count the number of copies of an allele present in a sample and divide by total number of alleles in sample.
Frequency of an allele= Number of copies of allele/ number of copies of all alleles at the locus

Question 12

Calculating allelic frequencies from the genotypic frequencies

Answer 12

Add the frequency of the homozygotes for each allele to half the frequency of the heterozygote.
p= f(A)= f(AA) + 1/2 f(Aa)
q = f(a) = f(aa) + 1/2 f(Aa)
Obtain the same o q values regardless if we use genotypic frequencies of numbers of genotypes.

Question 13

When was the Hardy-Weinberg law formulated?

Answer 13

1908 by G.H. Hardy and Wilhelm Weinberg

Question 14

Assumptions of the hardy weinberg for an autosomal locus with two alleles.

Answer 14

• if a population I’d large, randomly mating, and not affected by mutation, migration or natural selection, then the allelic frequencies of a population do not change and the genotypic frequencies stabilize after one generation in the proportions p^2, 2pq, and q^2

Question 15

1st implication of the HW law

Answer 15

A population cannot evolve if it meets the hardy Weinberg assumptions, because evolution needs change in the allelic frequencies of a population. This tells us that reproduction alone doesn’t lead to evolution, natural selection, mutation, migration are required for population to evolve.

Question 16

2nd implication of the HW law

Answer 16

When a population is in HW equilibrium, the genotypic frequencies are determined by the allelic frequencies. When population isn’t in equilibrium, we have no basis for predicting frequencies. It is only possible to determine the genotypic frequencies when the population is in HW equilibrium.

Question 17

3rd implication of HW law

Answer 17

A single generation of random mating produces the equilibrium frequencies of p^2, 2pq, and q^2. The genotypes being in HW proportions doesn’t prove that the population is free from natural selection, mutation and migration. Just means they haven’t acted since last random mating.

Question 18

How to determine if a populations genotypes are in HW equilibrium

Answer 18

The expected genotypic proportions under HW law must be compared with the observed genotypic frequencies.

Question 19

How does non random mating affect the genotypic frequencies of a population?

Answer 19

Non random mating affects the way in which alleles combine to form genotypes and alter the genotypic frequencies of a population.

Question 20

Two types of non random mating

Answer 20

Positive assorting mating- tendency for like individuals to mate. Ex. Humans exhibit positive assortative mating for height, tall people mate preferentially with tall people, short with short.
Negative assortative mating- tendency for unlike individuals to mate. Ex. Tall and short people would preferentially mate.

Question 21

Inbreeding def

Answer 21

Preferential mating between related individuals. This is an example of positive assortative mating for relatedness, but differs from other types of assortative mating because it affects all genes. Inbreeding causes a departure from the hardy Weinberg frequencies of p2, 2pq, q2.

Question 22

Outcrossing def

Answer 22

The avoidance of mating between related individuals

Question 23

Alleles identical by descent

Answer 23

The copies of the two alleles are descended from a single allele that was present in an ancestor. If we go back far enough, many alleles are likely identical by descent , we go back only a couple of generations.

Question 24

Inbreeding coefficient F def

Answer 24

A measure of the probability that the two alleles are identical by descent. Can range from 0 to 1. 0 indicating that mating in a large population is random, 1 indicates that all alleles are identical by descent.

Question 25

Self fertilization effects

Answer 25

With selfing, each homozygote produces progeny only of the same homozygous genotype, whereas only half of the progeny of a heterozygote will be like the parent. Selfing reduces the proportion of heterozygote in the population by half with each generation, until all genotypes in population are homozygous.

Question 26

Close inbreeding dangers

Answer 26

For most outcrossing species, close inbreeding is harmful because it increases the proportion of homozygotes and boosts the probability of lethal recessive alleles will combine and produce a harmful trait. The increased appearance of lethal and deleterious traits with inbreeding in called inbreeding depression

Question 27

Inbreeding depression

Answer 27

Increased appearance of lethal and deleterious traits. The more intense the inbreeding, the more severe the inbreeding depression.
Inbreeding depression is mostly studied in humans, and plants and animals. The negative effects are more severe in natural populations.
Ex. Children of first cousins have a 40% increase in mortality over children of unrelated people.

Question 28

Evolutionary forces cause changes in allelic frequencies- Mutation

Answer 28

Mutations are changes in DNA sequence, creating new alleles or variations of genes, often random and can be beneficial, neutral or deleterious. All genetic variants arise through mutation. Mutation can influence the rate one genetic variant increases at the expense of another.
When mutation is the only evolutionary force acting on a population, allelic frequencies change with the passage of time because some alleles mutate into others. Eventually they reach equilibrium, and the hardy Weinberg law tells us the frequencies will remain the same.
Change due to mutation in a single generation is extremely small and as frequency of p drops as a result of mutation, the amount of change becomes even smaller.

Question 29

Migration def

Answer 29

Process that may bring about change in allelic frequencies- the influx of genes from other populations. One assumption of HW, migration does not take place, but many natural populations do experience migration. O

Question 30

Overall effect of migration

Answer 30

1. It prevents populations from becoming genetically different from one another
2. It increases genetic variation within populations

Question 31

Effects of migration on allelic frequencies

Answer 31

Individuals in population 1 migrate to population 2 and reproduce, adding its alleles to the population. This migration is unidirectional 1 to 2. All conditions of HW law apply except absence of migration. After migration, population 2 consists of two types of individuals, some migrants, m of pop 2, so the frequency of allele a in migrants is q1.
With each generation of migration, the frequencies of the two populations become more and more similar, until they are the same, q1- q2=0.

Question 32

Overall effect of migration

Answer 32

Two major
1. Causes the gene pools of populations to become more similar.migration tends to keep populations homogeneous in their allelic frequencies.
2. Migration adds genetic variation to populations. Different alleles may arise in different populations owing to rare mutational events, and these alleles can be spread to new populations by migration, increasing genetic variation.

Question 33

Genetic drift def

Answer 33

Change in allele frequency
It’s in a population from generation to generation that occurs due to chance events. Due to a sampling error in selecting the alleles for the next generation from the gene pool of the current generation. Tends to be stronger in small populations.

Question 34

Sampling error def

Answer 34

Deviation from an expected ratio due to limited sample size.
Ex. Flipping a coin 10 times and not getting 50/50 heads and tails. But if you flipped a coin 1000 times, it would be closer to 50/50.
Sampling error arises when gametes unite to produce progeny. Many organisms lroduce a large number of gametes but, when population is small, limited number of gametes unite to produce the individuals of the next generation.