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Flashcards in Population genetics Deck (27):
1

What is population genetics

The study of the genetic composition of biological groups

2

How do changes in genetic composition occur? (7 points)

Natural selection
genetic drift
mutation
gene flow
mating structure
recombination
population size

3

Population genetics is applicable to:

Evolutionary biology
disease inheritance
conservation biology
ecology

4

Why is genetic diversity needed?

Allows for adaptation to environmental change
lessen the chances of inbreeding which can create and lock degenerative genetic traits into a population (Bulldogs and respiratory problems due to malformed facial features)

5

What is used as a marker for genetic diversity?

Mitochondrial DNA (mtDNA)

6

9 properties of mtDNA

– relatively small circular molecule
– ~16,000 bases in vertebrates
– it is haploid
– it is maternally inherited
– no recombination
– typically a higher mutation rate than nucleic DNA
– several thousand copies per cell
– no introns
– relatively few duplications

7

4 major evolutionary forces

Mutation ( + diversity)
Migration ( + diversity)
Selection ( +/- Diversity)
Drift ( - diversity)

8

Microsatellites

• Usually polymorphic even in small populations and endangered species
High polymorphism results from a high
mutation rate.
• Microsatellite mutation usually results in a
change in repeat number (an increase or
decrease of one repeat unit).
• Stepwise mutation model (SMM)

9

Basic measures of genetic diversity microsatellites

• Proportion of polymorphic loci
– P= # polymorphic loci/ total # loci
e.g. # polymorphic loci = 4, total # loci = 5
P = 4/5 = 0.8
• Average number of alleles
– A =Total number of alleles for all loci/total # loci
e.g. sum total alleles all loci = 2+3+3+5+5 = 18, total # loci = 5
A = 18/5 = 3.6
• Heterozygosity
– H = sum % heterozygous in each locus/total # loci
e.g. % hetero = (1/10) + (4/10) + (3/10) + (10/10) + (8/10) =
2.6, total # loci = 5
H = 2.6/5 = 0.52

10

Allele frequency

Proportion of genes in a population which are a given allele

11

Locus

A site on a chromosome occupied by a gene

12

Genotype

set of genes possessed by an individual; often specifically referring to a set of alleles at a particular locus

13

Genotype frequency

Proportion of a population possessing a particular genotype.

14

Hetrozygous

Possessing different alleles at focus (Aa)

15

Homozygous

Possessing the same allele at a locus (AA)

16

Hardy-Weinburg Equilibrium

If individuals in a population mate randomly, then genotypes carried in gametes are united randomly and genes are therefore also united randomly…
…the probability that a particular gamete carries a particular allele is the frequency of the allele in the population

17

Hardy-Weinburg Formulae

p2 + 2pq + q2 = 1
After one generation of random mating
the genotype and allele frequencies will
remain at these frequencies in perpetuity
in the absence of other forces

18

Assumptions of the H-W Equilibrium

• Mating is random
• The population is infinitely large
• No migration
• No mutation
• Equal probability of survival and
reproduction – no selection

19

Why is HWE used?

• Acts as a null basis to examine the occurrence of other phenomena
• We can detect departures from HWE due to:
- natural selection
- genetic drift
- non-random mating
- inbreeding

20

Genetic Drift

variation in the relative frequency of different genotypes in a small population, owing to the chance disappearance of particular genes as individuals die or do not reproduce.

21

Consequences of Genetic Drift

• Random changes in allele frequency (their direction is neutral)
• Loss of genetic diversity – fixation of alleles
• Diversification of populations that are isolated
• Heterozygosity decays.
• This decay is proportional to the initial
population size
• Genetic drift is weak in large populations

22

Migration

• Counteracts drift
• Changes allele frequencies
• Introduces new variation into populations
• Homogenizes variation between populations

23

Bottlenecks

A population reduced to a small size - the Amur tiger was reduced to a population of 50 by 1947 but by 2009 that number increased to around 900. Yet the effective population was only 27 - 35 individuals

24

Effective population

the number of individuals in a population who contribute offspring to the next generation.

25

Inbreeding

When mating occurs between individuals that are genetically related (mother to son, father to daughter, first cousins etc) it increases the chance of disadvantages mutations. As inbreeding continues these mutation can become fixed in the population.

26

Inbreeding depression

the reduced biological fitness in a given population as a result of inbreeding, or breeding of related individuals.

27

Causes of inbreeding depression

• Deleterious mutations are usually partially
recessive
• Inbreeding increases homozygosity
• Therefore deleterious recessive mutationsare expressed in a greater number of individuals