Population genetics Flashcards
What is population genetics
The study of the genetic composition of biological groups
How do changes in genetic composition occur? (7 points)
Natural selection genetic drift mutation gene flow mating structure recombination population size
Population genetics is applicable to:
Evolutionary biology
disease inheritance
conservation biology
ecology
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)
What is used as a marker for genetic diversity?
Mitochondrial DNA (mtDNA)
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
4 major evolutionary forces
Mutation ( + diversity)
Migration ( + diversity)
Selection ( +/- Diversity)
Drift ( - diversity)
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)
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
Allele frequency
Proportion of genes in a population which are a given allele
Locus
A site on a chromosome occupied by a gene
Genotype
set of genes possessed by an individual; often specifically referring to a set of alleles at a particular locus
Genotype frequency
Proportion of a population possessing a particular genotype.
Hetrozygous
Possessing different alleles at focus (Aa)
Homozygous
Possessing the same allele at a locus (AA)
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
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
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
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
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.
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
Migration
- Counteracts drift
- Changes allele frequencies
- Introduces new variation into populations
- Homogenizes variation between populations
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
Effective population
the number of individuals in a population who contribute offspring to the next generation.
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.
Inbreeding depression
the reduced biological fitness in a given population as a result of inbreeding, or breeding of related individuals.
Causes of inbreeding depression
• Deleterious mutations are usually partially
recessive
• Inbreeding increases homozygosity
• Therefore deleterious recessive mutationsare expressed in a greater number of individuals
