definitions Flashcards
(43 cards)
Population genetics
Evolution as the change of allele and genotype frequencies in populations over time
Polymorphism
More than one allele is present at a given locus within a single population of organisms
ie. the occurrence of different forms among the members of a population or colony
Hardy-Weinberg Principle
What happens when we apply Mendel’s law to a population
p2 + 2pq + q2 = 1
H-W assumptions
Mating is random Population size is infinite No mutations occur, so no new variants No migration No natural selection is operating
How do you test for deviation from HWE?
Chi-squared
Assortative mating
When mate choice is based on a genetically encoded phenotype
Certain phenotypes are attracted to each other
Phenotype is based on genes
Inbreeding
Mating with genetically related individuals
More in some societies
Plants do it lots
Non-random mating
Changes genotype frequencies (not allele frequencies)
Interacts with natural selection, leading to evolution
Positive assortative mating
Attracted to similar mates
Increases the number of homozygotes in a population
Eg. flower shape in plants- narrow for hummingbirds, wide for bates- pollen is moved between the same morphologies
Negative assortative mating
Opposites attract
Decreases homozygotes in a population
Eg. heterostyly in plants- alpine woodsorrel is monoecious, foraging insects collect pollen on different parts of body depending on flower morphology
Pin pollen is deposited on thrum flowers, thrum on pin
Allozymes
Enzymes that differ in electrophoretic mobility as a result of allelic differences in a single gene
Amino acid replacement changes the overall charge of the protein, so will have a different electrophoretic mobility
Rate of migration depends on size and charge of protein
Look at DNA nowadays…
Wahlund effect
The reduction of heterozygosity in a population caused by non-random breeding
Mutation
Any change in DNA of an organism
Only mutations occuring in reproductive (germline) cells are heritable
Creates heritable variation in a population
The environment effects the rate of mutation, but not which mutations occur
What causes spontaneous mutations
DNA polymerase infidelity
DNA polymerase slippage leading to insertion/deletion
Recombination and double strand break repair
Transposable elements- jumping around the genome
Rare in nature
How to measure rate of mutation
Start off with identical genomes
Go through multiple generations
Whole genome sequenced at the end
Compare to the original
Work out mutation rate from how many have occured in the time
10^-9 point mutations per base pair per generation
When does recombination occur
Process in which your parents mix their parents chromosomes pairs together
On non-homologous chromosomes as a result of segregaton
On homologous chormosomes as a result of crossing over
Genetic drift
Stochastic changes in allele frequency caused by random sampling of gametes to form offspring in finite populations
A violation of HW assumptions
Leads to loss of variation
Can lose alleles altogether
Large population = small drift fluctuations
Higher starting frequency, the higher the probability of being fixed
Genetic drift experiment in drosophila
8 males and 8 females observed for 19 generations
Brown phenotype is neutral
In the first generation, there was a 50/50 split
Over time, frequency changes
Either fixed for brown allele, or lost
Idealised population characteristics
Mating is random
All individuals are equally likely to produce the same number offspring
Number of breeding individuals is constant from one generation to the next
There are equal numbers of males and females (all of which can reproduce)
Why might effective population size be lower than census size?
Overlapping generations
Some individuals may contribute more genes than others- varying reproductive success
There is variation in the number of offspring by males and females- males contribute more
There are fluctuations in population size
Population bottleneck
Extreme reduction in population size that increases the effect of genetic drift
Alleles can be completely lost
Also increases inbreeding due to the reduced pool
Eg. Florida panther- nearly wiped out in the 20th century due to hunting, dropped to around 6 individuals
Late 20th century cats had 1/3 of the genetic diversity of the ancestors
Founder effect
Loss of genetic diversity when a new smaller population is founded from a larger population
Associated with bottleneck
Eg. bighorn sheep- Tiburon island sheep started from 20, less genetic variation than same species kept in Arizona
Inbreeding coefficient
F = 0, random mating, HWE
F = 1, self-fertilisation/full inbreeding, all homozygotes
0 < F < 1 = partial inbreeding, increase in homo and decrease in hetero
Leads to increase in homozygotes + decrease in heterozygotes
Inbreeding costs
More homozygotes created, bringing together recessive deleterious alleles
