genetic variation- dominant factors affecting Flashcards
factors affecting allele freq and HWE
HWE is calculated for each locus and each population
if hwe is not maintained, assumptions causing changes to allele freq need evaluation
main factors affecting
- gene flow/migration
- genetic drift + found effect
- non random mating/inbreeding
- selection
- mutation
migration or gene flow
gene flow- interchange of alleles between populations- change in allele frequencies
spread of favourable/ lethal mutations- mutations occur at the individual levels but peresist at the population level
gene flow is a natural consequence of mating
exogamy
when people marry outside of the community
most animals dont mate with close kin
humans have marriage rules specififying who can marry and where one must/can live
patrilocal- wife moves to husbands community
matrilocal- husband moves into wives community
endogamy
when people only marry within their own community
migration calcs
(change in)qr = m (qm - qr)
qr new = qr + (change)q
r= recipient pop
m=migrant pop
if there is migration from 1 to 2 and 2 to 1, the population will eventually become 1 population
effects of migration on HWE
pooling of migrants from different populations can lead to statistically significant deviations from HWE
would be a sig diff in the chi square value
random genetic drift
due to small population size- effect size is a lot larger
deviations in allele frequencies are simply due to chance
aka sampling error- chance element of which allele has been selected
smaller population= more noticeable effects of drift
changes are random
if population is small enough, effects of drift may swamp other evolutiinary forces, even selection
genetic drift occurs at gametic level
founder and bottleneck
establishment of a new population by a few founders who carry a small proportion of the total genetic variation in the parental population
results in the loss of alleles, reduction of variation and the increase in rare alleles
foinder effect can be traced to a historical event
bottleneck- when indivudals found a population, they carry with them a sample of the genetic variation of their population
non random mating
positive assortive mating (PAM)- individuals tend to mate only with others of like genotype
inbreeding is a type of PAM
net increase in homozygotes, decrease in heterozygotes
endogamy- inbreeding calc
inbreeding coefficient is a simple coefficient calculation to work out the level of inbreeding at a locus
F = (HE - HO) / HE
He= expected heterozygote freq (2pq)
Ho= observed heterozygosity (nH/N)
inbreeding
inbreeding increases recessive genes and diseases- homozygous= recessive
coefficient can be positive or negative
positive- inbreeding
negative- out breeding or gene flow
endogamous populations tend to marry within their own religion/cultural/social system
many example from different parts of the world
mutation
primary source of genetic variation
mutation rates are very low- 1 in a million for a gene, 1 in a billion for dna base
can generate lots of potential variation in a population- 7 billion humans would produce about 14 billion new alleles per generation
mutation numbers are low
due to repair mechanisms and repair enzymes
mutations can be forward (a to b allele) or backward (b to a allele)- maintaining balance
only small, slow change to allele frequencies in the populations
unless mutation rates are abnormally high, change in p and q is virtually 0
generally no major effect on HWE
mutation calcs
example
population- all homozygous for allele a
p=1.0 q=0
mutation = a to b
rate of mutation= u
(change in) q = up
q= fraction of p that mutated
selection
differential reproduction- those individuals best able to survive and reproduce will do so more often than others
selection is based on differences in fitness
certain alleles can increase or decrease fitness
whether an allele is selective will also depend on the genetic background and environment
