Inbreeding Flashcards
Definition of Inbreeding
Mating of individuals related by ancestry.
Inbreeding is measured as:
The probability that any two alleles at a locus (a certain section of the gene) are identical by descent.
Inbreeding in small populations
Inbreeding is unavoidable, through time all individuals in a small population become related to each other.
Example, Pink Pigeon
13 founders, all the rest of the population would be related to the original 13.
Why does inbreeding cause an issue?
Inbreeding reduces reproductive fitness. The more inbreed the species the higher the mortality rate.
Unit of measurement
F inbreeding coefficient.
F = Probability that ant two alleles at a particular locus in an individuals genome will be identical by shared descent.
Value between 0 and 1. 0 being unrelated, 1 being identical twins.
Calculating F from first principles
Assumptions: 1) All ancestors are un-related. 2) All ancestors have unique alleles. Inbreeding coefficients (F) can be averaged across a population.
Genetic consequences of inbreeding
Inbreeding decreased heterozygosity and increases homozygosity, in direct proportion to the inbreeding coefficient. In breeding changes the frequency of genotypes.
Changes of genotypes
Inbreeding changes the proportion of genotypes in a population from one generation to the next. But it does not change the allele frequencies. But in small populations, allele frequencies (p+q=1) will be liable to change due to genetic drift.
Estimating F from Hererozygosity
F=1-(H inbred/H expected). Meaning increased level of inbreeding closely reflects increased level of homozygosity.
Inbreeding exposes…
Rare deleterious alleles. When a population number becomes inbreed by sibling mating (F=0.25), the frequency of lethal recessive genetypes more than doubles. Californian Condor’s. Very inbreed and a proportion born dwarfed and die on hatching.
Rare alleles in Californian Condor’s.
Very inbreed and a proportion born dwarfed and die on hatching.
Most deleterious alleles…
are partly recessive. Inbreeding actually raises the these frequencies very rapidly . The effects of inbreeding, increases homozygosity, on a single locus is minimal.
Reducing reproductive fitness
Cumulative increase in lethal homozygotes, Cumulative increase in deleterious partially recessive alleles. This all reduces reproductive fitness.
Inbreeding is inevitable…
In small populations. Most species have evolved inbreeding avoidance mechanisms. However, given enough time, most individuals become related by descent.
Ancestors
Everyone has two parents, thats back one generation. The previous 10 generations are 1024 ancestors.
For an individual to not be inbred, it needs to have been produced from a population of MORE than 1024 individuals, all ancestors need to be completely unrelated.
Inbreeding theory
Inbreeding increases at a rate of 1/2Ne per gen. The effect is cumulative across generations.
Homozygotes can be formed in two ways:
1) 2 copies of the same allele can be drawn by chance 1/2N
2) By acquiring 2 identical alleles due to previous inbreeding. Probability = F
Inbreeding and loss of genetic diversity
increase in inbreeding per generation = loss of heterozygosity per generation. Rate of accumulation depends on population size.
Example, Isle Royale Grey Wolf
Island population in Lake Superior founded by single female. Island wolves have rare mtDNA genotype, implies single female founder. Island population is 5-7 generations old. Ne never higher than Ne=6. Island population has lost 40-65% heterozygosity. DNA data suggests that all wolves are as related (F=0.25) as siblings.
Norfolk Island Boobook Owl
One individual survived in the 1990’s. New Zealand Government decided to hybridise the last remaining individual with closely related species. Increases genetic representation of the threatened subspecies. But the costs ate high, 4 generations and the offsprings F=around 0.5%
Inbreeding benefits for small populations
Inbreeding increases the frequency of homozygotes, and so increases opportunity for deleterious alleles to be removed by selection, PURGING.
Purging
Works well for lethal alleles, but it is far less efficient at removing mildly deleterious alleles. (Hedrick, 1994). The effectiveness of purging depends of mutation, selection balance.
