LECTURE #7 : Inbreeding and The Founder Effect

TEXT : Hedrick, Chapter 7

An increase in genetic variability

*Definition of Outbreeding - Mating with individuals that are less closely related than those drawn by random chance.

A decrease in genetic variation

An increase in homozygosity at all loci

A reduction in fitness of inbred individuals (where fitness is equal to the capacity to pass on your genes to the next generation)

*Definition of Inbreeding - Mating with individuals that are more closely related than those drawn by random chance.

'f ' is the symbol for the 'inbreeding coefficient'

f = the probability that the two homologous alleles at a locus in an individual are identical by descent (I.B.D.)

identical by descent means that both alleles came from a common ancestor. (*note: this is as opposed to an allele that may be identical but is randomly drawn from an unrelated individual).

Determining the Inbreeding Coefficient from Pedigrees (SEE YOUR TEXTBOOK pgs. 105-108 FOR A MORE COMPLETE UNDERSTANDING OF HOW TO DETERMINE THIS)

The chain counting technique = A chain for a given common ancestor starts with one parent of the inbred individual, goes up the pedigree to the common ancestor (CA) and back down to the other partent.

The formula:

f =(1/2)^N

This value is the probability that the two alleles at a given locus are identical by descent.

 here, f =(1/2)⁵ = 1/32

here, f =(1/2)⁶ + f =(1/2)⁶ = 1/32

Inbreeding Depression - A reduction of fitness in a trait due to inbreeding.

"Population Bottleneck" = some condition that reduces the population down to a small number of individuals (these individuals make it through the bottleneck to 'found' a new populaiton -- i.e. this is why they are called 'founders' and therefore why the whole process together is called the 'founder effect'.)

EXAMPLE:

say that your starting population = 1000 individuals

at the 'bottleneck', say that n= 20 (this means that 20 individuals have survived some sort of challenge to the population, which could be a natural disaster, a resource fluctuation, etc.)

After the 'bottleneck', these 20 individuals begin to reproduce and 'found' (become the founders of…) a new population.

a population bottleneck could occur for any number of reasons -- i.e. natural disaster, disease, predation, competition, etc.

loss of alleles (those alleles which did not make it through the bottleneck)

changes in allele frequencies

reduction in fitness

possibility of allele fixation

all of those processes which affect small populations more dramatically than large will be more evident in the case of the founder effect. (i.e. genetic drift, sampling effect, inbreeding, etc.)

Example: Lions in the Ngorongoro Crater

Founding Population = A geographically isolated larger population is decimated by a plague of biting flies (Stomoxys calcitrans) down to 15 individuals.

ANOTHER EXAMPLE:

Amish Population of Lancaster County, Pennsylvania - high incidence of a recessive disorder known as Ellis-van Creveld syndrome.