Question

 

Part II
4. For a diploid species, assume one set of 100 demes, each with a constant size of 10 individuals, and another set of 100 demes, each with 100 individuals.

a) If in each deme the frequencies of neutral alleles A1 and A2 are 0.2 and 0.8, respectively, what fraction of demes in each set is likely to become fixed for allele A1 versus A2? Show your work below:

b) Assume that a neutral mutation arises in each deme. Calculate the probability that it will become fixed in a population of each size. In what fraction of demes do you expect it to become fixed?

c) If fixation occurs, how many generations do you expect it to take?

d) Compare your results for the two sets of demes. What effect does population size have on the results? Explain any differences.

5. Population size is a very important measurement in population genetics.

a) What is the difference between N and Ne? How are they measured/estimated?
Explain in your own words!

b) Ne is often smaller than N for 5 reasons. In your own words, describe these reasons below:

Answer 1

4. Fixation of allele in a population means existance of only one variant of allele in the population. These are mostly homozygous, as there is a loss of any other variant.

(a)

p(A1)= 0.2, p (A2)=0.8

p(A1A1)=0.4, p(A2A2)= 0.64

Fp(A1)=4%, Fp=64%

(b)

The probabilty of fixation of a new mutation is 1/2N

so, for set I,

F(p)= 1/2*1000= 0.0005

for set II,

F(p)= 1/2*10000=0.00005

The fraction of population with fixed mutation for set I is, = 0.05%

The fraction of population with fixed mutation for set II is, = 0.005%

(c) In one generation, 0.05% population is fixed, so it will take 2000 years for fixation. for set one

For set 2, it will take 20000 years.

(d) Large population size is more resistant to mutation.

5. (a) Ne is the EFFECTIVE POPULATION SIZE, which denotes the total number of individuals in the population which contribute to next generation offsprings. They are estimates of total breeders of the population. It is the ideal population size in which genetic drift and mutation rate effect is similar as real population set.It can be calculated as:

Ne= (4 Nm Nf ) / (Nm + Nf)

where, Nm- total number of breeing male, Nf- total number of breeding female.

N is the population size of the species, the total head count.

(b) Ne is affected by various circumstances, which makes it smaller than the N. Below here is listed the five reasons for it:

1. A species have fluctuating population size, which is measured as hormonic mean not arithmetic mean, that gives it a lower value.

2. The ratio of breeding male and female changes time to time, that affects the effective population size.(sex ratio)

3. Overlapping of generations also reduces effective population size.

4. structure of population, if it is decreasing, increasing or stabilizing.

5. Relative survival of offsprings.