Question

Case B: In this population, the frequency of the dominant allele, B, is 0.5 and the frequency of the recessive allele, b, is 0.5. However, in this situation any individual who is born with the dominant trait has a 50 % chance of not surviving to reproductive age. Assuming that mating is still random, what will happen to the allele frequencies (p and q) after 5 generations? (le. Will one allele increase or decrease?) Prediction (1 mark): Reason (2 marks):

Answer 1

For a given trait: Dominant allele B, has frequency of 0.5. Therefore p = 0.5; Recessive allele b, has frequency of 0.5. Therefore q = 0.5

Person with dominant trait (BB or Bb) has 50% chances of non-surviving to reproductive age.

Prediction : After 5 generation frequency of allele B will decrease.

Reason: As given in data allele B and allele b have equal frequency (0.5) in randomly mated population (p + q = 1). So population will have genotype frequencies as per Hardy Wienberg equation p² +2pq +q² = 1, where p² is frequency of BB (homozygous dominant), 2pq is frequency of Bb (Heterozygotes) and q² is frequency of bb (homozygous recessive). Persons with BB and Bb will express dominant phenotype and will have only 50% chances of survived up to reproductive age.

Since presence of B allele is increasing rate of early death in population. This population will not be part of random mating population, thus resultant frequency of B allele in population will keep reducing in forthcoming generations. Thus favoring allele b in population.

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