You are the proud owner of a purebred herd of Pietrain pigs. They have a high fr

Question

You are the proud owner of a purebred herd of Pietrain pigs. They have a high frequency of PSS. PSS has a major locus that controls its presence. There may be several modifier loci that affect the degree of PSS, but they only have an effect if the PSS allele is present. The pigs can be screened for the presence of the allele by exposure to halothane gas. Stress susceptible pigs become rigid. There is now also a DNA Marker to test for the PSS gene

Genetic model: HH= normal, Hh- normal, hh- exhibiti PSS (react to halothane)

You decide to test your herd for PSS. Figure out the gene frequencies for the PSS locus if the number of halothane reactor pigs out of 100 is 20. (When testing for a carrier use a probability level of 0.94)

Explanation / Answer

In the question, it is mentioned that HH (Homozygous Dominant)= Normal, Hh (Heterozygous)= Normal, and hh (Homozygous recessive)= exhibit PSS (react to halothane).

Here, H is the dominant allele while h is the recessive allele.

We know that (p+q)=1 [Where p= frequency of dominant allele and q= frequency of recessive allele]

We also know that (p2+2pq+q2)=1 [Where p2 = frequency of homozygous dominant, 2pq= frequency of heterozygous, and q2= frequency of homozygous recessive]

In the herd, the number of halothane reactor pigs (homozygous recessive) is 20 out of 100.

So, q2= (20/100)

So, q= 0.45

Therefore the frequency of the recessive allele= 0.45

Now, (p+q)=1

So, p= (1-q)= (1-0.45)= 0.55

Therefore the frequency of the dominant allele= 0.55

So, the frequency of H allele= 0.55 while the frequency of h allele= 0.45

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