S You discover a willow speci factory, and later determine that the PCB resistan

Question

S You discover a willow speci factory, and later determine that the PCB resistance is due to a single dominant allele. Homozygou condition is lethal. (3 pts) es (Salix sp.) growing in soil contaminated with toxic PCBs near an abandoned erminate in a. If 45% of the seeds from a randomly mating population of resistant weeds will g contaminated soil, while the rest die. What is the frequency of the PCB-resistance allele? 2 places after decimal) b. Among all the plants that germinate in contaminated soil, what proportion will be heterozygous? (2 places after decimal) c. Among all the plants that germinate, what proportion will be homozygous dominant? (2 places after decimal)

Explanation / Answer

Let PCB resistance is due to R dominant gene.The willow species will germinate if they homozygous dominant (RR) or heterozygous (Rr) while homozygous recessive (rr) are not viable.

a.

45% of seeds germinate i.e. 45% seeds are either homozygous dominant (RR) or heterozygous (Rr). Thus, 55% are homozygous recessive (rr).

If p and q denote the frequency of dominant and recessive alleles respectively, therefore,

f(rr) = q2 = 0.55

q = 0.74

now, p + q = 1

thus, p = 1 - 0.74 = 0.26

so, the frequency of dominant (PCB resistance) allele is 0.26

b.

Frequency of heterozygotes is given by 2pq

2 *0.26 * 0.74 = 0.38

Thus, 38% of the viable plants are heterozygotes.

Since viable plants are 45% only

Thus the proportion of heterozygotes = 38% of 45% = 17.1%

c.

Since viable plants are 45% only and among them the proportion of heterozygotes 17.1%

Thus the proportion of homozygous dominant = 45 – 17.1 = 27.9 %

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