Question

The fungus Phytophthora infestans caused the Irish potato famine because it infected potato crops and made them rot. Phytophthora infestans is only virulent when it can avoid the potato plant's defenses. A loss-of-function mutation in the Avirulence gene allows this to happen and causes potato disease, but only when fungal strains are homozygous recessive (aa), not when they are AA or Aa. You observe a potato crop where 75/150 plants have rotted from Phytophthora infestans. At the Avirulence locus, what is the frequency of the a allele?

a) 50%
b) 75%
c) 100%
d) Not enough information

Answer 1

The frequency of the recessive a allele causing potato blight in Phytophthora infestans cannot be determined from the information given, as only the phenotype frequency is known and assumptions on Hardy-Weinberg equilibrium may not hold without evidence.

Step-by-step explanation:

To calculate the frequency of the a allele in a population where Phytophthora infestans causes potato blight and rots the potato crops, we can use the Hardy-Weinberg equilibrium principle. However, we only have information about the phenotypes, not the genotypes. As the question states that the rot only occurs in plants infected by fungal strains that are homozygous recessive (aa), and 75 out of 150 plants have rotted, it's implied that these 75 plants owe their rotting to being infected by the aa genotype of the fungus. If we assume that this population is in Hardy-Weinberg equilibrium, the frequency of the homozygous recessive (aa) genotype (q²) is equal to the proportion of affected plants (75/150 = 0.5). To find the frequency of the recessive allele (a), we take the square root of the frequency of the homozygous recessive genotype. Thus, q = √0.5 which equals to approximately 0.7071, or 70.71%. However, because this is a frequency for the allele, we express it as a percentage by multiplying by 100. Therefore, the frequency of the a allele is approximately 70.71%, which is not an option provided in the question. Therefore, this implies the correct response is that there is not enough information to accurately calculate the allele frequency without further details about the population genotypes.