Bean plants may have different symptoms when infected with a virus. Some show lo

Question

Bean plants may have different symptoms when infected with a virus. Some show local lesions that do not seriously harm the plant; others show general systemic infection. The following genetic analysis was made:
P: local lesions x systemic infection
F1: all local lesions
F2: 785 local lesions : 269 systemic infection

What is the likely genetic basis of this difference in beans? Evaluate your hypothesis using a chi-square test. Assign gene symbols to the genotypes occurring in the genetic analysis. Design a testcross to verify your assumptions.

Explanation / Answer

The difference in beans is caused by a gene with two alleles, one for local lesions and the other for systemic infection. The allele for local lesions is dominant. Let S = local lesions and s = systemic infection. Then, in the analysis described, a plant which is homozygous for local lesions (SS) is crossed with a plant which is homozygous for systemic infection (ss). The F1 plants are all Ss and exhibit local lesions only. The F2 plants are 1/4 SS, 1/2 Ss, and 1/4 ss. 3/4 exhibit local lesions and 1/4 exhibit systemic infection. The analysis described can be considered a testcross where the parents were obtained by identifying plants which breed true for local lesions and for systemic infection. If the systemic infection allele was dominant, the F1 plants would have all exhibited systemic infection rather than local lesions.

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