Question

In soreflies (a hypothetical insect), the dominant allele, L, is responsible for resistance to a common insecticide called Loritol. Another dominant allele, M, is responsible for the ability of soreflies to sing like birds. A true-breeding mute, Loritol-resistant sorefly was mated to a true-breeding singing, Loritol-sensitive sorefly, and the singing, Loritol-resistant female progeny were test-crossed with true-breeding wild-type (i.e., mute, Loritol-sensitive) males. Of the 400 total progeny produced, 117 were mute and Loritol-resistant, 114 could sing and were Loritol-sensitive, 83 could sing and were Loritol-resistant, and 86 were mute and Loritol-sensitive. What will be the results of a chi-square test for independent assortment?

a) The genes assort independently
b) The genes are in complete linkage
c) The genes exhibit partial linkage
d) The genes show complete repulsion

Answer 1

The given progeny numbers from the test cross suggest that the genes for resistance to Loritol and the ability to sing in soreflies exhibit partial linkage rather than independent assortment. A chi-square test would confirm a significant deviation from the expected 1:1:1:1 ratio, indicating that the correct answer is 'c) The genes exhibit partial linkage'.

Step-by-step explanation:

To determine if the genes for resistance to Loritol (L) and the ability to sing (M) in soreflies assort independently, we can use a chi-square test for the progeny's phenotype ratios from the test cross. If genes L and M were located on different chromosomes or far apart on the same chromosome, their alleles would assort independently, and we would expect equal numbers of the four phenotypic classes. However, the progeny numbers (117 mute and Loritol-resistant, 114 singing and Loritol-sensitive, 83 singing and Loritol-resistant, and 86 mute and Loritol-sensitive) deviate from a 1:1:1:1 ratio expected for independent assortment.

Mendel's law of independent assortment states that each pair of alleles segregates independently of any other pair of alleles during gamete formation. When applied to dihybrid crosses, we expect offspring in the ratios of 9:3:3:1 for the different phenotypic classes, if independent assortment occurs. However, the given progeny numbers suggest that alleles L and M do not sort independently, which indicates the presence of gene linkage.

Using the given progeny numbers in a chi-square test will reveal a significant deviation from the expected 1:1:1:1 ratio. Since the deviation is significant and not due to random chance, the correct answer to this question would be 'c) The genes exhibit partial linkage', meaning L and M are located on the same chromosome and are close enough to each other that they do not assort independently, but are occasionally separated by crossing over during meiosis.