A pair of alleles P and p determines spot colour in the purp

A pair of alleles, P and p determines spot colour in the purple sea hare, an attractive sea slug that is a popular addition to a saltwater aquarium. PP and Pp sea hares have light purple spots, and pp sea hares have bright yellow spots. To avoid the environmental impact to coral reefs caused by the poorly regulated and destructive ornamental trade, a local aquarium shop owner wants to breed some sea hares to sell. Because they are attractive, yellow-spotted purple sea hares are in high demand and can be sold for more money than the drab purple sea hares. However, true-breeding yellow-spotted sea hares breed poorly (i.e. not very fertile). So, the shop owner purchases a pair of purple-spotted sea hares, and they mate and produce, in the F1, 31 purple and 9 yellow-spotted offspring (which are sold). Given these results, what is the best strategy for this breeder to produce more yellow-spotted sea hares? (hint: you need to calculate the expected frequency of the yellow-spotted offspring for both scenarios below – show your work) Circle the best breeding strategy: a. Random crosses between pairs of F1s b. F1 backcross to parents

Solution

Given that the cross between two purple seeds produced 31 purple and 9 yellow-spotted offspring; means, the parents are heterozygotes.

Pp* Pp -à PP (1/4, purple), Pp (1/2, purple), pp (1/4, yellow spots) --à F1

Random back cross between F1 purple sea hares will not result in yellow spots because, it also contains sea hares homozygous for the P allele. As the parents are heterozygotes, back cross of F1 progeny to the parents likely to produce more yellow sea hares.