You examine a worm that has two genders: males that produce sperm and hermaphrodites that produce both sperm and eggs. The diploid adult has four homologous pairs of chromosomes that undergo very little recombination

Question

Given a choice, the hermaphrodites prefer to mate with males, but just to annoy the worm, you pluck a hermaphrodite out of the wild and fertilize its eggs with its own sperm. Assuming that all the resulting offspring are viable, what fraction do you expect to be genetically identical to the parent worm? Assume that each chromosome in the original hermaphrodite is genetically distinct from its homolog.
(a) all
(b) none
(c) 1/16
(d) 1/256

Answer 1

(c) Because each chromosome is genetically distinct from its homolog, the parent is heterozygous for each chromosome and thus can produce 24 = 16 types of egg and 24 = 16 types of sperm. Any of the eggs produced will be able to give rise to an adult that is identical to the parent, but to do so it must be fertilized by the right type of sperm. For each type of egg, only one of the 16 possible sperm will produce a diploid that is identical to the parent. Therefore, one out of 16 of the offspring should be identical to the parent. In other words, a sexually reproducing organism with several heterozygous chromosomes has a relatively high probability of producing genetically distinct offspring even when the parent mates to itself.