Instructions

A virgin Colombian rainbow boa gave birth to a litter of parthenogenetic babies. The genotype of the living baby snakes was determined at several genes for which the mother snake was heterozygous. Each baby was homozygous at these genes, but the babies were homozygous for different alleles. There are several ways that parthenogenetic offspring, with a mother but no father, can be produced: (A) A diploid primary oocyte can develop into a baby instead of dividing by meiosis. (B) The two cells produced at the end of meiosis II (a haploid egg and a polar body) may fuse to produce a diploid cell that develops into a baby. (C) The haploid egg produced by meiosis may undergo DNA replication to produce two identical sister chromatids joined at the centromere. The cell begins to divide by mitosis and the sister chromatids move apart to become separate chromosomes. Instead of completing mitosis, the sister chromatids remain, now as homologous chromosomes in a diploid cell that develops into a baby. (a) Based on the information above, select a form of parthenogenesis that may have led to the baby snakes. (Simply select letter A, B, or C. The correct answer to (b) will differ, depending on which form of parthenogenesis you choose.) (b) If the researchers investigated the baby snakes’ genotype at two additional genes for which the mother snake was heterozygous and found that two of the babies were heterozygous (i.e., one baby was heterozygous for each of the additional genes), would you have to modify your answer to part (a)? Explain your answer. (c) In the lecture on Sex and the Komodo Dragon, I described a fourth form of parthenogenesis: A haploid cell formed by meiosis I fused with the first polar body to yield a diploid egg. I said the outcome of this form of parthenogenesis was complex and would be ignored in further discussion. Imagine that a diploid egg (now a zygote) is formed by this form of parthenogenesis. The zygote divides by mitosis as it grows into an embryo. This first mitotic division will produce two daughter cells that are not genetically identical. Clearly explain why this is so.

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