13. (2pts) A strain of Drosophila is heterozygous for a paracentric chromosome i

Question

13. (2pts) A strain of Drosophila is heterozygous for a paracentric chromosome inversion that spans a large region of chromosome 2 in which crossovers typically occur in meiosis. This inversion does not disrupt normal gene function of any of the genes on chromosome 2 but a developing fly embryo requires two copies of all the genes within the inversion region to be viable. You cross two flies that are both heterozygous for the inversion. What is the expected percentage of progeny that will be viable (alive) and inviable (dead embryos)?

100% inviable progeny

75% inviable progeny and 25% viable progeny

50% inviable progeny and 50% viable progeny

25% inviable progeny and 75% viable progeny

100% viable progeny

100% inviable progeny

Explanation / Answer

When crossing over will occur in meiosis-I, in the adjacent non-sister chromatids, some genes will be missing and some will be in two copies in the same chromatid, due to this paracentric inversion. The other two non-sister chromatids where recombination is not occurring are normal and contain all the genes. So, after the meiosis is over in both the parental cells half of the gametes are normal and half miss some genes i.e 50% of the gametes of both parents are normal.

If the zygote is formed by the fusion of two normal gametes, it will survive and since the normal gametes in total are 50%, then chances of a viable progeny is also 50%.

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