Question

Non disjunction involving the x chromosomes may occur during oogenesis and produces two kinds of eggs. If normal sperm fertilize these two types which of the following pairs of genotypes are possible

Answer 1

Explanation:

non dis junction

Nondisjunction in Translocation Heterozygotes

Organisms heterozygous for a reciprocal chromosome translocation are prone to higher frequency of abnormal meiotic disjunction, including nondisjunction. Only alternative disjunction, combining either both translocated chromosomes or both intact homologs in the gamete, leads to a balanced, euploid genome. Adjacent I and adjacent II disjunctions combine one translocated and one intact chromosome from the pachytene translocation cross and result in partial nullisomy of one translocated chromosome associated with partial disomy of the other chromosome involved in the rearrangement (Figure 2). Nondisjunction can occur also in the 3:1 form when three or one chromosomes involved in the translocation cross (Figure 2) enter the secondary gametocyte. The resulting N+1 aneuploid gamete contains an extra chromosome composed of two chromosomes involved in the translocation. After fertilization with a normal gamete, this extra chromosome gives rise to tertiary trisomy of the embryo. The N−2 aneuploid gamete, if functional, results in preimplanation lethality when fused with an euploid gamete.

Sign in to download full-size image

Figure 2. Chromosome disjunction in meiosis of a reciprocal translocation heterozygote. Only alternant (Alt.) disjunction results in balanced gametes. Recombination between a centromere and the translocation break (not shown in the picture) results in uneven chromatids, one of which can end in unbalanced, adjacent I product and the other can yield a balanced gamete with alternative disjunction. Nondisjunction (3:1) can lead to tertiary trisomies.

The unbalanced gametes occur in translocation heterozygotes with a frequency of approximately 50% and result in inviable embryos. The phenomenon is referred to as ‘semisterility’ in mice, since the translocation heterozygotes display about half of the normal number of pups in their litters. Human reciprocal translocation carriers can have a family history of frequent spontaneous abortions.