Question

During oogenesis in an animal species with a haploid number of 6, one dyad undergoes nondisjunction during meiosis II. Following the second meiotic division, the dyad ends up intact in the ovum. How many chromosomes are present in (a) the mature ovuam and (b) the second polar body? (c) Following fertilization by a normal sperm, what chromosome condition is created?

Answer 1

The mature ovum will contain 6 chromosomes, and the second polar body will also contain 6 chromosomes. Following fertilization by a normal sperm, a diploid chromosome condition is created in the zygote.

Step-by-step explanation:

The mature ovum, or egg, resulting from the oogenesis process will have a haploid number of chromosomes, which is 6 in this animal species. So, (a) the mature ovum will contain 6 chromosomes.

On the other hand, the second polar body is a small, non-functional cell that will eventually break down. It will also have a haploid number of chromosomes, which is 6 in this case. Therefore, (b) the second polar body will also contain 6 chromosomes.

When the mature ovum is fertilized by a normal sperm, the two haploid nuclei derived from the sperm and ovum combine to form a diploid zygote with a total of 12 chromosomes. This creates a diploid chromosome condition in the resulting zygote.

During oogenesis in an animal species with a haploid number of 6, nondisjunction during meiosis II results in an ovum and a polar body with abnormal chromosome numbers. In scenario (a), the mature ovum will have 7 chromosomes due to the intact dyad ending up in the ovum. Specifically, it receives one extra chromosome from the nondisjoined dyad. In scenario (b), the second polar body will have the usual haploid number, which is 6, assuming it gets no part of the nondisjoined dyad. Finally, in scenario (c), following fertilization by a normal sperm with 6 chromosomes, the resulting zygote would have a chromosome condition with a total of 13 chromosomes: 6 from the sperm and 7 from the ovum, creating an aneuploid condition.

Answer 2

Nondisjunction is defined as the failure of the chromosomes to seperate during cell division. An animal with a haploid number of 6 (diploid number of 12) undergoes meiosis, and at the meiosis II (cells are already haploid from the meiosis I), nondisjunction occurs. The mature ovum will contain 6+1 chromosomes (n+1) because there is an extra chromosome from the nondisjunction. The second polar body will contain 5 chromosomes (normal would be 6) because the second polar body did not receive that extra chromosome due to nondisjunction. In the event of fertilization of the abnormal ovum with a normal sperm, the zygote will be a diploid (12 chromosomes) normally but since the ovum as an extra chromosome from the nondisjunction then the zygote will have an extra chromosome (13 chromosomes). One pair of chromosomes has an extra single chromosome and this is called a trisomy.