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

Verified step by step guidance

1. Step 1: Understand the normal chromosome number and meiotic divisions. The species has a haploid number (n) of 6, meaning each gamete normally has 6 chromosomes after meiosis. Meiosis I separates homologous chromosomes, and meiosis II separates sister chromatids.
2. Step 2: Analyze the nondisjunction event during meiosis II. Nondisjunction means the sister chromatids of one dyad fail to separate. As a result, one daughter cell (the ovum) receives both chromatids (the intact dyad), while the other (the second polar body) receives none from that dyad.
3. Step 3: Determine the chromosome number in the mature ovum. Since the ovum normally has 6 chromosomes, but here it receives an extra chromatid from the nondisjoined dyad, calculate the total chromosomes by adding the extra chromatid to the normal haploid number.
4. Step 4: Determine the chromosome number in the second polar body. Because the sister chromatids failed to separate, the second polar body will lack the chromatids from that dyad, so subtract those from the normal haploid number to find its chromosome count.
5. Step 5: Assess the chromosome condition after fertilization. The ovum with the abnormal chromosome number is fertilized by a normal sperm with 6 chromosomes. Combine these to determine the zygote's chromosome number and describe the resulting aneuploid condition (e.g., trisomy or monosomy).

Key Concepts

Meiosis and Chromosome Number

Meiosis is a two-step cell division process that reduces the chromosome number by half, producing haploid gametes from diploid cells. In species with a haploid number of 6, each gamete normally contains 6 chromosomes. Understanding the stages of meiosis and how chromosomes segregate is essential to determine chromosome numbers in gametes.

Nondisjunction During Meiosis II

Nondisjunction is the failure of sister chromatids to separate during meiosis II, resulting in one gamete with an extra chromosome and another missing that chromosome. This error leads to abnormal chromosome numbers in the resulting cells, affecting the genetic content of the ovum and polar bodies.

Fertilization and Aneuploidy

Fertilization combines the haploid gametes from each parent to restore the diploid chromosome number. When a gamete with an abnormal chromosome number (due to nondisjunction) fuses with a normal gamete, the resulting zygote exhibits aneuploidy, a condition with an abnormal number of chromosomes, which can cause developmental abnormalities.