Kuliev and Verlinsky (2004) state that there was a relatively high number of separation errors at meiosis I. In these cases the centromere underwent a premature division, occurring at meiosis I rather than meiosis II. Regarding chromosome 21, what would you expect to be the chromosome 21 complement in the secondary oocyte in which you saw a single chromatid (monad) for chromosome 21 in the first polar body? If this secondary oocyte was involved in fertilization, what would be the expected consequences?

Verified step by step guidance

1. Understand the context: The problem involves errors during meiosis I, specifically premature separation of sister chromatids. Normally, homologous chromosomes separate during meiosis I, and sister chromatids separate during meiosis II. However, in this case, the centromere prematurely divides during meiosis I, leading to abnormal chromosome segregation.
2. Analyze the chromosome 21 complement in the secondary oocyte: If a single chromatid (monad) for chromosome 21 is observed in the first polar body, it means that one of the sister chromatids of chromosome 21 was segregated into the first polar body during meiosis I. This indicates that the secondary oocyte will have the other sister chromatid of chromosome 21, resulting in a single chromatid (monad) for chromosome 21 in the secondary oocyte.
3. Consider the consequences of fertilization: During fertilization, the secondary oocyte will combine with a normal haploid sperm, which contributes one full chromosome 21. Since the secondary oocyte contains only a single chromatid for chromosome 21, the resulting zygote will have an abnormal chromosome 21 complement.
4. Predict the zygote's chromosome 21 complement: The zygote will have one full chromosome 21 from the sperm and only a single chromatid (monad) from the oocyte. This results in an imbalance in the chromosome 21 complement, which could lead to developmental abnormalities or non-viability.
5. Summarize the expected consequences: The abnormal chromosome 21 complement in the zygote could lead to genetic disorders or developmental issues. For example, if the zygote ends up with a monosomy (only one copy of chromosome 21), it is typically lethal. Alternatively, if the chromatid duplicates post-fertilization, it could lead to mosaicism or other chromosomal abnormalities.

Key Concepts

Meiosis and Chromosome Segregation

Meiosis is a specialized form of cell division that reduces the chromosome number by half, resulting in four haploid cells. During meiosis I, homologous chromosomes are separated, while meiosis II separates sister chromatids. Errors in this process, such as premature centromere division, can lead to aneuploidy, where gametes have an abnormal number of chromosomes.

Chromosome 21 and Down Syndrome

Chromosome 21 is one of the smallest human chromosomes and is associated with Down syndrome when there is an extra copy (trisomy 21). If a secondary oocyte has a single chromatid for chromosome 21, it indicates that one chromatid was retained while the other was segregated into the first polar body. This can lead to fertilization with an abnormal chromosome complement, potentially resulting in Down syndrome.

Polar Bodies and Oocyte Development

Polar bodies are small cells produced during oocyte development that typically do not develop into ova. They are a byproduct of meiosis, allowing the oocyte to retain most of the cytoplasm and genetic material. The presence of a single chromatid in the first polar body suggests that the secondary oocyte may have an abnormal chromosomal arrangement, which can affect the genetic outcome of fertilization.