Assume that you were examining a first polar body and noted that it had one copy (dyad) of each chromosome except chromosome 21. Chromosome 21 was completely absent. What would you expect to be the chromosome 21 complement (only with respect to chromosome 21) in the secondary oocyte? What consequences are likely in the resulting zygote if the secondary oocyte was fertilized?

Verified step by step guidance

1. Understand that the first polar body and the secondary oocyte are products of meiosis I, where homologous chromosomes are separated.
2. Recognize that the absence of chromosome 21 in the first polar body suggests nondisjunction occurred during meiosis I, leading to both copies of chromosome 21 being retained in the secondary oocyte.
3. Predict that the secondary oocyte will have two copies of chromosome 21, as both homologous chromosomes failed to separate and were not distributed to the first polar body.
4. Consider the fertilization of the secondary oocyte by a normal sperm, which contributes one copy of chromosome 21, resulting in a zygote with three copies of chromosome 21.
5. Conclude that the resulting zygote will likely have Trisomy 21, which is associated with Down syndrome, due to the presence of an extra chromosome 21.

Key Concepts

Meiosis and Polar Bodies

Meiosis is a specialized type of cell division that reduces the chromosome number by half, resulting in gametes. During oogenesis, the primary oocyte undergoes meiosis to produce a secondary oocyte and polar bodies. Polar bodies are small cells that typically do not develop into ova and are discarded. The first polar body contains half the chromosome complement of the primary oocyte, but in this case, it lacks chromosome 21.

Chromosomal Abnormalities

Chromosomal abnormalities occur when there is a deviation from the normal number or structure of chromosomes. In this scenario, the absence of chromosome 21 in the first polar body suggests that the secondary oocyte may also lack this chromosome. If the secondary oocyte is fertilized, the resulting zygote may have an abnormal chromosomal complement, leading to conditions such as Down syndrome, which is caused by an extra copy of chromosome 21.

Fertilization and Zygote Formation

Fertilization is the process where a sperm cell unites with an oocyte to form a zygote, which contains genetic material from both parents. If the secondary oocyte, which is missing chromosome 21, is fertilized, the zygote will have an incomplete set of chromosomes. This can lead to developmental issues or genetic disorders, as the zygote may not have the necessary genetic information for normal development.