An interesting procedure has been applied for assessing the chromosomal balance of potential secondary oocytes for use in human in vitro fertilization. Using fluorescence in situ hybridization (FISH), Kuliev and Verlinsky (2004) were able to identify individual chromosomes in first polar bodies and thereby infer the chromosomal makeup of "sister" oocytes. Assume that when examining a first polar body you saw that it had one copy (dyad) of each chromosome but two dyads of chromosome \(21 .\) What would you expect to be the chromosomal 21 complement in the secondary oocyte? What consequences are likely in the resulting zygote, if the secondary oocyte was fertilized?

Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. Meiosis produces sex cells (gametes) like eggs and sperm. During meiosis, the homologous chromosomes (one from each parent) pair up and exchange genetic material (crossing over). Then, the homologous chromosomes are separated into different cells (Meiosis I). Those cells then undergo the second round of separation (Meiosis II) to separate sister chromatids into four haploid daughter cells. In oogenesis (formation of eggs), the division is unequal, creating a large secondary oocyte and a smaller first polar body. In this exercise, we have been informed that in the given first polar body, there are two dyads (copied sister chromatids) of chromosome 21.

A secondary oocyte is formed from the primary oocyte after the first meiotic division (Meiosis I). Since we are informed that the first polar body has two dyads (sister chromatids) of chromosome 21, it means that both sister chromatids of one homologous chromosome pair have gone to the polar body. As a result, the secondary oocyte will have none of the sister chromatids of chromosome 21. Therefore, the chromosomal 21 complement in the secondary oocyte is zero.

If the secondary oocyte with no copy of chromosome 21 gets fertilized, it will form a zygote that has only one copy of chromosome 21, as it will receive one copy of the chromosome from the sperm cell. This condition is known as Monosomy 21. Monosomy 21 is a rare chromosomal abnormality and can lead to severe developmental problems and miscarriage. Most embryos with Monosomy 21 do not develop into viable pregnancies. In conclusion, the chromosomal 21 complement in the secondary oocyte is zero. If this secondary oocyte is fertilized, it would have Monosomy 21, which is a rare chromosomal abnormality and can lead to severe developmental problems and miscarriage.