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?

Verified step by step guidance

1. Understand the context: The first polar body is a byproduct of meiosis I, and its chromosomal composition reflects the segregation of homologous chromosomes during this division. The secondary oocyte, which is the product of meiosis I, will have the complementary set of chromosomes to the first polar body.
2. Analyze the chromosomal makeup of the first polar body: The first polar body has one copy (dyad) of each chromosome, but it has two dyads of chromosome 21. This indicates that during meiosis I, both homologous chromosomes 21 segregated into the first polar body, leaving none in the secondary oocyte.
3. Infer the chromosomal complement of the secondary oocyte: Since the first polar body received both homologous chromosomes 21, the secondary oocyte will lack chromosome 21 entirely. This is due to nondisjunction, an error in chromosome segregation during meiosis I.
4. Predict the consequences for the zygote: If the secondary oocyte is fertilized by a normal sperm (which contributes one copy of chromosome 21), the resulting zygote will have only one copy of chromosome 21 (monosomy 21). Monosomy 21 is typically lethal and results in early embryonic loss.
5. Summarize the implications: The chromosomal imbalance observed in the first polar body suggests that the secondary oocyte is not viable for producing a healthy zygote. This highlights the importance of chromosomal screening in in vitro fertilization procedures to ensure the selection of genetically balanced oocytes.

Key Concepts

Chromosomal Complement

The chromosomal complement refers to the complete set of chromosomes present in a cell. In humans, this typically includes 23 pairs, totaling 46 chromosomes. Each gamete, such as an oocyte, carries half of this complement, meaning it has 23 chromosomes. Understanding the chromosomal complement is crucial for assessing genetic balance and potential abnormalities in fertilization.

Fluorescence In Situ Hybridization (FISH)

Fluorescence in situ hybridization (FISH) is a molecular cytogenetic technique used to detect and localize the presence or absence of specific DNA sequences on chromosomes. By using fluorescent probes that bind to specific chromosome regions, FISH allows researchers to visualize chromosomal abnormalities, such as aneuploidy, which is the presence of an abnormal number of chromosomes. This technique is particularly useful in reproductive genetics to assess the chromosomal status of oocytes.

Aneuploidy and Its Consequences

Aneuploidy refers to the condition of having an abnormal number of chromosomes, which can lead to various genetic disorders. In the context of the question, if the secondary oocyte has two dyads of chromosome 21, it indicates a potential trisomy 21 condition (Down syndrome) in the resulting zygote if fertilized. This can result in developmental delays, physical growth issues, and increased risk of certain health problems, highlighting the importance of chromosomal balance in reproduction.