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?

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 predicting genetic outcomes 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. In the context of the question, FISH allows researchers to visualize individual chromosomes and assess their number and structure, which is essential for evaluating chromosomal abnormalities in oocytes and polar bodies.

Consequences of Chromosomal Abnormalities

Chromosomal abnormalities, such as aneuploidy, can lead to significant consequences in the resulting zygote. If a secondary oocyte has an extra chromosome 21, it may result in Down syndrome (trisomy 21) upon fertilization. Understanding these potential outcomes is vital for assessing the risks associated with in vitro fertilization and genetic counseling.