Question

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?

Short answer

Answer: 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.

Step-by-step solution

Understanding Meiosis and Polar Bodies

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.

Determining Chromosome 21 Complement in Secondary Oocyte

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.

Consequences of Fertilizing the Secondary Oocyte

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.

Key concepts

Chromosomal Abnormalities

Chromosomal abnormalities occur when there are alterations in the number or structure of chromosomes. These changes can affect development and may lead to genetic disorders. In the scenario provided in the exercise, we observe a specific type of chromosomal abnormality involving chromosome 21 during meiosis.
Meiosis is essential for creating gametes, which carry half of the genetic material of an organism. If something goes wrong while chromosomes separate, it results in gametes with abnormal numbers of chromosomes. Such abnormalities often happen due to errors in the segregation of chromosomes, leading to conditions like monosomy and trisomy.

• Monosomy refers to the presence of only one chromosome from a pair, rather than two.
• Trisomy implies having three instances of a chromosome instead of the usual pair.

When a secondary oocyte with no copies of chromosome 21 is fertilized, the resulting zygote ends up with a single copy of the chromosome from the sperm. This is known as Monosomy 21 and can cause significant developmental challenges and often results in miscarriage.

In Vitro Fertilization

In vitro fertilization (IVF) is a sophisticated method to assist reproduction, wherein egg cells are fertilized with sperm outside the human body. This technique is often utilized by couples facing fertility challenges.
During IVF, eggs are retrieved from the ovaries and then exposed to sperm in a lab setting to achieve fertilization. Once fertilization happens, the formed embryos are monitored for several days before being assessed for quality.
One crucial aspect of IVF is ensuring that chromosomally balanced embryos are selected for transfer to the uterus to improve the potential for a successful pregnancy.
Techniques like pre-implantation genetic testing allow for the screening of embryos for certain chromosomal abnormalities, thus minimizing the chances of genetic disorders. This helps in identifying embryos with the proper genetic makeup before implantation, enhancing the likelihood of childbirth.

Fluorescence In Situ Hybridization (FISH)

Fluorescence In Situ Hybridization, abbreviated as FISH, is a powerful tool that allows researchers to visualize specific chromosomes or parts of chromosomes using fluorescent probes. This technique helps in identifying chromosomal abnormalities, among other applications.
FISH works by making use of fluorescently labeled DNA probes that bind to particular chromosomal regions, highlighting those areas under a fluorescent microscope.
In the context of the exercise, FISH is utilized to assess the chromosomal composition of first polar bodies during the IVF process. This provides valuable insights into the genetic health of potential secondary oocytes and can inform decisions about embryo selection.
The ability to specifically visualize distinct chromosomes can quickly reveal anomalies, such as extra copies or missing chromosomes. This enhances our understanding of the embryo's potential development and assists in guiding clinical decisions on embryo viability and transfer.