1. Errors in chromosome segregation: During meiosis I or meiosis II, the chromosomes may fail to separate properly. This can result in one daughter cell having an extra copy of a chromosome (trisomy) while the other has a missing copy (monosomy).
2. Nondisjunction: This is the most common cause of aneuploidy in human eggs. It specifically refers to the failure of homologous chromosomes or sister chromatids to separate during meiosis, leading to the formation of gametes with an abnormal chromosome number.
- In meiosis I nondisjunction, the homologous chromosomes fail to separate during the first meiotic division. As a result, one daughter cell receives both homologous chromosomes, while the other daughter cell receives none.
- In meiosis II nondisjunction, the sister chromatids fail to separate during the second meiotic division. One daughter cell receives both sister chromatids, while the other daughter cell receives none.
3. Premature loss or gain of chromosomes: Rarely, errors can occur during the development of the egg before meiosis, leading to a premature loss or gain of chromosomes. This may result in aneuploid eggs from the start of meiosis.
4. Aging effects: The risk of aneuploidy increases with advancing maternal age. As women age, their eggs are more likely to experience chromosomal abnormalities due to factors like the decline in oocyte quality, reduced efficiency of DNA repair mechanisms, and hormonal changes.
5. Genetic factors: Certain genetic variations or rearrangements can also predispose individuals to an increased risk of aneuploidy. For example, translocations or inversions involving multiple chromosomes may disrupt the normal segregation pattern during meiosis.
Aneuploid eggs can lead to miscarriage or can result in the development of offspring with chromosomal disorders such as Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), Patau syndrome (trisomy 13), and Klinefelter syndrome (XXY) or Turner syndrome (X). Preimplantation genetic testing (PGT) is a technique used in assisted reproductive technologies to identify aneuploid embryos before implantation, thus, improving the chances of a successful pregnancy and reducing the risk of genetic disorders.
