Final answer:
Non-disjunction during meiosis can lead to gametes with abnormal numbers of chromosomes, such as the extra chromosome 21 that causes Down syndrome. This typically happens due to issues with the spindle checkpoint during cell division. The risk of non-disjunction and, consequently, Down syndrome increases with maternal age.
Step-by-step explanation:
Non-disjunction is the event during meiosis when homologous chromosomes or sister chromatids fail to separate properly. This can occur in either meiosis I or meiosis II leading to gametes with an abnormal number of chromosomes. One well-known consequence of non-disjunction is Down syndrome, also known as trisomy 21. Individuals with Down syndrome have an extra copy of chromosome 21, leading to a total of three instead of the usual two. The likelihood of non-disjunction events increases with the maternal age, particularly in women over 36, which is why the risk of having a child with Down syndrome also increases with maternal age.
The spindle checkpoint is a crucial regulator during cell division, ensuring that chromosomes are correctly aligned before they are separated. A problem with this checkpoint during meiosis can result in non-disjunction, leading to the extra chromosome characteristic of Down syndrome. The presence of an additional chromosome 21 causes a range of physical and intellectual disabilities, with the severity varying widely among individuals.