Final answer:
Down syndrome is a genetic condition caused by an extra copy of chromosome 21 due to nondisjunction during meiosis. Faulty spindle checkpoints during cell division can lead to this disorder, and the risk increases with maternal age. Down syndrome survivors present with intellectual and physical impairments.
Step-by-step explanation:
Understanding Down Syndrome
Down syndrome is a chromosomal disorder characterized by the presence of an extra copy of chromosome 21, a condition known as trisomy 21. During cell reproduction, particularly meiosis, chromosomes should separate evenly into gametes (sperm and egg cells). However, in cases leading to Down syndrome, nondisjunction occurs, meaning chromosome 21 fails to separate properly. As a result, a gamete with an extra chromosome 21 can be formed. When this gamete fuses with a normal gamete during fertilization, the resulting zygote will have three copies of chromosome 21, causing Down syndrome.
Several factors can contribute to nondisjunction, including problems with the spindle checkpoint during cell division. A faulty spindle checkpoint could mean that cells do not properly monitor the alignment and separation of chromosomes. Additionally, the frequency of nondisjunction increases with the age of the mother, making advanced maternal age a risk factor. Most embryos with extra chromosomes do not survive; however, Down syndrome is an exception whereby individuals can survive into adulthood, albeit with varying degrees of intellectual and physical impairments.