What causes down syndrome?
Group of answer choices
trisomy
nondisjunction of homologues
nondisjunction of chromatids monosomy
The correct answer and explanation is:
The correct answer is nondisjunction of homologues.
Down syndrome is caused by the presence of an extra chromosome 21, making a total of 47 chromosomes instead of the usual 46. This extra chromosome results from a genetic error called nondisjunction, which happens during cell division, specifically during meiosis.
Nondisjunction occurs when the chromosome pairs do not separate properly during the formation of eggs or sperm. Normally, a pair of chromosomes should be divided into separate cells, but in nondisjunction, both chromosomes of a pair might end up in the same cell. When this happens with chromosome 21, it leads to the egg or sperm having an extra chromosome 21. Upon fertilization, the embryo will inherit three copies of chromosome 21, instead of the usual two, resulting in trisomy 21 (the scientific name for Down syndrome).
This specific nondisjunction of homologues happens during meiosis I, when the homologous chromosomes (chromosome 21 from both parents) fail to separate properly. Nondisjunction of chromatids, which occurs during meiosis II, is less common for Down syndrome but can also cause errors in chromosome separation. However, the majority of Down syndrome cases result from the nondisjunction of homologues in meiosis I.
It is important to note that monosomy, where one chromosome is missing, is not a cause of Down syndrome. In fact, monosomy 21, where only one copy of chromosome 21 is present, is not compatible with life, and such a condition typically results in miscarriage. Therefore, nondisjunction of homologues is the primary cause of Down syndrome, leading to the characteristic features of the condition, including intellectual disabilities and physical traits like a flat facial profile and distinct eye shapes.