A species could gain negative traits through evolution. So, evolution does not always mean progress.
Answer:. Nondisjunction during meiosis I in either the male and female gamete
The failure of homologous chromosome to separate at mitosis, or failure of sister chromatids to separate at Anaphase of meiosis I or ii is called non-disjunction.
In this cell division anomaly the unsegregated chromosomes are separated into cells as single chromosomes, therefore extra copies are therefore inherited.This result is one of the cell having an extra copies of chromosomes than others( chromosomes 21.
If this extra chromosome copies is segregated into a sperm or egg, the resulting zygote cells will have extra copy as chromosome 21, and hence Down syndrome.
→It is more common in Meiosis 1 than ii,because of longer duration of the first stage of division than meiosis ii,given rooms to errors.
→Most common in women(with age increase) than men because of exchange of telomeres in them,which worsen with age. Their Meiotic mechanism which weaken with age, and more prone to errors compare to men,
Explanation:
