Question

chromosomal inheritance: cause of downs syndrome
in 95% of cases of downs syndrome, there is an extra piece of chromosome 21 in every cell. this condition is the result of
multiple choice
independent assortment.
improper crossing over.
nondisjunction.
achiasmate segregation.

Explanation:

Down's syndrome is a chromosomal disorder. Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. In the case of Down's syndrome (trisomy 21), nondisjunction during meiosis (usually in egg cells) leads to an extra copy of chromosome 21. Independent assortment refers to the random distribution of homologous chromosomes during meiosis, which is a normal process for genetic variation. Improper crossing - over involves the exchange of genetic material between non - sister chromatids, which can cause genetic recombination but not an extra chromosome. Achiasmate segregation is a less common and more complex form of chromosome segregation that is not the main cause of the extra chromosome 21 in most Down's syndrome cases.

Answer:

nondisjunction.