State the cause and symptoms of Down's syndrome. Name and explain the event responsible for causing this syndrome.

Explanation

Cause. Down.s syndrome is an autosomal aneuploidy caused by the presence of an extra chromosome number 21 i.e., trisomy of 21st. The person possesses 47 chromosomes (45+XY or 45 + XX). Symptoms. (i) Round face, broad forehead. (ii) Permanently opened mouth and protruding tongue. (iii) Projecting lowerlip, shortneck. (iv) Flat hands with stubby fingers. (v) Furrowed tongue. (vi) Mongolian type eyelid fold. (vii) Little IQ. Events responsible. This genetic disorder due to autosomal abnormality is a result of non disjunction of chromosomes of pair 21 and pass in a single egg during oogenesis. The egg possesses 24 chromosomes instead of 23 and the offspring has 47 chromosomes.