Write short notes on any two of the following:
Down.s syndrome 

Explanation

Down.s Syndrome
It is associated with the trisomy of 21st chromosome. It was described by Langdon Down in 1886. The person suffering from this have broad forehead, short and broad neck, short and stubby fingers, a simian line in the palm and a wide gap between first and second fingers in feet.
The person also suffers from severe mental retardation because of malformation of central nervous system. The patient has 47 chromosomes, (i.e. an extra 21st chromosome). Trisomy arises due to the non-disjunction (non-separation) of 21st chromosome during cogenesis. Thus, the egg has 24 chromosomes instead of 23. Fertilisation of such egg with a normal sperm results in trisomy of 21st chromosome.