What is Down's syndrome? Give its symptoms and cause. Why is it that the chances of having a child with Down's syndrome increases if the age of the mother exceeds forty years?

Introduction to Down's syndrome

Down syndrome is a genetic disorder that occurs due to an error in cell division that generates an extra copy of chromosome 21 in an individual.

Symptoms of Down's syndrome

• Round face
• Broad forehead
• Short statured with small round head
• Furrowed tongue and partially opened mouth
• Broad palm with a characteristic palm crease
• Retareded physical, psychomotor, and mental development
• Low intelligence

Causes of Down's Syndrome

Down syndrome is a genetic disease caused due to trisomy of 21${}^{st}$ chromosome.

• Trisomic condition arises due to the formation of n+1 male of female gamete by non-disjuction and the subsequent fertilisation by normal (n) gamete.
• Presence of an extra chromosome number 21 leads to trisomy (2n+1), i.e., three chromosomes of the same type.

This is an example of aneuploidy which is characterised by the presence of an abnormal number of chromosomes in a cell. Here, the chromosome number is 2n+1, i.e., 46+1 = 47.

Chances of having a child with Down's syndrome increases with age

Chances of having a child with Down's syndrome increases, if the age of the mother exceeds forty years because older eggs are at a greater risk of improper chromosome division.

The primary oocytes are arrested in the diplotene stage of prophase-I in the foetal stage. When there is a long period for prophase prior to fertilisation, the chromosomes of the egg cells which are arrested in the prophase-I, may become unpaired. Longer duration in Prophase-I leads to nondisjunction of chromosomes. In this way, ageing affects the chromosome behaviour in meiosis.