Identify chromosomal disorder caused due to non-disjunction of 21st chromosome and enlist its characteristics.

Answer: Down’s syndrome (trisomy 21; 2n = 47).

Non-disjunction of chromosome 21 during gamete formation (usually in the mother, more often with higher maternal age) gives a zygote with three copies of chromosome 21. This is Down’s syndrome (mongolism), karyotype 45 + XX or 45 + XY (total 47).

Characteristics:

• Short stature with a small, round head and flat face.
• Broad flat nose, slanting eyes with an epicanthal fold, partly open mouth with a furrowed, protruding tongue.
• Mental retardation and delayed physical and psychomotor development.
• Broad palm with a characteristic single transverse palm crease (simian crease); short, stubby fingers.
• Congenital heart defects are common; reduced life span.