Zoology - Principles of Inheritance and Variation Question with Solution | TestHub

The above symptoms are seen in Down's syndrome. Down's syndrome is due to an extra chromosome number 21. The patient's cells have 3 copies of 21st chromosome (Trisomy 21) instead of homologous pair. It occurs due to failure of separation of homologous chromosomes or non-disjunction during meiosis (gamete formation). Symptoms of Down's syndrome:

(i) Patients show Mental retardation

(ii) Due to poor skeletal development, they have short stature and relatively small skull

(iii) Their face is typically flat with rounded flat nose

(iv) They have an open mouth, protruding tongue and a high arched palate.

(v) Most children show typical facial features with a fold of skin (epicanthal skin fold) over the inner corner of the eye. This results in downward slanting of eyelids.

(vi) They have flat hand with characteristic crease which runs all the way across the palm (Simian crease).