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CBSE Class 12-science Answered

Explain how does trisomy of the 21st chromosome occurs in humans. List any four characteristic features in an individual suffering from it.
Asked by Topperlearning User | 16 Jun, 2014, 03:57: PM
answered-by-expert Expert Answer

Down’s syndrome is also known as Trisomy 21. It is caused by the presence of an extra chromosome no. 21. Both the chromosomes of the pair 21 pass into a single egg due to non-disjunction during oogenesis in the mother’s ovary. Thus, the egg has 24 chromosomes instead of 23, and the offspring has 47 chromosomes (45+XY in male and 45+XX in female) instead of 46 chromosomes (44+XX or XY).

Characteristic features of Down’s syndrome:

(i) Short statured with small round head

(ii) Partially open mouth with protruding furrowed tongue

(iii) Broad palm with characteristic palm crease

(iv) Slow mental development

Answered by | 16 Jun, 2014, 05:57: PM
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