ICSE BoardEnglish MediumSTD 10BiologyGenetics1 Mark
Question
Define the following terms: Down's syndrome
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Answer
Down's syndrome : It is caused by the trisomy of the 21 st chromosome. The 21 st pair of chromosome does not split equally during oogenesis thus, producing gametes with either $n-1$ and $n+1$ chromosomes, not the normal ' $n$ ' number of chromosomes. These upon fertilisation by a normal spermatozoa produce a zygote with three 21st chromosomes. Zygotes with less than normal number of chromosomes ( $2 n -1$ ) fail to develop while the zygote with more than normal number $(2 n+1)$ causes severe abnormalities such as Down's syndrome, characterised by mental retardation, reduced resistance to diseases, congenital heart abnormalities.
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