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(A) Down's syndrome is a chromosomal disorder caused by the presence of an additional copy of chromosome number $21$ (trisomy of $21$).The frequency o

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Above $35$ years

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(A) Down's syndrome is a chromosomal disorder caused by the presence of an additional copy of chromosome number $21$ (trisomy of $21$).The frequency of this syndrome is known to be correlated with maternal age.For women aged $25$ years or under,the incidence is approximately $1/700$ births.As maternal age increases,the risk of non-disjunction during oogenesis increases,leading to a higher probability of producing aneuploid gametes.The frequency increases significantly to about $1/100$ for women of age $40$ and up to $1/10$ for women of age $45$. Therefore,the risk is notably higher when the maternal age is above $35$ years.

List-$I$	List-$II$
$A.$ Down's syndrome	$I.$ $11^{th}$ chromosome
$B.$ $\alpha$-Thalassemia	$II.$ $X$ chromosome
$C.$ $\beta$-Thalassemia	$III.$ $21^{st}$ chromosome
$D.$ Klinefelter's syndrome	$IV.$ $16^{th}$ chromosome

Frequency of Down's syndrome increases when the maternal age is

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