Down syndrome is caused by an extra copy of c | vedclass

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(A) Down syndrome is a chromosomal disorder caused by trisomy of chromosome $21$. An affected mother has the genotype $21, 21, 21$ (trisomy) in her ge

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(A) Down syndrome is a chromosomal disorder caused by trisomy of chromosome $21$. An affected mother has the genotype $21, 21, 21$ (trisomy) in her germ cells,while a normal father has $21, 21$ (disomy). During meiosis,the mother produces two types of gametes: $50\%$ with two copies of chromosome $21$ $(21, 21)$ and $50\%$ with one copy of chromosome $21$ $(21)$. The father produces gametes with one copy of chromosome $21$ $(21)$. When these gametes fuse: $1$. $50\%$ of the zygotes will have three copies of chromosome $21$ $(21, 21 + 21 = 21, 21, 21)$,resulting in Down syndrome. $2$. $50\%$ of the zygotes will have two copies of chromosome $21$ $(21 + 21 = 21, 21)$,resulting in normal offspring. Therefore,$50\%$ of the offspring will be affected.

Down syndrome is caused by an extra copy of chromosome $21$. If an affected mother and a normal father produce offspring,what percentage of the offspring will be affected by this disorder (in $\%$)?

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