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Down syndrome

Down syndrome occurs once in approximately 700 live births. The vast majority of these cases are due to trisomy 21 resulting from a non-disjunction event. In rare cases, Down syndrome is caused by a reciprocal translocation between chromosome 21 and the tip of chromosome 14 (or some other autosome) in the germ line. No genes on chr14 are moved to chr21 and nearly all the genes on chr21 are added to the tip of chr14 (i.e., a Robertsonian translocation). Because Down syndrome females with this translocation are fertile, they can pass the translocation chromosome to their descendents, resulting in a high incidence of Down syndrome in their families. Assuming that the translocation chromosome can pair with both chr14 and chr21 in meiosis-I, please estimate the frequency at which the translocation chromosome is transmitted to gametes in these Down syndrome females. (It might help if you draw out the different kinds of pairing options that can occur during meiosis.)

Most Down syndrome conceptions do not survive to birth. What is the overall frequency of Down syndrome children born to Down syndrome mothers who carry these translocations?

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