Hello everybody! I have a question that I would like some help understanding. 1:

Question

Hello everybody! I have a question that I would like some help understanding.
1: I would like to know the probability of a female with Down syndrome (when mated with a normal 46 chromosome male) having a child WITH down syndrome. This should have something to do with the gametes formed in Meiosis, however, I am unsure how the trisomy splits and what my gametes will be.

2: With a similar situation above, what are the odds that both a male and a female with downsyndrome having a child together with down syndrome (assuming 48 chromosomes is lethal) So ONLY the surviving offspring with 2n=47. Thank you for the help!

Explanation / Answer

Well, In answering this I am assuming that the Down Syndrome you are talking about is Trisomy 21 only, because there is a Down Syndrome that is caused by Translocation of chromosomes 14 and 21... and this can have a carrier type. This one gets very confusing... But in simple Trisomy assuming no crossing over or further non-disjunction occurs: The Mother is +++ The father is ++ for chromosome 21 that is the mother has 1 extra one so during meiosis one all her gametes are +++, but after meiosis 2 there are 2 genotypes ++ or + (this is assuming this genetic dissorder does not put her at a greater risk for non-disjunction in meiosis 2 causing gamete production of +++ and 0, if this is the case they would not have any viable offspring) the father's gamets will be + and + so the arrangement of these will be (do a punnet square) 2 +++ (trisomic) 2 ++ (diploid) So the probability is 1/2 that the offspring will exhibit trisomy 21 Down Syndrome. Now when both parents have trisomy of 21 the gamete formation is + and ++ for both the mom and dad so (simple punnet square) 1 ++ (diploid) 2+++ (trisomic) 1++++ (tetraploidy is always lethal in humans and is seen in about 5% of spontaneous abortions) so the probability that the viable offspring will be 2n + 1 = 47 Trisomic at chromosome 21 is 2/3 This assumes there is no mutation occurs as a result of the initial non-disjunction and is inherited on chromosome 21 that inhibits the tolerance of the trisomic zygote.

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