An adult woman has the 14:21 Robertsonian translocation. What is the probability that she will give birth to a child with Down syndrome?

Question

(a) 66%

(b) 25%

(c) 75%

(d) 33%

Answer 1

Robertsonian translocations occur when two chromosomes fuse and the arm of one is completely translocated to the other. These are large scale translocations. Relative to chromosomes 14 and 21, there would be four products of meiosis for this woman Assume the father has no aberrations on these chromosomes, so there are four possibilities for offspring occurring in equal probabilities:

1. Normal chromosomes 14 and 21.from both parents. These offspring are normal
2. Normal chromosomes from the father, balanced translocated chromosomes from the mother. Her chromosome 14 has the extra piece 21 because of the translocation, but the chromosome 21 from her in this case is shortened, so the offspring have the normal complement of genes and are phenotypically normal.
3. Normal chromosomes 14 and 21 from the father. Normal 21 and chromosome 14 with the extra piece of 21 from the translocation from the mother. These offspring will have three copies of the translocated region and if that region is will exhibit Down Syndrome the same as if they had three copies of chromosome 21.
4. Normal chromosomes 14 and 21 from the father. Normal 14 and a shortened 21 from the mother. These offspring will be similar to 21 monosomy, and although not actually Down syndrome will also exhibit effects of genetic imbalance.

So the phenotypic probabilities are 50% unaffected,;25 % trisomy 21 (Down syndrome,;25% monosomy 21.

Hope this helps