Down syndrome

[student12]

A question in my texts that I am unable to answer...any help mostly appreciated.

A 45-year-old woman gave birth to a child with Down syndrome. The child is her sixth and the first to show the condition.
a What is the most likely cause of trisomy-21 in this case?
b if the first child of a mother of 30 years of age had Down syndrome, what further information would you need to determine the chance that a second child would suffer from the condition?

My answers:
a The most likely cause of trisomy-21 would be nondisjunction b/c firstly it is her sixth child (suggesting that it is not within her phenotype), secondly her age.
b ?????

[weesper]

About 2-3% of cases of Down's are due to Robertsonian translocations and these can be familial ie they can be transmitted in the germline. This would mean that to give her a fair estimate of the chance of delivering another child afflicted with Down's syndrome you would need to know the family history first of all and secondly do a karyotype study to find out about the genetic makeup of the parents and whether one of them might carry a Robertsonian translocation (most often t 14;21). Hope this helps take care mxj