Question

Healthy couple have three children, one with Angelman syndrome (due to gene mutation), one with Prader Willi syndrome (due to gene mutation) and one normal. they plan to have a fourth child, what is the probability of this child to be: normal, with Prader Willi syndrome and with Angelman syndrome?

The gender of the child is not determined by the questioner

Answer 1

Ans 1- There is less than 1% chance that the fourth child will have any of the two disorder (Angelman syndrome or Prader Wili syndrome, if the affected child has a gene deletion or uniparental disomy, up to 50% if the affected child has a mutation of the imprinting control region, and up to 25% if a parental chromosomal translocation is present.

Prader Willi syndrome is caused by  father's chromosome 15 is deleted or  the person has two copies of chromosome 15 from their mother and none from their father.

Angelman syndrome is caused by due to a lack of function of part of chromosome 15 inherited from a person's mother.

So in the given case they have already 2 childrens with both cases, therefore there must be deletion of chromosome 15 and genes causing these disorders, from both the parents.

So the probability of gettiing a normal child is one.