Help pls with this question

The ratio of monozygotic to dizygotic twins is constant at 1:2.

The risk of Down Syndrome (DS) occurring in a single foetus pregnancy at 40 years is 1 in 110.


Questions

Calculate the risk of Down Syndrome if a 40 year old woman is carrying MZ twins.

My answer for this would be 1/110?

Calculate the risk of Down Syndrome if a 40 year old woman is carrying DZ twins for one and both twin.

My answer for this would be:

1/110 times 1/3

Assume she does not know if twins are MZ or DZ and calculate risk for at least one Down Syndrome child.

I don't know how to do this one...


Can someone tell me if I am going in the right track? thank you and to show me how to do the last one...

thank you

Hello, americo74!

Is this the original wording of the problem?
\(\displaystyle \;\;\)It is not clearly written.

The ratio of monozygotic to dizygotic twins is constant at 1:2.

Click to expand...

\(\displaystyle P(MZ)\,=\,\frac{1}{3},\;\;P(DZ)\,=\,\frac{2}{3}\)

The risk of Down Syndrome (DS) occurring in a single foetus pregnancy at 40 years is 1 in 110.

Click to expand...

\(\displaystyle P(DS)\,=\,\frac{1}{110},\;\;P(\sim DS)\,=\,\frac{109}{110}\)

(a)Calculate the risk of Down Syndrome if a 40 year old woman is carrying MZ twins.

Click to expand...

"The risk of DS": Does this mean: "Given that the woman is carrying MZ twins,
\(\displaystyle \;\;\)what is the probability that at least one infant has DS?
If so, why didn't they say so?

(b) Calculate the risk of Down Syndrome
if a 40 year old woman is carrying DZ twins for one and both twins.

Click to expand...

Exactly where does the last phrase belong?

I believe it was meant to say:
Calculate the risk of DS for one or both twins if the woman is carrying DZ twins.

Assume she does not know if twins are MZ or DZ
and calculate risk for at least one Down Syndrome child.

Click to expand...

Finally, they use the phrase "at least one".
\(\displaystyle \;\;\)Very sloppy writing!

thank you Champ!

I know the writing of it was very sloppy! :wink: