# A low number statistics probability-problem (genetics)

1. Oct 31, 2013

### B0b-A

A probability problem (genetics), not homework, honest.

3 of 5 siblings are affected by an unknown inherited disease*.

That this outcome is higher than 1 in 4 which is typically the case in recessive traits suggests that the disease is more likely to be dominant than recessive.

Given 3 of 5 siblings are affected how much more likely is it that this genetic illness is dominant rather than recessive ?.

Like I said this isn't homework so I can't confirm if any answer given is correct.

[ *adult onset : inheriting the trait would not make their birth less likely ].

2. Oct 31, 2013

### Staff: Mentor

Without knowing the status of the parents, I don't think this problem works. In particular, the 1/4 is expected only in a case where a dominant trait would not appear at all.

With assumptions about their health, you can calculate the likelihood for both hypotheses and compare them.

3. Nov 1, 2013

### B0b-A

In this case only one of the parents of the five siblings is affected (symptomatic).
(sorry forgot to mention that ).

4. Nov 1, 2013

### Staff: Mentor

Then there is no reason to expect a rate of 1/4.

Based on the information that neither zero nor all childen got symptoms, you can determine the genetic composition of both parents in both cases and solve the problem.

5. Nov 2, 2013

### B0b-A

OK now I see 1in 4 is only expected in recessive inheritance where both parents were unaffected carriers: both carry one copy of the disease gene. But in this case one parent was affected.

If it were dominant the affected parent cannot have two copies of the disease gene, otherwise all the children would be affected.

If recessive the unaffected parent must have a copy of the disease gene, otherwise no children would be affected.

Now my possible punnet squares are giving me 1 in 2 odds that a child would be affected, whether it was dominant or recessive inheritance.

So that it affected 3 of 5 siblings doesn't make it more likely to be dominant than recessive (?)

Last edited: Nov 2, 2013
6. Nov 2, 2013

### Staff: Mentor

Correct.

You could argue that the information "some do not get the trait, some do" itself is some information, but then you need some estimate about the total rate of the trait in the population.