In couples where each person is heterozygous for the sickle...

In couples where each person is heterozygous for the sickle-cell gene, there is a probability of 0.25 that any child of the couple will actually have the disease, and a probability of 0.75 that any child will not have the disease. For a population of families of six children in which both parents are heterozygotes, find the probability than in a randomly selected family: (a) none of the children will have the disease? (b) one or more of the children will have the disease?

Transcript

00:01 Where you have a couple heterozygous for the sickle cell gene, the probability of a child having the disease is 0 .25, not having the disease is 0 .75.

00:11 So i'm going to start with the kind of biology background to this particular question.

00:16 So everybody gets two copies of this gene.

00:20 You get two copies of any particular gene, excluding sex chromosomes.

00:25 So you can have a dominant copy, which is normal, for the system.

00:31 Cell gene, and you can have a recessive copy, which gives you sickle cell anemia.

00:37 If you have two recessive copies, you have sickle cell anemia.

00:42 If you have at least one normal copy, it is dominant and you don't get the condition.

00:49 So what we're looking at here, we're looking at a couple where each parent has one normal copy and one sickle cell copy.

00:58 So the parents don't have sickle cell anemia, but they are capable of passing this copy on.

01:04 And it's completely random, which copy you get from each parent, 50 -50 chance.

01:10 So there are four possible combinations you can get here, and all are equally likely.

01:18 You could get the dominant one from each.

01:23 You could get the dominant from this parent, recessive from this one, dominant from this parent, recessive from this one, or you could get the recessive from each.

01:34 And each of these is equally likely to happen, 25%.

01:38 All of these are normal.

01:40 You don't get the condition.

01:42 This one, you get sickle cell.

01:44 So that's where this is coming from.

01:48 It's all to do with inheritance.

01:50 So for each child, the probability they inherit two recessive all and therefore have sickle cell disease is 0 .25.

01:58 In this case, they have six children...