00:01
We're looking at a family with three children.
00:03
Both parents are carriers for cystic fibrosis.
00:06
What is the chance they have no affected children? so the parents both have a copy of the dominant allele and a copy of the recessive.
00:16
So this is the cross that has happened.
00:18
And in this question, the first thing you need to do is work out the chance of a single child not being affected.
00:25
Let's do a ponnet square.
00:29
So we put the parental alleles on the outside.
00:32
And then inside the square, we're going to be combining them to get all of the possible outcomes and their relative frequencies.
00:39
Because each child here is getting one allule from each parent.
00:45
So here the possibilities.
00:46
Which of these represent a child who does not have cystic fibrosis? well it's these three squares.
00:54
All of these don't have two copies of a recessive allele.
00:58
This one down here, this one has fibrosis.
01:02
So for a single child, it's three over four of being unaffected...