Question

if couple III-7 III-8 have another child what is the probability it will have the syndrome

Answer 1

@rherdman1 Still there? I can help you.

Answer 2

yeah thanks

Answer 3

Ok so when we look at this we can tell that the trait is going to be recessive, but how? We know and understand this because not every generation is being affected, looks like every 2 generations is being effected.

1. Trait is Recessive

Now, every male has this trait, but why not females? Well this is going to be X-linked trait meaning that a 'x' chromosome that is inherited is in other words "Defective" and carries the bad gene.

So when we look at III-7 and III-8 neither are affected, but they have kids who are affected, how is this done? Well, remember 1. This trait is recessive, and the only way the kids can get the trait is if the male gets one of the "x" chromosomes.


Example:



And the reason I know the Female has 1 big X and 1 small x is because the only way you can be affected by this is if you get a small 'x' chromosome, and because the III-8 male (Father) will donate a "Y" chromosome to the son, his "X" chromosome won't matter for the male.

And the reason the Female is never effected is because (REMEMBER: one X chromosome comes from mom, and if the baby is a female, then the MALE (III-8) will only have to offer the baby its X chromosome (Which is not defective).

Now because the males will automatically get the "Y" Chromosome from the Male (Father III-8) that mean the only way the male could be affected is if the female (MOM III-7) has a defective gene small 'x'

Example.



Now look at the question, it asks us what is the probability the next child will have down syndrome, as long as the pair has a big 'X' then it will override the small 'x'. So the baby will not be affected as long as it is ---> 'XX' or 'Xx' or 'XY' but will be affected if it is ----> 'xY'

So look at the table and the chances are 1/4 to get a baby (Male or female) 25% to have down syndrome.

Hope that helped, please give medal if it did :)