Question

Please see attachment. There is a passage to read and what I do not understand is how they are getting the numbers in the diagram. I would like an explanation about that

Answer 1

What I am not understanding is how are they getting these numbers?

Answer 2

Ignoring, for the moment, the calculations, the initial numbers are the probabilities that a specific offspring will inherit a specific chromosome from a specific parent. Mom has two chromosomes, she passes one on to her child, the probability of the child getting one chromosome is 0.5 and the probability of it getting the other chromosome is 0.5.

That part should be pretty cut and dried...

Answer 3

Ok that makes sense, BUT WHY DO THEY MULTIPLY THE TWO NUMBERS? <-- sorry, I do not intend this to mean "yelling", but this is where I was stuck at.

Answer 4

Like when they do 0.5*0.5 = 0.25. <-- why do they do this, and how does this make sense?

Answer 5

No prob about the yelling, I shake my fists in the air above my head from time to time.

Let's say that Mom has two kids as in the first picture. The probability that kid A gets chromosome 1 is 0.50 as discussed above; the probability that kid B gets the same chromosome is also 0.50. These are independent events of equal probability.

But when you consider the probability that both kid A and kid B have got that same chromosome, you have to multiply. (0.5)*(0.5) = 0.25.

Answer 6

Ok first, may I show you as to how I am picturing this? I want to show you my visual interpretation of what I think how they got the 0.25

Answer 7

Actually forget it. I am going to read you what you said.

Answer 8

:D Take your time.

Answer 9

Why does it make sense to multiply in the first place?

Answer 10

I understand that it is a rule, but why multiply?

Answer 11

So is it saying that the probability that they share a gene is 25%?

Answer 12

for the half-sibling case

Answer 13

Yes, that is it.

Answer 14

how? Look this is what I drew. One sec.

Answer 15

Let me quickly photocopy it

Answer 16

I will call you back. I do not want you to keep waiting. I will message you OK?

Answer 17

Thank you for your help so far

Answer 18

Cool.

Answer 19

Ok it is attached. Hopefully it turned out to be legible.

Answer 20

I would solve the two parent problem by initially dividing it up into two runs of the one parent-two kid problem solved above and then combining the results by adding at the end.

As discussed above, the degree of relatedness between two kids with the same Mom but different Dads is 0.25. You can see that as the contribution to genetic sameness between kids A and B made by Mom.

Similarly, you can now assume that the same two kids, A and B, have the same Dad but different Moms. In that case, the degree of genetic sameness is the same - 25%. That's Dad's contribution to the sameness of kids A and B.

Now assume that both Mom and Dad contribute to the genetic sameness of the SAME two kids. Mom still contributes 0.25 and Dad still contributes 0.25 - so for the total you have 0.25 + 0.25 = 0.50 in total.

Take your time with that, it is by no means an obvious concept.

Answer 21

My result is not consistent with the first visual. The first visual says that the degree of relatnedness between half siblings is 25%, while the degree of relatedness between the full siblings is 50%.

Why is it that I am not getting 50%? I keep getting 25

Answer 22

You are setting up a harder problem than you need to.

In the shared kids case, both Mom and Dad contribute to the genetic similarity of the kids. Instead of trying to compute the entire thing in one gulp, it is simpler to divide it up. Look separately at the contribution made by Mom and the contribution made by Dad - then add them up to get the total genetic sameness of the kids.

To measure the contribution made just by one parent, look at the case where it is the only contributing factor: the half-siblings case. It's the one we already looked at above: the genetic sameness (that is, the degree of relation) due to just a common Mom is 0.25. Similarly, the genetic sameness due just to Dad is 0.25.

The total contribution made by both parents is the sum of Mom's and Dad's contribution, which is 0.50.

It's what they were trying to draw out in the picture of the 2 siblings case in the book.