How is it possible to not be genetically related to one of your biological grandparents?

Question

My biology teacher asked me this and I have no idea! Will someone explain it?

Answer 1

Here's why.

Your father (like any human being) has 23 pairs of chromosomes. In each pair, one is from his father (your grandfather), and one from his mother (your grandmother).

When it comes time for dad to make sperm cells, this involves the process of meiosis. Each sperm cell gets one chromosome from each of those 23 pairs ... but *which* chromosome the sperm cell gets is determined randomly. So it is *possible* that a sperm cell may get all 23 chromosomes that your dad got from his father (your grandfather), and none of the chromosomes that came from his mother (your grandmother).

So that sperm cell is carrying none of the DNA that your dad got from his mother (your grandmother). If that is the sperm cell involved in the fertilization that caused you, then you would technically not have any DNA from your paternal grandmother.

Now, while this is *possible*, it is extremely unlikely. In order for a sperm cell to get all 23 chromosomes from one parent, the odds are the same as flipping a quarter 23 times and getting all tails. That is 1 in 2^23 ... or 1 in 8,388,608.

But still ... dad makes a *lot* of sperm cells ... so it is remotely *possible*.

What often confuses people is the fact that every person gets exactly a 50-50 split from each parent ... but this does not mean a 25-25-25-25 split from each grandparent. It could be, say 50-0-30-20.


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Another interesting point: If you are male (XY), it is *possible* to be genetically unrelated to your paternal grandmother. But you are *guaranteed* to have at least one chromosome from your paternal grandfather ... namely your Y chromosome.

If you are female (XX), the opposite is true. You are guaranteed that one of your X chromosomes came from your paternal grandmother.

Finally, what happens on your mothers side? Basically the same thing w.r.t. chromosomes in the nucleus. The egg cell that made you may have no chromosomes from your maternal grandfather, or possibly no chromosomes from maternal grandmother. This is true whether you are male or female.

But you are still absolutely *guaranteed* to be genetically related to your maternal grandmother ... through mitochondrial DNA. This is DNA outside the nucleus and is always inherited entirely from your mother (and she got it from her mother). This is true whether you are male or female.

Answer 2

There are rare cases of twins being born who each had different fathers... I don't know if that would count? If you were the progeny of such, your twin's father would be different from your own which would mean that you would both have different paternal grandparents.

I think I just confused myself.

Answer 3

unless your addopted or have some strange geens i don't know if thats possible.