X. We all have an X chromosome. Some of us even have two. How we get it, where we get it from and how it comes to us is similar but different.
Females have two X chromosomes, they receive one from each parent. When a mother gives her X chromosome to a child it is a recombination of the two X chromosomes she received from HER parents.
A male can only have one X chromosome, the one he receives from his mother. His father can only give him a Y chromosome.
A father can only give his X chromosome to a daughter, and because he has only one there is no recombination. The X chromosome he passes to his daughters is an exact copy of the recombined X chromosome which he received from his mother. If he only has sons, it can’t be passed on at all and the inheritance stops.
We still get other autosomal DNA though from our female ancestors – just not always the X.
This chart shows where my X DNA comes from. Coloured to see where it is passed on and where it is not. Pink inherited from mothers and blue inherited from fathers. There is no colour where no X can be inherited because the child in the next generation was male.
It also shows as a percentage how that is combined each generation. For example for me, each parent contributing 50% to make a total of 100% X DNA in 2 chromosomes. The 50% from my mother is made up from the recombination of her two chromosomes from her parents, nominally 25% each. The 50% from my father is that same 50% he received from his mother.
How
crazy is it to see that for my paternal grandmother, the X chromosome from her
father (which makes up half of the X I inherited from my father) came direct
from his mother and in turn half of that again came from his mother’s
grandmother! That means I have nominally 12.5% X DNA from one of my 4 x great
grandmothers. I don’t even know her name.
