It is simple, and complicated, at the same time.
The simple part: Males have XY for the 23rd chromosomes. Females have XX.
The a little bit complicated part: Males get their X from their mother, and a Y from their father, and females get one X from each parent.
This means that the possible X contributors for a persons X are limited, compared to the full set of ancestors for autosomal DNA on chromosome 1-22.
This chart, from Blaine Bettinger, at www.thegeneticgenealogist.com shows the *average* contribution of each ancestor for a male.
You can see all the ancestors who DO NOT contribute to the X at all. This chart is for a male, but for a female just start in the pink block of the second ring.
For a male, the X comes from up to 2 out of his 4 grandparents, but for a female, the 2 Xs come from up to 3 out of her 4 grandparents. Only her paternal grandfather is eliminated. For males, only 3 of their 8 grandparents can be contributors to their X, for female, 5 out of their 8 great grandparents can be contributors to their two Xs. As you go further back in the generations, the number of possible ancestors who contributed to the X is much smaller than the actual number of ancestors.
Because of the lesser number of possible contributors, relationships may appear/be predicted closer than they actually are based on the number of cM shared on the X.
At some date in the future, when I have tested my 6 children I will post some more charts showing how much actual contribution we have in our results.
No comments:
Post a Comment