Which Grandparent Are You Most Related to?

 

Your family tree says you inherited 25 percent of your ancestry from each. Genetics says you didn’t.

 

genes

What makes you who you are genetically? The easy answer is your family. The longer answer begins with the fact that all humans have two parents (at least for now), and usually four distinct grandparents (there are unfortunate exceptions). Genetically you are a recombination of four separate individuals. But that does not mean you have an equal contribution from four separate individuals.

But that does not mean you have an equal contribution from four separate individuals. Humans normally carry 23 pairs of chromosomes: 22 autosomal pairs and one pair of sex chromosomes, either two copies of the X for a female or an X and a Y in the case of males. By Mendel’s law of segregation you receive one copy of each pair from your mother (via the egg), and one copy from your father (via the sperm). This means exactly half of your genome derives from each parent.

Things begin to get more complicated going back two generations. One might think that of the 44 autosomal chromosomes you would receive 11 from each of the four grandparents. (For simplicity we’ll leave the sex chromosomes out for now. If you are a female, you receive one X from each parent, while if you are a male you receive an X from your mother and a Y from your father, who got it from his father.) But while the proportion of one’s inheritance from parents is fixed by exact necessity, the fraction from grandparents is governed by chance. For each of the chromosomes you inherit from a given parent, you have a 50 percent chance of gaining a copy from your grandfather and a 50 percent chance of gaining a copy from your grandmother. The laws of independent probability imply that there is a 1 in 4 million chance that all of your maternal or paternal chromosomes could come from just one grandparent!* What’s more, genetic recombination means that chromosomes aren’t purely from one grandparent or the other; during the cell divisions that produce sperm and eggs, chromosomes exchange segments and become hybrids. You almost certainly have different genetic contributions from your four grandparents.

But this is not just abstract theorizing. Imagine that you could know that 22 percent of the genome of your child derives from your mother, and 28 percent from your father. Also imagine that you know that 23 percent of the genome of your child derives from your partner’s mother, and 27 percent derives from your partner’s father. And you could know exactly how closely your child is related to each of its uncles and aunts. This isn’t imaginary science fiction, it is science fact.

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