May 19, 2023

Siblings Don’t Always Share 50 Percent Of Their Genes

You’ve probably heard somewhere that siblings share half of their genes with one another. That’s, like, Genetics 101, right? Actually, not quite. Thanks to the randomness of chromosome segregation and a process called recombination, siblings’ genomes are not always 50 percent the same.

This figure is actually an average, as Our World in Data researcher Saloni pointed out recently on Twitter. So, while you and your sibling probably share around 50 percent of your genes, the actual number is likely a little different. 

Genetic inheritance

To understand why that is, you first need to know a little bit about genetic inheritance. 

As humans, our DNA is coiled into 23 pairs of chromosomes – 46 chromosomes in total. Twenty-two of these pairs are called autosomes, and the final pair are sex chromosomes (XX or XY). One chromosome in each pair is inherited from our mother and the other from our father.

For this to happen, cells must first undergo a process called meiosis to produce gametes (egg or sperm cells). During meiosis, the number of chromosomes in the parent cell is reduced by half: a cell with 46 chromosomes produces four gametes, each containing just 23 chromosomes, one from each pair.

When the egg and sperm (each with 23 chromosomes) then fuse during reproduction, an embryo with a complete set of 46 chromosomes is formed.

But before the chromosome pairs get split apart, a sort of genetic reshuffling occurs. This is known as recombination. Autosomes line up in their pairs and exchange bits of genetic information, resulting in each egg and sperm cell having its own unique combination of genes.

For more info, read an article by Maddy Chapman published in the web site at:

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