How paired chromosomes trade DNA
Before an egg or sperm cell is made, it copies its chromosomes and then pairs the two versions of each chromosome: one inherited from the mother and one from the father. While paired, matching chromosomes can break at corresponding points and rejoin with the opposite partner. This exchange, called crossing over, gives a chromosome a stretch of DNA from each inherited version.
The exchange is orderly rather than a random swap of unrelated pieces. The matching chromosomes carry genes in the same general sequence, so the trade usually changes which versions of nearby genes travel together without changing the chromosome’s overall set of genes.
Why siblings receive different combinations
After these exchanges, the paired chromosomes separate into developing reproductive cells. Each pair is also sorted independently of the others: the maternal or paternal version can go into a particular egg or sperm. Crossing over and this independent sorting work together, creating many possible combinations of family DNA.
That is why two siblings can inherit different bundles of genetic variants even from the same parents. The process reshuffles inherited material; it does not create a wholly new genome. The result is that a chromosome passed to a child may be a mosaic of segments from the parent’s own two inherited copies—one reason family resemblance can come in such varied combinations.
