Reproduction (1 of 6) Egg, sperm, and zygote
Eggs and sperm carry only half the usual number of chromosomes — just 23 unpaired chromosomes, carrying one version of each gene. When the egg and sperm get together, the baby receives the normal 23 matched pairs.
Reproduction (3 of 6) Recombination
Producing eggs and sperm is our first opportunity for mixing and matching genes. When the mother makes an egg, her chromosomes first find their matched partners and exchange some genes with each other. That's called recombination. Because of this shuffling, genes from the mother's mom and genes from the mother's father can wind up next to one another on the same stretch of genes. (The same thing happens in the father's sperm.)
Reproduction (6 of 6) Zygote with recombinant genes
When egg and sperm meet, the baby inherits a combination of genes that is totally unique: it carries versions of genes from all 4 grandparents plus any mutations that occurred when the mother and father were making the egg and sperm.