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 (2 of 6) Chromosome duplication
When eggs and sperm are produced, the parent cell first copies each chromosome, leaving the duplicate pairs attached to one another.
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.)