Sexual reproduction commonly alternates between a haploid gamete stage and a diploid zygote stage.

A gamete is a reproductive cell that contains one complete chromosome set. Meiosis produces haploid products by separating homologous chromosome pairs and then sister chromatids.

Fertilization is the fusion of two compatible haploid gametes. Their nuclei contribute chromosome sets to one new cell, the zygote.

If each gamete contains one chromosome set, written $n$, then fertilization restores two sets:

$$n+n\rightarrow2n.$$

For each chromosome type, the zygote therefore receives one homolog from one gamete and one homolog from the other.

For example, if a species has three chromosome types, one gamete might contain one copy of chromosomes 1, 2 and 3, while the other gamete contributes the corresponding second copies. The zygote again contains two homologous sets.

Schematically:

diploid germ-line cell
        ↓ meiosis
haploid gametes
        ↓ fertilization
 diploid zygote

Fertilization does not undo the chromosome reshuffling created by meiosis. The particular gametes that fuse are samples from the varied products generated by homolog assortment and crossover.

The central inheritance logic is therefore physical: meiosis separates homologous chromosome copies into different gametes, and fertilization combines one haploid chromosome set from each gamete to restore paired homologs in the zygote.