Alleles are passed from parents to offspring through sex cells (egg and sperm), where each parent contributes one allele for each gene to the child during fertilization.

The Basic Idea

In most organisms (including humans), genes come in pairs, and each version of a gene is called an allele.

When parents make sex cells (egg or sperm), those pairs are split so each sex cell gets only one allele from each gene pair.

When an egg and a sperm join at fertilization, the offspring again ends up with two alleles for each gene: one from the mother’s egg and one from the father’s sperm.

Step‑by‑step: How alleles move

  1. During formation of sex cells (meiosis), the two alleles for a gene in a parent separate, so each egg or sperm carries just one allele.
  1. Which allele goes into which sex cell is largely random, so different eggs or sperm may carry different alleles.
  1. At fertilization, one egg and one sperm fuse, combining their alleles and giving the offspring a new pair of alleles for each gene.
  1. The combination of alleles the offspring receives (its genotype) then influences its traits (its phenotype), often following Mendelian patterns like dominant and recessive.

One‑sentence explanation (often what teachers want)

The alleles were passed from parents to offspring when each parent’s alleles separated into their sex cells, and then one allele from each parent combined at fertilization to form the offspring’s genotype.

TL;DR: Parents first split their allele pairs when making eggs and sperm, then one allele from each parent comes together at fertilization, giving the offspring two alleles for each gene.

Information gathered from public forums or data available on the internet and portrayed here.