law of segregation

Picture a parent holding a pair of socks, one of each colour, and tucking exactly one sock into each gift bag — never both, never neither. Every gamete (egg or sperm) gets just one of the parent's two alleles for a gene, chosen at random. That is the law of segregation: the two members of an allele pair separate cleanly during the formation of sex cells.

Because each gamete carries a single allele, the offspring's pair is rebuilt at fertilization — one allele from the egg, one from the sperm. A heterozygous parent therefore passes each of its two alleles to half its gametes on average, which is what produces Mendel's tidy ratios when crosses are tallied.

Mendel inferred this rule purely from breeding results, decades before chromosomes were understood. We now know the physical basis: during meiosis, homologous chromosomes — and the alleles they carry — are pulled to opposite poles and end up in different gametes. The law holds for genes on the chromosomes; segregation can be distorted in rare cases where one allele cheats the meiotic lottery.