Law of Segregation and the Test Cross

Mendel's Law of Segregation — sometimes also called the Law of Purity of Gametes — is the second of his three great laws, and it states that the two alleles of a gene present together in a heterozygous individual do not blend, merge, or contaminate one another in any way; instead, they remain entirely distinct throughout that individual's life and then physically separate (segregate) from each other during the formation of gametes, so that each individual gamete carries only ONE of the two alleles, never both together. This law is precisely what Mendel's F2 generation demonstrated so convincingly: when he allowed his uniformly tall, heterozygous (Tt) F1 plants to self-pollinate, the resulting F2 generation did not remain uniformly tall, nor did it show some blended, intermediate height — instead, dwarf plants reappeared, fully and unmistakably dwarf, in a ratio of one dwarf plant to every three tall plants (a 3:1 phenotypic ratio), and, more tellingly still, the underlying genotypic ratio was 1 TT : 2 Tt : 1 tt. The recessive t allele, therefore, had evidently not been lost, diluted or altered in any way while it sat hidden inside the heterozygous F1 generation — it re-emerged completely intact in the homozygous recessive (tt) quarter of the F2, exactly as the principle of segregation, rather than any kind of blending inheritance, predicts.

The physical, cellular basis of the Law of Segregation lies in the behaviour of homologous chromosomes during meiosis, specifically at anaphase of meiosis I: the two members of each homologous chromosome pair — one originally inherited from the individual's mother, the other from its father, each carrying one allele of every gene located on that particular chromosome — are pulled apart and move to opposite poles of the dividing cell. Because this separation happens for every homologous pair, the four haploid gametes ultimately produced from one meiotic division each end up carrying exactly one allele of each gene, never two copies of the same gene together, which is exactly the physical event that Mendel's abstract "segregation of factors" describes at the level of visible cell biology (a connection that was only worked out decades after Mendel's own work, by Sutton and Boveri — see §4.8). …