Law of Segregation

The Law of Segregation states that every diploid organism carries two copies of each gene, called alleles, which are separated into gametes randomly during meiosis. This principle explains how traits pass from parents to offspring and why offspring receive one allele from each parent. It accounts for the predictable inheritance patterns Mendel observed in his pea plant experiments.

When two heterozygous parents (each carrying one dominant and one recessive allele) reproduce, their gametes randomly combine. This random segregation produces offspring with a 1:2:1 genotypic ratio, resulting in 75% displaying the dominant phenotype and 25% displaying the recessive phenotype. The dominant allele masks the recessive allele in heterozygous offspring, creating the characteristic 3:1 ratio.

Mendel crossed pea plants differing by single traits and observed that one parental phenotype disappeared in the F1 generation, then reappeared in the F2 generation. The F2 generation showed a 3:1 phenotypic ratio: 75% dominant and 25% recessive. This pattern revealed that traits are controlled by segregating alleles that separate during gamete formation and recombine randomly in offspring.

Alleles segregate during meiosis because they are located on homologous chromosomes that separate into different gametes. Scientists now understand that the separation of chromosomes during meiosis accounts for the segregation of parental alleles. This chromosomal basis explains how the Law of Segregation operates at the molecular level.

The presence of a dominant allele masks the recessive allele, determining the organism's phenotype. A homozygous dominant individual (two dominant alleles) and a heterozygous individual (one dominant, one recessive allele) both display the dominant phenotype. Only homozygous recessive individuals (two recessive alleles) display the recessive phenotype.

Mendel's model consists of four parts: first, alternative gene versions called alleles account for trait differences; second, organisms inherit two gene copies, one from each parent; third, dominant alleles mask recessive alleles; and fourth, the two alleles separate during gamete formation. This last principle is the Law of Segregation, which explains how traits segregate across generations.

Crossing two heterozygotes (Pp × Pp) produces a 1:2:1 genotypic ratio: 25% homozygous dominant (PP), 50% heterozygous (Pp), and 25% homozygous recessive (pp). The phenotypic ratio is 3:1, with 75% displaying the dominant phenotype and 25% displaying the recessive phenotype, demonstrating the Law of Segregation in action.