Law of Segregation

Gregor Mendel performed monohybrid crosses between pea plants that differed in a single trait, such as pod color. He saw one parental phenotype disappear in the first filial, or F1, generation.

The F1 pea plants contain both male and female reproductive structures, allowing them to self-pollinate. When these plants were self-crossed, the missing parental phenotype, which was not expressed in the F1 generation, reappeared in the F2 generation.

In the F2 generation, offspring with the dominant phenotype made up about 75% of the population, while offspring with the recessive phenotype made up about 25%.

From these experiments, Mendel concluded that traits pass from one generation to the next through pairs of heritable factors, now known as genes.

Mendel proposed the Law of Segregation. This law states that every diploid organism carries two variants of each gene, called alleles. These alleles separate randomly into gametes during meiosis. So, when organisms reproduce, each offspring receives one allele from each parent.

Because allele segregation is random, heterozygous parents carrying two different alleles, like the plants of the F1 generation, are equally likely to pass on either the dominant allele or the recessive allele. This explains the three-to-one phenotypic ratio seen in a monohybrid cross between two heterozygotes.

Based on this ratio, 25% of the offspring are expected to have a homozygous dominant genotype and show the dominant phenotype. Another 50% are expected to be heterozygous and show the dominant phenotype. The remaining 25% are expected to have a homozygous recessive genotype and show the recessive phenotype.