Mendel's Laws: Dominance, Segregation, Independent

Gregor Mendel, working with garden pea plants in the 1860s, discovered the basic rules by which traits pass from parents to offspring. Three of his conclusions — the Law of Dominance, the Law of Segregation and the Law of Independent Assortment — form the foundation of classical genetics.

These laws matter because they let us predict the outcome of crosses: which traits will appear in children, in what proportions, and why hidden traits can reappear in later generations. Plant breeders, doctors and genetic counsellors still use them today.

Key Concepts

• Gene and allele: a gene controls a trait; alleles are its alternative forms, such as T (tall) and t (dwarf).
• Dominant allele: expressed even in a single copy (Tt appears tall). Recessive allele: expressed only in two copies (tt appears dwarf).
• Genotype and phenotype: the allele combination (TT, Tt, tt) versus the visible trait (tall or dwarf).
• Homozygous: both alleles identical (TT or tt). Heterozygous: alleles different (Tt).
• Law of Dominance: in a heterozygote, only the dominant allele shows in the phenotype.
• Law of Segregation: the two alleles of a gene separate during gamete formation, so each gamete carries only one allele.
• Law of Independent Assortment: alleles of different genes assort into gametes independently of each other (for genes on different chromosomes).

Formulas

Key ratios derived from the laws. For a monohybrid cross Tt x Tt: genotypic ratio 1 TT : 2 Tt : 1 tt and phenotypic ratio 3 tall : 1 dwarf. For a dihybrid cross YyRr x YyRr (two independently assorting genes), the phenotypic ratio is 9 : 3 : 3 : 1. The number of gamete types a genotype can produce is 2 raised to the power n, where n = the number of heterozygous gene pairs. Example: YyRr has n = 2, so 2 x 2 = 4 gamete types (YR, Yr, yR, yr). These ratios assume complete dominance, no linkage and random fertilisation.

Worked Example

Cross two heterozygous tall pea plants, Tt x Tt. Each parent makes gametes T and t in equal numbers (segregation). Combine them: T from father + T from mother = TT (tall); T + t = Tt (tall, dominance); t + T = Tt (tall); t + t = tt (dwarf). Out of 4 combinations, 3 are tall and 1 is dwarf. If the pair produces 400 offspring, expected dwarfs = 1/4 x 400 = 100 plants, and expected tall plants = 3/4 x 400 = 300 plants.

Common Misconceptions

• "Dominant means more common or stronger." Dominance only describes expression in a heterozygote; a recessive trait like blood group O can be very common in a population.
• "The recessive allele disappears in hybrids." It is hidden, not destroyed — it reappears in the tt offspring of two Tt parents.
• "Independent assortment applies to all gene pairs." It holds for genes on different chromosomes; genes close together on one chromosome are linked and tend to be inherited together.

Real-World Applications

Plant breeders use these laws to predict and select crop varieties with desired traits, such as disease resistance. Genetic counsellors estimate the risk of recessive disorders — for example, two carrier parents (Aa x Aa) have a 1 in 4 chance of an affected child (aa). Animal breeders similarly plan matings to fix useful traits.

Mendel's laws state that dominant alleles mask recessive ones, allele pairs segregate into gametes, and different genes assort independently, together explaining the predictable ratios seen in inherited traits.