Principles of Inheritance and Variation
Syllabus Unit: Genetics and Evolution
Why it matters
Chapter overview
Gregor Mendel's pea-plant experiments gave biology its first quantitative laws of heredity, and everything from a single blended flower colour to a full human pedigree of X-linked disease builds on the same two rules. This topic moves from the monohybrid and dihybrid cross through the exceptions Mendel never saw (blending, co-dominance, multiple alleles) to how chromosomes, linkage and sex determination shape inheritance in real organisms — a core NEET-scoring block.
1Mendel's Laws & the Monohybrid Cross
Mendel's pea experiments gave biology its first two laws of heredity, and every single-trait cross since has followed the same 3:1 pattern.
Learning Objectives
- State Mendel's Law of Dominance and Law of Segregation in your own words.
- Predict the F1 and F2 genotypic (1:2:1) and phenotypic (3:1) ratios of a monohybrid cross.
- Distinguish genotype from phenotype and homozygous from heterozygous.
- Explain why a test cross is used to reveal an unknown genotype.
Key Terms
Gregor Mendel used the garden pea (**Pisum sativum**) because it is self-pollinating, easy to cross-pollinate by hand, and has several traits — like plant height, seed shape and flower colour — that exist as two clear, contrasting forms with no in-between. He worked with true-breeding lines for each trait before crossing them.
Law of Dominance
When a true-breeding tall (**TT**) plant is crossed with a true-breeding dwarf (**tt**) plant, every F1 offspring is **Tt** and is tall — the dominant allele T fully masks the recessive allele t in the heterozygote. This is Mendel's first law.
Law of Segregation
During gamete formation, the two alleles of a gene (here T and t) separate so that each gamete carries only one. When F1 (Tt) self-pollinates, the alleles recombine randomly to give a **1 TT : 2 Tt : 1 tt** genotypic ratio, which appears as a **3 Tall : 1 Dwarf** phenotypic ratio in F2.