Cue Column | Note Column
Key Questions | Main Content
Why did Mendel choose peas? | Self-pollinating → controlled crosses possible; contrasting traits (7 pairs) easily tracked; short generation time; produces large numbers of offspring for statistical analysis; 7 traits on 7 different chromosomes — fortuitous for discovering independent assortment.
What is Mendel's First Law? | Law of Segregation: The two alleles for any trait separate (segregate) during gamete formation. Each gamete carries only ONE allele. Restored at fertilisation. Proved by 3:1 phenotypic ratio in F2 of monohybrid cross.
What is Mendel's Second Law? | Law of Independent Assortment: Alleles of different genes on non-homologous chromosomes assort independently during gamete formation. Proved by 9:3:3:1 F2 ratio in dihybrid cross. EXCEPTION: does not apply to linked genes.
What is the Law of Dominance? | When organisms with contrasting alleles are crossed, F1 shows only the dominant trait. Recessive trait reappears in F2 . EXCEPTION: incomplete dominance, co-dominance.
Key Ratios | Monohybrid F2 phenotypic: 3:1 | Monohybrid F2 genotypic: 1:2:1 | Dihybrid F2 phenotypic: 9:3:3:1 | Dihybrid test cross: 1:1:1:1 | Monohybrid test cross: 1:1
What is a test cross? | Cross unknown dominant phenotype with homozygous recessive (tt). If all offspring dominant → TT. If 1:1 dominant:recessive → Tt. Reveals hidden genotype.
Diagram Reference
Summary (Bottom)
Mendel's three laws (Dominance, Segregation, Independent Assortment) provide the mathematical framework for predicting inheritance. Key exceptions: incomplete dominance and co-dominance modify the Law of Dominance; linkage modifies Independent Assortment. The 3:1 and 9:3:3:1 ratios are Mendel's hallmark predictions, testable in NEET by Punnett square calculations.