Mendel's Laws of Heredity

Mendel crossed peas with clear traits and found the laws of dominance, segregation, and independent assortment. The first hybrid generation (F₁) shows only the dominant trait, and in F₂ from hybrid × hybrid the ratio is dominant:recessive = 3:1. Genotype ratios are AA:Aa:aa = 1:2:1, and a Punnett square makes the cross easy to see. On this page, switch between Punnett square and dominance modes to check the outcomes of a cross.

Mendel's Pea Experiments

🌱 Intuition — Why Peas?

①Peas have short generations and clear, distinct traits

②Round vs wrinkled, yellow vs green

③Mendel crossed thousands of plants and uncovered the rules

Punnett Square & Dominance

Mendel's Three Laws

📚 Three Laws Summary

①Law of Dominance: F₁ shows only the dominant trait

②Law of Segregation: F₂ shows 3:1 dominant:recessive

③Law of Independent Assortment: traits inherit independently

Summary

Law of Segregation

F₂ phenotype = dominant : recessive = 3 : 1

Aa × Aa → AA : Aa : aa = 1 : 2 : 1

Law of Dominance

F₁ (Aa) → only dominant trait shown

Dominant allele masks recessive

Law of Independent Assortment

F₂ phenotype ratio = 9 : 3 : 3 : 1

Two trait pairs inherit independently

🎯 Exam Points

①Even one dominant allele (A) shows the dominant trait (AA, Aa)

②Recessive only shows as aa

③F₂ ratio 3:1 → segregation

④Distinguish genotype 1:2:1 vs phenotype 3:1

⑤Pure (AA, aa) vs hybrid (Aa)

Examples and Unit Test

Example 1

When round (A) is dominant over wrinkled (a), what are the phenotype ratio and the genotype ratio of the offspring from a hybrid cross Aa × Aa?

A Punnett square for Aa × Aa gives offspring AA : Aa : aa = 1 : 2 : 1 (genotype ratio).

Any seed with at least one A is round (dominant), so round (AA, Aa) : wrinkled (aa) = 3 : 1 (phenotype ratio).

▸ Phenotype round : wrinkled = 3 : 1 / genotype AA : Aa : aa = 1 : 2 : 1

By the law of segregation, crossing hybrids gives a 3 : 1 dominant-to-recessive ratio in F₂.

Example 2

A pure round pea (AA) is crossed with a pure wrinkled pea (aa). What are the genotype and phenotype of the first hybrid generation (F₁), and why?

From AA × aa, every F₁ has the single genotype Aa.

Since A is dominant over a, all of F₁ appear as round peas (law of dominance).

▸ F₁ = all Aa, all showing round peas / law of dominance

The F₁ of a pure cross shows only the dominant trait, and all are hybrids (Aa).

Unit Test

Which statement about Mendel's laws is correct?

①The offspring phenotype ratio of a hybrid Aa × Aa is 1 : 1

②A genotype of Aa shows the recessive trait

③The first hybrid generation of pure AA × aa all show the dominant trait

④The recessive trait appears when the genotype is AA

⑤The phenotype ratio of the second hybrid generation is always 1 : 2 : 1

▸ ③ The first hybrid generation of pure AA × aa all show the dominant trait

The phenotype ratio of Aa × Aa offspring is 3 : 1 (① wrong, 1:2:1 is the genotype ratio), and Aa has an A so it shows the dominant trait (② wrong).

The recessive trait appears only as aa (④ wrong), and the F₂ phenotype ratio is 3 : 1 (⑤ wrong, 1:2:1 is the genotype); the F₁ of pure AA × aa all show the dominant trait (③ correct, law of dominance).

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