What is Mendelian Inheritance?
Mendelian inheritance describes how traits pass from parents to offspring through discrete units called genes, following predictable ratios discovered by Gregor Mendel's pea plant experiments. It forms the foundation of classical genetics.
Mendelian inheritance states that traits are controlled by pairs of alleles, one dominant and one recessive, that separate during gamete formation (Law of Segregation) and combine independently (Law of Independent Assortment), producing predictable ratios like 3:1 in a monohybrid cross.
- 1↓P GenerationCross true-breeding purple (AA) × white (aa) pea plants.
- 2↓F1 GenerationAll offspring are Aa (purple) — the dominant trait masks the recessive one.
- 3↓F1 Self-CrossAa × Aa plants are crossed to produce the F2 generation.
- 4F2 GenerationGenotype ratio 1 AA : 2 Aa : 1 aa → phenotype ratio 3 purple : 1 white.
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Step-by-step worked examples
A pea plant heterozygous for flower color (Aa, purple dominant) is crossed with another Aa plant. Out of 200 offspring, how many are expected to have white flowers?
Genotype ratio from Aa × Aa = 1 AA : 2 Aa : 1 aa. Phenotype ratio = 3 purple : 1 white. Recessive (white) fraction = 1/4 = 25%. Expected white offspring = 200 × 0.25 = 50.
A true-breeding tall plant (TT) is crossed with a true-breeding short plant (tt). What are the genotype and phenotype of the F1 generation?
Cross TT × tt → all offspring get one T and one t. F1 genotype = Tt (heterozygous) for all offspring. Since tall (T) is dominant, F1 phenotype = 100% tall.
In an F2 generation of 320 pea plants from an Aa × Aa cross, how many are expected to be heterozygous (Aa)?
Genotype ratio = 1 AA : 2 Aa : 1 aa (total 4 parts). Heterozygous fraction = 2/4 = 50%. Expected Aa plants = 320 × 0.5 = 160.
Flashcards
Quick quiz
Q1.In an Aa × Aa cross, what is the expected phenotypic ratio?
Q2.Mendel's Law of Segregation states that…
Q3.TT × tt cross produces F1 offspring that are…
Q4.Out of 400 Aa × Aa offspring, how many are expected to show the recessive phenotype?
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Common mistakes
Assuming traits blend, like paint colors mixing. — Correct: Mendelian traits follow discrete inheritance — alleles stay distinct, they don't blend.
Thinking the F1 generation shows a 3:1 ratio. — Correct: The 3:1 ratio appears in the F2 generation, not F1 (which is uniform Aa).
Confusing genotypic and phenotypic ratios. — Correct: Genotypic ratio for Aa × Aa is 1:2:1; phenotypic ratio is 3:1 because AA and Aa look the same.
Applying independent assortment to genes on the same chromosome. — Correct: Independent assortment only holds for genes on different chromosomes (or far apart on the same one).
FAQ
What are Mendel's laws of inheritance?
The Law of Segregation (allele pairs separate into gametes) and the Law of Independent Assortment (different genes assort independently).
What is the formula for Mendelian inheritance ratios?
A monohybrid cross (Aa × Aa) gives a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio.
What is an example of Mendelian inheritance?
Crossing a purple-flowered pea plant (Aa) with another Aa plant produces about 75% purple and 25% white offspring.
How do you calculate Mendelian inheritance ratios?
Multiply the total offspring by the expected fraction from the cross (e.g., 1/4 for recessive in Aa × Aa) to get the expected count.