What Are Gregor Mendel's Laws of Inheritance?
Gregor Mendel's three laws explain how traits pass from parents to offspring. They form the foundation of genetics: the Law of Segregation, the Law of Independent Assortment, and the Law of Dominance.
Mendel's laws state: (1) Alleles segregate equally to gametes, (2) different genes assort independently, and (3) dominant alleles mask recessive ones. These explain inheritance patterns in living organisms.
- •Each parent has 2 alleles (A/a)
- •Alleles separate during gamete formation
- •Each gamete gets 1 allele (A or a)
- •Offspring gets 1 from each parent (Aa)
- •Different genes assort independently
- •Trait for pea height: Tall (T) vs short (t)
- •Trait for pea color: Yellow (Y) vs green (y)
- •Gametes contain all combinations: TY, Ty, tY, ty
Step-by-step worked examples
A tall pea plant (Tt) crosses with a short plant (tt). What are the possible offspring?
Tall parent (Tt) produces gametes: T and t Short parent (tt) produces gametes: t and t Punnett square: | t | t | --+---+---| T | Tt| Tt| t | tt| tt| Result: 50% tall (Tt), 50% short (tt)
Two heterozygous tall plants (Tt × Tt). What ratio of tall to short?
Each parent (Tt) produces gametes: T and t Punnett square: | T | t | --+---+---| T |TT | Tt| t | Tt| tt| Result: TT (1), Tt (2), tt (1) → 3 tall : 1 short
In a dihybrid cross (TtYy × TtYy), how many phenotypes?
Independently assort height (T/t) and color (Y/y). Each trait shows 3:1 ratio (tall:short and yellow:green). 3 × 3 = 9 phenotypic classes: 9 tall-yellow : 3 tall-green : 3 short-yellow : 1 short-green
Flashcards
Quick quiz
Q1.Law of Segregation means…
Q2.A heterozygous tall (Tt) × homozygous short (tt) gives…
Q3.Independent assortment applies to…
Q4.In a 9:3:3:1 ratio, how many genes?
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Common mistakes
Mendel's laws apply to all organisms equally. — Correct: They apply best to diploid organisms with simple Mendelian traits; some traits are non-Mendelian (multiple genes, environment).
Dominant alleles are more common. — Correct: Dominance is about expression, not frequency. Recessive alleles can be common.
Heterozygotes show a blend of both traits. — Correct: Heterozygotes typically show the dominant phenotype; the recessive allele is masked.
Two tall parents always produce tall offspring. — Correct: If both are Tt (heterozygous), 25% of offspring can be short (tt).
FAQ
What is Gregor Mendel's Law of Segregation?
During gamete formation, alleles for a trait separate so each gamete receives only one allele.
What is the difference between genotype and phenotype?
Genotype is the genetic makeup (Tt); phenotype is the visible trait (tall).
How does a Punnett square help predict inheritance?
It shows all possible allele combinations from each parent's gametes.
Why is Mendel's work important to modern genetics?
His laws explain basic inheritance patterns and form the foundation of predictive genetics.