What is Mendelian Genetics?
Mendelian genetics describes how traits pass from parents to offspring through discrete units called alleles, following patterns Gregor Mendel discovered from pea plant crosses. Two core laws — segregation and independent assortment — explain predictable ratios of genotypes and phenotypes.
Mendelian genetics states that each parent contributes one of two alleles for a trait (law of segregation), and alleles for different genes on different chromosomes are inherited independently of each other (law of independent assortment), producing predictable Punnett-square ratios like 3:1 and 9:3:3:1.
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Step-by-step worked examples
A monohybrid cross: Aa × Aa. What genotype and phenotype ratios appear in the offspring?
Each parent produces gametes A and a with equal probability (law of segregation) Punnett square: AA, Aa, Aa, aa → genotype ratio 1 AA : 2 Aa : 1 aa If A is dominant, phenotype ratio is 3 dominant : 1 recessive
A testcross: Aa × aa. Predict the offspring ratio.
The heterozygote makes gametes A and a (1/2 each); the homozygous recessive parent makes only a Punnett square: Aa, Aa, aa, aa → genotype ratio 1 Aa : 1 aa Phenotype ratio is 1 dominant : 1 recessive — testcrosses reveal an unknown genotype this way
A dihybrid cross: AaBb × AaBb, with A and B on different chromosomes. Find the phenotype ratio for A_B_ (both dominant).
By independent assortment, P(A_) = 3/4 and P(B_) = 3/4 P(A_ and B_) = P(A_) × P(B_) = 3/4 × 3/4 = 9/16 The full dihybrid ratio is 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb
Flashcards
Quick quiz
Q1.Which law explains why alleles separate into different gametes?
Q2.In Aa × Aa, what fraction of offspring is expected to show the recessive phenotype?
Q3.For a dihybrid cross AaBb × AaBb, what fraction of offspring is aabb?
Q4.Why is a testcross (Aa × aa) useful?
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Common mistakes
A dominant allele is always more common in a population. — Correct: Dominance describes how alleles interact in a heterozygote, not their frequency in a population.
Aa × Aa always gives exactly 3 dominant and 1 recessive offspring. — Correct: 3:1 is the expected ratio over many offspring; small litters can deviate from it by chance.
Independent assortment applies to any two genes. — Correct: It only holds for genes on different chromosomes (or far apart on the same one) — linked genes don't assort independently.
Genotype and phenotype ratios are always the same. — Correct: With complete dominance, genotype ratio 1:2:1 corresponds to phenotype ratio 3:1, not the same numbers.
FAQ
What is Mendelian genetics?
It's the study of how traits are inherited through discrete alleles following Mendel's law of segregation and law of independent assortment.
What is the formula behind Mendelian genetics probability?
For independently inherited traits, the combined probability of two phenotypes is P(A) × P(B), the product rule applied to Punnett-square outcomes.
What are examples of Mendelian inheritance?
Pea plant seed shape and color, monohybrid crosses (3:1 ratio), and dihybrid crosses (9:3:3:1 ratio) are classic examples.
How do you calculate Mendelian genetics ratios?
Use a Punnett square to list all possible gamete combinations, or multiply individual trait probabilities together for genes that assort independently.