Biology
Mendelian genetics & Punnett squares — quick study summary
Mendelian genetics describes how single-gene traits pass from parents to offspring. Mendel's three laws: segregation (alleles separate during gamete formation), independent assortment (different genes sort independently), and dominance (a dominant allele masks a recessive one in heterozygotes). Punnett squares predict offspring ratios. Monohybrid cross between heterozygotes (Aa × Aa) gives a 3:1 phenotypic ratio; dihybrid (AaBb × AaBb) gives 9:3:3:1.
Key points
- Allele = a variant form of a gene; genotype = allele combo; phenotype = observable trait
- Homozygous = AA or aa; heterozygous = Aa
- Law of segregation: each gamete gets one allele per gene
- Monohybrid Aa × Aa → 1 AA : 2 Aa : 1 aa (3:1 phenotype)
- Dihybrid AaBb × AaBb → 9 : 3 : 3 : 1 phenotypic ratio
Practice quiz
Click each question to reveal the answer.
1. Two heterozygous parents (Aa × Aa) are crossed. What phenotypic ratio do their offspring show, assuming A is dominant?
- 1:1:1:1
- 3:1
- 9:3:3:1
- 1:2:1
Answer: 3 dominant : 1 recessive
Punnett square: 1 AA, 2 Aa, 1 aa. AA and Aa both show the dominant phenotype, aa shows recessive — 3:1.
2. If a tall (Tt) plant is crossed with a short (tt) plant, what fraction of offspring are tall?
Answer: 1/2 (50%)
Tt × tt → 1 Tt : 1 tt. Half the offspring carry T and are tall.
3. What is the difference between genotype and phenotype?
Answer: Genotype is the genetic makeup (alleles); phenotype is the observable trait
Two organisms can share a phenotype (e.g. both tall) but have different genotypes (TT vs Tt).
Last reviewed: May 2026