Chapter 8 · Biology
Heredity
Mendel's ratios are probability, and probability can be computed. The module generates the full Punnett square for any parent genotypes and reports exact genotypic and phenotypic ratios.
Punnett Square Lab — Monohybrid Cross
Pick the parent genotypes; the gametes, square, genotypic ratio and phenotypic ratio are generated live.
Each gamete carries only one allele (law of segregation); fertilisation is random, so each box is equally likely.
Parent 1
Parent 2
- Cross
- Tt × Tt
- TT
- 1/4
- Tt
- 2/4
- tt
- 1/4
- Phenotype
- 3/4 Tall, 1/4 Dwarf
- Phenotypic ratio
- 3 : 1
Formula sheet
Monohybrid F₂ phenotypic ratio
3 : 1
Tt × Tt gives 3 tall : 1 dwarf.
Monohybrid F₂ genotypic ratio
1 : 2 : 1
TT : Tt : tt.
Dihybrid F₂ ratio
9 : 3 : 3 : 1
RrYy × RrYy — proves independent assortment.
Test cross
Unknown × homozygous recessive
All dominant offspring → homozygous parent; 1:1 → heterozygous.
Sex determination
XX = female, XY = male
The father's sperm decides the sex of the child; 50 % chance each.
Number of gamete types
2ⁿ, n = number of heterozygous gene pairs
RrYy gives 2² = 4 gamete types.