What is a Punnett Square?
A Punnett square is a simple graphical tool invented by geneticist Reginald Punnett in 1905. It visualizes all possible combinations of maternal and paternal alleles to predict the probability of traits in offspring.
Key Genetics Vocabulary You Must Know
- •Allele: A version of a gene (represented by a single letter, like B or b).
- •Dominant Allele (Capital letter, e.g. B): Strong trait. Only one copy is needed to show the physical trait.
- •Recessive Allele (Lowercase letter, e.g. b): Weak trait. Can only show if both alleles are recessive (bb).
- •Homozygous: Having two of the same alleles (BB = homozygous dominant, bb = homozygous recessive).
- •Heterozygous: Having two different alleles (Bb = heterozygous carrier).
- •Genotype: The letter combination (BB, Bb, or bb).
- •Phenotype: The physical appearance (e.g. Purple flower or White flower).
Worked Example 1: Heterozygous Cross (Bb × Bb)
This is the most famous cross in biology (Gregor Mendel's F2 generation).
Problem:
In pea plants, Purple flowers (P) are dominant over white flowers (p).
Cross two heterozygous purple plants (Pp × Pp).
What are the genotype and phenotype probabilities?
Step 1: Set up the 2×2 grid
Place Parent 1 (P and p) along the top, and Parent 2 (P and p) along the left:
| P | p | |
|---|---|---|
| P | PP (Purple) | Pp (Purple) |
| p | Pp (Purple) | pp (White) |
Step 2: Tally the 4 boxes (Genotypes)
Each box represents 25% (14) of the offspring:
- •PP (Homozygous Dominant): 1 box = 25%
- •Pp (Heterozygous): 2 boxes = 50%
- •pp (Homozygous Recessive): 1 box = 25%
Genotype Ratio: 1 PP : 2 Pp : 1 pp
Step 3: Tally the physical traits (Phenotypes)
- •Any plant with at least one P will be Purple:
- •Two boxes are bb (Brown fur) ⟹ 24 = 50%
Final Answer: There is exactly a 50% chance that an offspring will have brown fur.
Punnett Square Summary Cheat Sheet
| Cross Type | Parent Genotypes | Offspring Phenotypes | Offspring Genotype Ratio |
|---|---|---|---|
| Homozygous Dom × Recessive | BB × bb | 100% Dominant | 100% Bb |
| Heterozygous × Heterozygous | Bb × Bb | 75% Dominant, 25% Recessive | 1 BB : 2 Bb : 1 bb |
| Heterozygous × Recessive | Bb × bb | 50% Dominant, 50% Recessive | 2 Bb : 2 bb (1:1) |
| Homozygous Dom × Heterozygous | BB × Bb | 100% Dominant | 2 BB : 2 Bb (1:1) |
Probability Does Not Guarantee Exact Litter Counts!
If two parents have a 25% chance of a blue-eyed child, having 4 children does NOT guarantee exactly 1 blue-eyed child. Each baby has an independent 25% chance, just like flipping a coin!