Punnett Squares

Key Concepts: Punnett Squares

What is a Punnett Square?

• A Punnett square is a chart used to determine the probability of different genotypes and phenotypes appearing in offspring.

• It helps predict inheritance patterns based on Mendelian genetics.

How to Use a Punnett Square

1. Write the alleles of one parent’s gametes along the top.

2. Write the alleles of the other parent’s gametes along the side.

3. Fill in the boxes by combining the alleles from the rows and columns.

4. Determine the genotypic ratio (e.g., PP : Pp : pp).

5. Determine the phenotypic ratio based on dominant and recessive traits.

Monohybrid Cross Example

A monohybrid cross involves one gene with two alleles.

Example: Ss × ss (S = long stems, s = short stems)

• Genotypic Ratio: 2 Ss : 2 ss (1:1)

• Phenotypic Ratio: 2 long : 2 short (50% long, 50% short)

Determining Missing Alleles in a Punnett Square

• The missing allele must be "a" because we see "Aa" in the squares where the unknown allele is involved.

• Final Answer: The missing letters are "A" and "a".

Monohybrid vs. Dihybrid Crosses

• Monohybrid Cross: Involves one trait (e.g., flower color: Pp × Pp).

• Dihybrid Cross: Involves two traits (e.g., pod color and pod shape: GgFf × GgFf).

Dihybrid Cross Example: AaBb × AaBb

Each parent has the alleles AaBb, so their gametes can be:

• AB, Ab, aB, ab

A Punnett square for a dihybrid cross has 16 boxes because each parent can pass four different gamete combinations.

Mendel’s Dihybrid Cross Example

• Seed Color: Yellow (Y, dominant) vs. Green (y, recessive).

• Seed Shape: Round (R, dominant) vs. Wrinkled (r, recessive).

• Cross: RrYy × RrYy

Phenotypic Ratio of Offspring (F2 Generation)

• 9 Yellow, Round (YR)

• 3 Yellow, Wrinkled (Yrr)

• 3 Green, Round (yyR)

• 1 Green, Wrinkled (yyrr)

Final Ratio: 9:3:3:1