October 2026 - Mendel's Laws & Punnett Squares Study Guide

Mendel's two laws

• Segregation — the two alleles for a trait separate during gamete formation, so each gamete carries one of them. This is why a heterozygous parent produces two kinds of gamete in equal numbers.
• Independent assortment — alleles for different traits are inherited independently of one another. Eye colour does not travel with height.

How to draw a Punnett square

Put one parent's two alleles across the top and the other parent's down the side. Fill each of the four boxes with the letter from its row and the letter from its column, writing the capital first. The four boxes are the four equally likely offspring genotypes.

For a Bb × Bb cross, the boxes come out BB, Bb, Bb, bb. Three of the four carry at least one dominant allele, giving a 3:1 phenotype ratio, while the genotypes divide 1:2:1. Those two ratios describe the same square, and mixing them up is the commonest error here.

The crosses worth recognising on sight

Working backwards from the phenotype

Beyond simple dominance

Two patterns that do not follow the standard rules, and which the exam names explicitly.

• Incomplete dominance — the heterozygote is intermediate. A red and a white flower give pink.
• Codominance — both alleles show fully and separately. AB blood type is the standard example.
• Sex-linked traits sit on the X chromosome, which is why some conditions appear far more often in males, who have only one X.

Terms to know

Allele

One version of a gene.

Dominant

An allele expressed whenever it is present, written as a capital.

Recessive

An allele expressed only when both alleles are recessive.

Homozygous

Two identical alleles.

Heterozygous

Two different alleles. Also called a carrier for a recessive condition.

Punnett square

A grid showing the possible offspring genotypes from a cross.

Segregation

Mendel's law that allele pairs separate during gamete formation.

Independent assortment

Mendel's law that different traits are inherited independently.

Incomplete dominance

The heterozygote shows an intermediate phenotype.

Codominance

Both alleles are fully expressed.

What the TEAS asks most

The same core ideas appear in different wording. If you can answer these without stopping to think, you have what this section requires.

Key points

• Segregation separates allele pairs into gametes; independent assortment separates different traits.
• A Bb × Bb cross gives 3:1 phenotypes and 1:2:1 genotypes.
• Draw the square rather than reasoning it out — it is faster and it does not slip.
• A recessive child of two unaffected parents means both parents are carriers.
• Ratios are probabilities for each offspring independently, not guaranteed outcomes.