O-Level Biology: Genetic Crosses & Punnett Squares

Q: What is the difference between an allele and a gene?

A gene is a section of DNA that codes for a specific string of amino acids to make a protein (e.g., the gene for eye color). An allele is an alternative VERSION of that gene (e.g., the allele for blue eyes vs the allele for brown eyes).

Q: What do homozygous and heterozygous mean?

Homozygous means an organism has two identical alleles for a particular gene (e.g., BB or bb). Heterozygous means they have two different alleles for that gene (e.g., Bb).

Q: What is a phenotype vs genotype?

A genotype is the genetic makeup of an organism, represented by letters (like 'Bb'). The phenotype is the physical characteristic that is actually expressed and visible as a result of that genotype (like 'Brown eyes').

Q: How does a test cross work?

A test cross is used to find out if an organism showing a dominant trait is homozygous (BB) or heterozygous (Bb). You breed it with a homozygous recessive (bb) organism. If any offspring show the recessive trait, the unknown parent MUST be heterozygous.

A Punnett square diagram showing a genetic cross between a heterozygous and homozygous recessive parent.

How do you construct a Punnett square for a heterozygous genetic cross?

If both parents are heterozygous (e.g., Bb), split their alleles on the outside of a 2x2 grid. One parent's 'B' and 'b' on the top, the other's 'B' and 'b' on the left side. Then combine the letters into the four inner boxes. You will always get a 1:2:1 genotype ratio (1 BB, 2 Bb, 1 bb), which results in a 3:1 phenotype ratio (3 dominant traits, 1 recessive trait).

1. The 6 Essential Vocab Words
2. How to Draw and Read a Punnett Square
3. The Test Cross Trap
4. Frequently Asked Questions

Genetics questions are pure logic puzzles. Once you know the rules vocabulary, you can solve any monohybrid cross the examiner throws at you. This guide from our Ultimate O-Level Biology Guide focuses on the mechanical layout of the Punnett square and the all-important Test Cross.

1. The 6 Essential Vocab Words

Before drawing boxes, you must speak the language. If a question tells you "a homozygous dominant male breeds with a heterozygous female", you need to know exactly what letters to write down.

Gene vs Allele: A gene is a piece of DNA for a trait (Eye Colour). An allele is a version of that gene (Blue or Brown). You always inherit TWO alleles for every gene (one from mom, one from dad).
Dominant (B): An allele that is ALWAYS expressed in the physical appearance if it is present. It masks the recessive allele.
Recessive (b): An allele that is only expressed physically if there is NO dominant allele present (i.e., you need two copies of it).
Genotype: The genetic letter code (e.g., BB, Bb, or bb).
Phenotype: The physical result of the code (e.g., Brown Fur or White Fur).
Homozygous vs Heterozygous: Homozygous means both alleles are the same (BB or bb). Heterozygous means they are different (Bb).

2. How to Draw and Read a Punnett Square

Let's cross two Heterozygous parents for fur color. 'B' = Brown (Dominant), 'b' = White (Recessive). Both parents are Bb.

Parent 1 Gametes: B and b
Parent 2 Gametes: B and b

Draw a 2x2 grid. Put Parent 1's letters on top of the columns. Put Parent 2's letters next to the rows. Then drag the letters into the intersecting boxes.

	B	b
B	BB	Bb
b	Bb	bb

Extracting the Ratios

Genotype Ratio: 1 BB : 2 Bb : 1 bb (or 25% : 50% : 25%).
Phenotype Ratio: 3 Brown : 1 White (75% probability of a brown offspring, 25% probability of white).

3. The Test Cross Trap

You have a guinea pig with Brown fur. Because Brown is dominant, you don't know its genotype. It could be homozygous dominant (BB) or heterozygous (Bb). How do you find out?

The Solution: A Test Cross

You MUST breed the unknown brown guinea pig with a Homozygous Recessive (bb) white guinea pig.

Scenario A: If the unknown parent was BB, then (BB x bb) means 100% of the offspring will be Bb (Brown).
Scenario B: If the unknown parent was Bb, then (Bb x bb) means 50% of the offspring will be Bb, and 50% will be bb (White).

📋 From the Desk of Sarah Mitchell

If a question asks "How can the breeder determine the dog's genotype?", you write "Perform a test cross by breeding it with a homozygous recessive individual. If any offspring display the recessive phenotype, the original dog must be heterozygous." Easy 3 marks.

Frequently Asked Questions

What is the difference between an allele and a gene?▼

A gene is the broad category of DNA for a trait (e.g. hair color). An allele is the specific version of that trait (e.g. blonde or black).

What do homozygous and heterozygous mean?▼

Homozygous = two identical alleles (HH or hh). Heterozygous = two different alleles (Hh).

What is a phenotype vs genotype?▼

Genotype is the letter code (Bb). Phenotype is what it looks like physically (Brown eyes).

How does a test cross work?▼

Breed an unknown dominant organism with a homozygous recessive one. The offspring's appearances will reveal the unknown parent's genotype.

Stop Guessing, Start Scoring

Get instant access to 500+ CAIE-aligned practice questions, worked solutions, and AI-powered mock exams across all O-Level subjects.

Access the Study Portal →Prepare with the Oracle Engine

Genetic Crosses: Defeating the Punnett Square