Homozygous recessive means having two copies of the recessive allele, such as bb, while homozygous dominant is BB. Free notes and 10 questions answered.
Updated 20 Mar 2026 · Revise Right Team
Genetic vocabulary and basic inheritance (Part 2/2) is part of Reproduction in GCSE AQA Higher Biology.
3 min
Reading time
27
Lessons in this topic
GCSE AQA Higher
Pathway
Reviewed by Revise Right
Content review
What this lesson covers
Homozygous recessive means having two copies of the recessive allele, such as bb, while homozygous dominant is BB. Free notes and 10 questions answered.
Define key genetic terms including allele, genotype, phenotype, homozygous and heterozygous
Explain the difference between dominant and recessive alleles using specific examples
Apply genetic vocabulary to describe inheritance patterns in single-gene traits
Analyse genetic crosses using correct terminology and predict offspring characteristics
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Use this lesson to stay inside Reproduction while connecting the detail back to the wider Inheritance, variation and evolution topic.
Inheritance, variation and evolution
Reproduction
GCSE AQA Higher Biology
Revision notes
Read the core explanations from Genetic vocabulary and basic inheritance (Part 2/2) before testing yourself from memory.
1. The Language of Genetics
You've learned that genes control characteristics. But here's where it gets interesting — most genes come in different versions. Think of it like having different flavours of the same ice cream. The gene is 'ice cream', but the flavours (vanilla, chocolate, strawberry) are what we call alleles.
Every characteristic you can see — from fur colour in mice to whether you're colour-blind — is controlled by these different gene versions working at the molecular level.
Trap: Many students confuse genotype and phenotype. Remember: genotype is the genetic code (what's written in DNA), phenotype is what you observe (what you see with your eyes).
2. Dominant vs Recessive Alleles
Not all alleles are created equal. Some are bossy and always get their way, whilst others are quiet and only show up when the bossy one isn't around. This is the key to understanding inheritance patterns.
Let's see this in action with fur colour in mice. The allele for brown fur (B) is dominant over the allele for white fur (b). This means:
• BB = brown fur (two dominant alleles)• Bb = brown fur (one dominant allele is enough)• bb = white fur (only when no dominant alleles present)
3. Homozygous vs Heterozygous
We need two more terms to complete our genetic vocabulary. These describe whether an organism has matching or different alleles for a trait.
Here's a memory trick: Homozygous = same alleles. Heterozygous = different alleles. Think of 'heterogeneous' meaning mixed or varied.
4. Real Examples in Action
Let's apply this vocabulary to two real examples that examiners love: fur colour in mice and red-green colour blindness in humans.
Worked example: Analysing Mouse Fur Colour Inheritance
Two mice are crossed. One is homozygous dominant for brown fur (BB), the other is homozygous recessive for white fur (bb). Describe the genotype and phenotype of their offspring.
Parent 1: BB (homozygous dominant)
Parent 2: bb (homozygous recessive) — Both parents are homozygous - they have matching alleles
Parent 1 can only produce B gametes
Parent 2 can only produce b gametes — Each parent can only pass on one type of allele
All offspring will be Bb — Every offspring gets B from parent 1 and b from parent 2
All offspring will have brown fur — B is dominant, so Bb genotype produces brown phenotype
Answer: Genotype: 100% Bb (heterozygous). Phenotype: 100% brown fur.
Red-green colour blindness in humans follows the same pattern, but it's more complex because the gene is on the X chromosome. The allele for normal colour vision (N) is dominant over the allele for colour blindness (n).
5. Exam Technique and Common Mistakes
Genetics questions are mark-heavy in GCSE exams, but students often lose marks on basic vocabulary. Here's how to nail the terminology every time.
Exam depth extension: In genetic vocabulary and basic inheritance (part 2/2), connect the language of genetics to dominant vs recessive alleles using clear cause-and-effect statements. When you explain biological processes, include what changes, why it changes, and the consequence for cells, organisms, or populations. If calculations appear, show each step, include units, and end with a sentence that interprets the result in biological context.
Common exam mistakes to avoid:
• Writing 'BB' when asked for phenotype (that's genotype - phenotype would be 'brown fur')• Saying an organism is 'dominant' instead of 'homozygous dominant'• Forgetting that heterozygous individuals show the dominant trait• Mixing up the definitions of genotype and phenotype
Master this vocabulary and you'll have the foundation for understanding all inheritance patterns. These terms appear in every genetics question, so getting them right is essential for exam success.
Sources and content standards
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Red-green colour blindness is controlled by a gene on the X chromosome, which makes it more complex than fur colour in mice. The allele for normal colour vision (N) is dominant over the allele for colour blindness (n).
What does homozygous recessive mean?
Homozygous recessive means having two copies of the recessive allele, such as bb. It is the only genotype that shows the recessive phenotype, for example white fur in mice, because there is no dominant allele present.
What does homozygous dominant mean?
Homozygous dominant means an organism has two identical dominant alleles for a trait, such as BB. Homozygous recessive means two identical recessive alleles, such as bb. An organism should be described as 'homozygous dominant', not just 'dominant'.
Why is colour blindness more complex to inherit?
Red-green colour blindness is more complex to inherit because the gene is on the X chromosome. The allele for normal colour vision (N) is dominant over the allele for colour blindness (n).
What offspring do you get from crossing BB and bb?
Crossing a homozygous dominant BB parent with a homozygous recessive bb parent gives 100% Bb (heterozygous) offspring, because one parent can only produce B gametes and the other only b gametes. With brown fur (B) dominant in mice, all the offspring have brown fur.
Which genotypes give brown fur in mice?
In mice, the allele for brown fur (B) is dominant over the allele for white fur (b), so BB and Bb both give brown fur, while only bb gives white fur. Heterozygous individuals always show the dominant trait.
Do heterozygous organisms show the dominant trait?
Heterozygous organisms show the dominant trait. A mouse with genotype Bb has brown fur, because one dominant B allele is enough; forgetting this is a common exam mistake.
What gametes can a BB parent make?
A BB parent can only produce B gametes, and a bb parent can only produce b gametes, because each parent can only pass on one type of allele when it is homozygous. Crossing them gives offspring that are all Bb.
Is it correct to say an organism is dominant?
An organism should not just be called 'dominant'; the correct terms are homozygous dominant (BB), heterozygous (Bb) or homozygous recessive (bb). Only alleles are dominant or recessive.
What is the difference between genotype and phenotype in an answer?
In an answer, genotype is written with letters, such as BB, while phenotype describes the observable characteristic, such as brown fur. Writing 'BB' when asked for a phenotype is a common mistake.