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An allele is a different version of the same gene that controls the same characteristic but may produce different traits. Free notes and 12 questions answered.
Genetic vocabulary and basic inheritance (Part 1/2) is part of Reproduction in GCSE AQA Higher Biology.
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An allele is a different version of the same gene that controls the same characteristic but may produce different traits. Free notes and 12 questions answered.
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Use this lesson to stay inside Reproduction while connecting the detail back to the wider Inheritance, variation and evolution topic.
Read the core explanations from Genetic vocabulary and basic inheritance (Part 1/2) before testing yourself from memory.
Why do you have your mum's eyes but your dad's nose? The answer lies in tiny structures inside every cell that carry the instructions for making you. These instructions are passed from parents to children through reproduction, but the language of genetics has specific terms we need to master first.
Trap: Many students think chromosomes and genes are the same thing. Actually, chromosomes are like books, and genes are individual chapters within those books. Humans have 46 chromosomes (23 pairs), but thousands of genes.
Now we zoom in further. Each chromosome contains hundreds of genes, and this is where inheritance gets interesting. Think of genes as recipes for making proteins, but like any recipe, there can be different versions.
Here's the key insight: you inherit two copies of each gene - one from each parent. These copies might be identical, or they might be different versions (alleles). For example, the gene for eye colour has different alleles - one for brown eyes, one for blue eyes.
When you have two different alleles for the same gene, which one wins? This is where dominance comes in. Some alleles are bossy - they always get their way. Others are quiet - they only show up when the bossy one isn't around.
We use letters to represent alleles. Capital letters (like B) represent dominant alleles, lowercase letters (like b) represent recessive alleles. If you have Bb, the dominant B allele masks the recessive b allele.
Trap: Students often think dominant means 'more common' or 'better'. Actually, dominant just means it shows up even with one copy. Some dominant alleles cause genetic disorders - being dominant doesn't make them good or common.
Remember, you get two alleles for each gene - one from mum, one from dad. These two alleles might be identical twins or completely different. We have special terms for these situations.
Think of it like this: 'homo' means same, 'hetero' means different. Homozygous = same alleles, heterozygous = different alleles. This distinction is crucial because it affects what characteristics are expressed.
Here's the final piece of the puzzle. Your genotype is like a recipe written in genetic code. Your phenotype is the actual cake that gets baked. The same recipe (genotype) usually produces the same cake (phenotype), but sometimes the environment can affect the final result.
Worked example: Working Out Phenotype from Genotype
A person has genotype Bb for eye colour, where B = brown eyes (dominant) and b = blue eyes (recessive). What is their phenotype?
Answer: Brown eyes (phenotype) from genotype Bb
Trap: Don't confuse genotype and phenotype in exam answers. Genotype uses letters (Bb), phenotype describes what you can see (brown eyes). Examiners will mark you wrong if you mix these up.
Genetics questions are mark-heavy in GCSE exams, but they follow predictable patterns. Master the vocabulary and you'll unlock the marks. Here's how examiners think and what they're looking for.
Exam depth extension: In genetic vocabulary and basic inheritance (part 1/2), connect the building blocks of inheritance to genes and their variations 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: mixing up genotype and phenotype, forgetting that dominant doesn't mean common, and using the wrong case letters for alleles. Practice using the vocabulary precisely - it's the difference between full marks and losing easy points.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
An allele is a different version of the same gene that controls the same characteristic but may produce different traits. Two copies of each gene are inherited, one from each parent; for example, the gene for eye colour has an allele for brown eyes and one for blue eyes.
A gamete is a reproductive cell, a sperm or egg, that contains half the normal number of chromosomes. Gametes pass genetic instructions from parents to children, so each child inherits one copy of each gene from each parent.
Genotype is the combination of alleles an organism has for a particular gene, written using letters such as Bb. Phenotype is the observable characteristic that results from the genotype, such as brown eyes. A person with genotype Bb, where B is brown and dominant, has brown eyes.
A genotype is the combination of alleles an organism has for a particular gene, written using letters, for example Bb. The phenotype is the observable characteristic that results, such as brown eyes.
Homozygous means having two identical alleles for a particular gene, such as BB or bb. 'Homo' means same, while heterozygous means having two different alleles, such as Bb.
Heterozygous means having two different alleles for a particular gene, such as Bb. 'Hetero' means different, and in a heterozygous organism the dominant allele is expressed in the phenotype.
Homozygous means having two identical alleles for a particular gene, such as BB or bb, while heterozygous means having two different alleles, such as Bb. 'Homo' means same and 'hetero' means different.
A dominant allele is expressed in the phenotype even when only one copy is present and is written as a capital letter, such as B. A recessive allele is only expressed when two copies are present and is written as a lowercase letter, such as b; in Bb, the dominant B masks the recessive b.
Dominant alleles are written with a capital letter, such as B, and recessive alleles with a lowercase letter, such as b. In Bb, the dominant B allele masks the recessive b allele.
Dominant does not mean 'more common' or 'better'; it just means the allele shows up in the phenotype even when only one copy is present. Some dominant alleles cause genetic disorders.
If B is the dominant allele for brown eyes and b the recessive allele for blue eyes, genotype Bb gives the phenotype brown eyes. The dominant B allele is expressed and masks the recessive b allele.
Chromosomes and genes are not the same: chromosomes are like books, and genes are individual chapters within them. Humans have 46 chromosomes (23 pairs) but thousands of genes.