Genetic vocabulary and basic inheritance (Part 1/2) Revision Notes | GCSE AQA Higher Biology

1. The Building Blocks of Inheritance

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.

2. Genes and Their Variations

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.

3. Dominant vs Recessive - The Power Struggle

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.

4. Homozygous vs Heterozygous - Same or Different?

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.

5. Genotype vs Phenotype - The Instructions vs The Result

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?

1. B (dominant) and b (recessive) — One of each type of allele
2. Dominant allele B will be expressed — Dominant alleles mask recessive ones
3. Brown eyes — The observable characteristic

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.

6. Exam Technique - Getting Full Marks

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.