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

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.

1. Parent 1: BB (homozygous dominant) Parent 2: bb (homozygous recessive) — Both parents are homozygous - they have matching alleles
2. Parent 1 can only produce B gametes Parent 2 can only produce b gametes — Each parent can only pass on one type of allele
3. All offspring will be Bb — Every offspring gets B from parent 1 and b from parent 2
4. 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.