Genetic vocabulary and basic inheritance (Part 2/2) Revision Notes | GCSE OCR Foundation Biology

What this lesson covers

Learn key genetic terms including gamete, chromosome, gene, allele, dominant, recessive, homozygous, heterozygous, genotype and phenotype. Understand ho...

• 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

Key ideas to keep in view

Use this lesson to stay inside B5.1 Inheritance while connecting the detail back to the wider B5: Genes, inheritance and selection topic.

• B5: Genes, inheritance and selection
• B5.1 Inheritance
• GCSE OCR Foundation Biology

Lesson notes preview

Read the core explanations from Genetic vocabulary and basic inheritance (Part 2/2) before testing yourself from memory.

• 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.
• Different forms of the same gene that control variations of a characteristic. For example, the gene for eye colour has alleles for brown eyes, blue eyes, green eyes.
• 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.
• The combination of alleles present in an organism for a particular trait. This is the genetic 'recipe' written in DNA.
• The observable characteristics expressed by an organism based on its genotype. This is what you actually see — brown fur, blue eyes, or colour blindness.
• 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).
• 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.
• An allele that is always expressed in the phenotype, even if only one copy is present. Represented by a capital letter (e.g., B for brown fur).
• An allele that is only expressed if two copies are present (no dominant allele present). Represented by a lowercase letter (e.g., b for white fur).
• 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:

Continue through the topic

Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.