A practical lesson on showing the outcomes of genetic crosses. It defines the key terms, explains dominant and recessive alleles, then works through several flower colour gene diagrams before introducing the Punnett square, with a question on black and white rats to finish.
What this presentation covers
- genotype, phenotype, gamete, allele, dominant and recessive
- how genes control characteristics
- worked gene diagrams for RR x rr, Rr x Rr and Rr x rr
- Punnett squares
- a practice question on coat colour in rats
Level: GCSE
Slides: 15
Format: PowerPoint (.pptx)
Teaching notes
How to use it. A 15 slide GCSE lesson (Inheritance, variation and evolution in AQA) that builds up from key words to full gene diagrams. Pupils often find it easier to draw a Punnett square after they have seen the gene diagram version, so keep the order. Ask them to give the answer as a ratio, a fraction and a percentage, because exam questions use all three.
Key words
- Allele: a different version of the same gene
- Dominant: an allele that is expressed even if only one copy is present
- Recessive: an allele that is only expressed when two copies are present
- Genotype: the alleles an organism has, such as Rr
- Phenotype: the characteristic that results, such as red flowers
Common misconception: “Dominant alleles are always the most common in a population.” Dominant only means the allele shows when one copy is present. Some dominant alleles are very rare, such as the one that causes Huntington’s disease.
Quick check questions
- Two Rr plants are crossed. What fraction of the offspring will be rr?
- What ratio of red to white flowers would you expect from Rr × Rr?
- A cross of Rr × rr gives what ratio of phenotypes?
Show answers
- One quarter (25%).
- 3 red : 1 white.
- 1 : 1, so half show the dominant feature and half the recessive one.
Related lessons: Gregor Mendel | Meiosis | DNA Structure
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