Genetic crosses (AQA GCSE Biology): Revision Notes
Genetic crosses
What are genetic crosses?
Genetic crosses help us predict what traits offspring will have. Scientists use two main tools to work this out: genetic diagrams and Punnett square diagrams.
These tools show us the chances of different characteristics appearing in the next generation by analysing how genes pass from parents to offspring.
Genetic diagrams
A genetic diagram shows how genes pass from parents to their offspring. These visual representations are essential for understanding inheritance patterns and making predictions about offspring characteristics.
Key terms:
- Alleles - different versions of the same gene
- Heterozygous - having two different alleles for a trait
- Dominant allele - shows when present (capital letters)
- Recessive allele - only shows with two copies (lowercase letters)
Worked Example: Flower Colour Inheritance
When both parent plants are heterozygous for flower colour, we can predict their offspring.
The setup:
- R = purple flowers (dominant allele)
- r = white flowers (recessive allele)
- Cross: Rr × Rr
The results:
- RR = purple flowers
- Rr = purple flowers (dominant allele shows)
- rr = white flowers
Punnett square diagrams
Punnett squares are grid-like diagrams that make genetic crosses easier to understand. They show all possible combinations of gametes (sex cells) from both parents in a systematic, organised way.
How to use a Punnett square:
- Write the possible gametes from one parent across the top
- Write the possible gametes from the other parent down the side
- Fill in each box by combining the alleles
- Count the different types to work out probabilities
Calculating probability
You can work out the chances of getting certain traits by counting the combinations in your Punnett square. This mathematical approach allows you to make precise predictions about inheritance patterns.
Different ways to show probability:
Using the flower colour example where 3 out of 4 offspring have purple flowers:
- Fraction: 3/4
- Decimal: 0.75
- Percentage: 75%
- Ratio: 3:1 (purple to white)
The actual results might be different from predictions, especially when there are only a few offspring. Genetic probability shows expected outcomes, but chance variation can affect real results.
Worked examples
Worked Example 1: Seed Pod Colour
The situation: Green seed pods (G) are dominant over yellow seed pods (g).
The cross: Gg × gg
The working:
- Parent 1 gametes: G, g
- Parent 2 gametes: g, g
- Offspring combinations: Gg, gg, Gg, gg
The result: Half the offspring (50%) will have yellow seed pods (gg). This happens because the recessive allele can only show when there are two copies present.
Worked Example 2: Rabbit Fur Colour
The situation: Black fur (B) is dominant over brown fur (b).
The cross: Bb × Bb (two heterozygous rabbits)
The working:
- Both parents' gametes: B, b
- Punnett square results: BB, Bb, Bb, bb
The results:
- 1/4 (25%) with black fur, homozygous dominant (BB)
- 1/2 (50%) with black fur, heterozygous (Bb)
- 1/4 (25%) with brown fur, homozygous recessive (bb)
- Phenotype ratio: 3:1 (black to brown fur)
Tips for exam success
Understanding genetic crosses requires practice and attention to detail. Here are the most important strategies to help you succeed:
Key Strategies for Success:
- Always show your working in Punnett squares
- Remember that dominant alleles use capital letters
- Check your probability calculations add up to 1 (or 100%)
- Practice drawing genetic diagrams step by step
- Label all parts of your diagrams clearly
- Double-check your gamete combinations
Remember!
Genetic crosses form a fundamental part of understanding inheritance. The key concepts work together to help you predict and explain how traits pass from parents to offspring.
Essential Concepts to Remember:
- Genetic crosses predict inheritance patterns using diagrams and mathematical calculations
- Punnett squares show all possible offspring combinations in a clear grid format
- Probability can be expressed in many ways - fractions, decimals, percentages, or ratios
- Dominant alleles mask recessive ones - you need two recessive alleles for the trait to show
- Practice makes perfect - work through lots of examples to build confidence