Genetic crosses (AQA GCSE Biology Combined Science): Revision Notes
Genetic crosses
Genetic crosses help us predict what traits offspring will inherit from their parents. Scientists use special diagrams to work out the likely outcomes when organisms reproduce.
What are genetic crosses?
A genetic cross shows what happens when two organisms with different traits reproduce together. We can predict the results using two main methods:
- Genetic diagrams - show the breeding process step by step
- Punnett square diagrams - show all possible combinations in a grid
These tools help us calculate the probability (chance) that offspring will have certain characteristics.
Understanding genetic crosses is fundamental to predicting inheritance patterns. These prediction tools are used by plant breeders, animal breeders, and geneticists to understand how traits pass from one generation to the next.
Understanding alleles and inheritance
Alleles are different versions of the same gene. Each parent has two alleles for each characteristic.
- Dominant alleles are shown with capital letters (like R)
- Recessive alleles are shown with lowercase letters (like r)
When an organism has two different alleles for a trait, it is called heterozygous. For example, a plant with alleles R and r is heterozygous for that characteristic.
The convention of using capital letters for dominant alleles and lowercase letters for recessive alleles was established by Gregor Mendel, the father of genetics. This system makes it easy to identify which traits will be expressed in offspring.
Genetic diagrams
Genetic diagrams show how alleles pass from parents to offspring. They display:
- The parent plants and their alleles
- The gametes (sex cells) each parent can produce
- All possible combinations in the offspring
Worked Example: Heterozygous Purple-Flowered Plants
When two heterozygous purple-flowered plants (Rr) reproduce:
- Each parent can make gametes with either R or r
- The offspring can be RR (purple), Rr (purple), or rr (white)
This shows us that even though both parents have purple flowers, some offspring can have white flowers due to the recessive alleles combining.
Punnett square diagrams
A Punnett square is a grid that shows all possible genetic combinations. Here's how to use one:
Step-by-step Punnett Square Method:
- Write one parent's gametes across the top
- Write the other parent's gametes down the side
- Fill in each box with the combination of alleles
- Count the different types of offspring
Worked Example: Crossing Two Rr Parents
| R | r |
|---|----|----|
| R | RR | Rr |
| r | Rr | rr |
This shows us the possible offspring: RR, Rr, Rr, and rr.
The Punnett square reveals that we get a 3:1 ratio of purple to white flowers, even though both parents are purple.
Working out probabilities
From the Punnett square above, we can calculate probabilities:
- 1 out of 4 combinations are RR (purple flowers)
- 2 out of 4 combinations are Rr (purple flowers)
- 1 out of 4 combinations are rr (white flowers)
So the probability of purple flowers = = 0.75 = 75% = 3:1 ratio
The probability of white flowers = = 0.25 = 25%
Remember that these probabilities represent the expected outcomes over many offspring. With small numbers of offspring, the actual results might vary significantly from the predicted ratios due to chance.
Worked examples
Worked Example 1: Seed Pod Colour
- G (green pods) is dominant to g (yellow pods)
- Cross: Gg × gg
- Only gg offspring will have yellow pods
- This happens when the recessive allele g is expressed (no dominant G present)
Punnett Square:
| G | g |
|---|----|----|
| g | Gg | gg |
| g | Gg | gg |
Result: 50% green pods (Gg), 50% yellow pods (gg)
Worked Example 2: Rabbit Fur Colour
- B (black fur) is dominant to b (brown fur)
- Parents with genotypes Bb × Bb
- Brown fur only appears in bb offspring (1 in 4 chance = 25%)
Punnett Square:
| B | b |
|---|----|----|
| B | BB | Bb |
| b | Bb | bb |
Result: 75% black fur (BB, Bb), 25% brown fur (bb)
Key tips for genetic crosses
Essential Rules for Genetic Cross Problems:
- Always use the correct symbols (capitals for dominant, lowercase for recessive)
- Check your Punnett square has all possible combinations
- Remember that actual results might differ from predictions, especially with small numbers of offspring
- Dominant alleles mask recessive ones - you need two recessive alleles to see the recessive trait
Key Points to Remember:
- Genetic crosses predict inheritance patterns using diagrams and probability
- Punnett squares show all possible genetic combinations in offspring
- Dominant alleles (capitals) mask recessive alleles (lowercase)
- Probability can be expressed as fractions, decimals, percentages or ratios
- Two recessive alleles are needed to show a recessive trait