Family trees (AQA GCSE Biology Combined Science): Revision Notes
Family trees
Family trees are diagrams that help us understand how traits pass from parents to children across generations. Scientists use them to study inheritance patterns and work out how genetic conditions are passed down through families.
What are family trees?
Family trees show the inheritance of alleles through different generations of a family. They use simple symbols to represent family members and whether they have certain traits or conditions.
Key symbols for reading family trees:
- Squares = males
- Circles = females
- Filled/shaded shapes = people who have the condition
- Empty shapes = people who don't have the condition
How to analyse a family tree
When looking at a family tree, you need to work out the inheritance pattern. This means figuring out whether a trait is caused by a dominant or recessive allele.
Worked Example: Cystic Fibrosis Inheritance
Cystic fibrosis (CF) is a genetic condition caused by a recessive allele. This means:
- You need two copies of the CF allele to have the condition
- If you only have one copy, you're a carrier but don't have symptoms
Looking at the family tree example:
- Ethan has cystic fibrosis, so he must have two recessive alleles
- His parents (Arun and Beth) don't have CF but had a child with it
- This means both parents must be heterozygous (carriers)
- They each have one normal allele and one CF allele
Understanding inheritance patterns
Recessive conditions
Recessive genetic conditions have distinctive patterns that help us identify them in family trees:
- Both parents must carry the allele for a child to be affected
- Parents are usually carriers (heterozygous) but don't show symptoms
- The condition can "skip" generations
Common Mistake to Avoid: Don't assume that if parents don't have a condition, their children can't have it. Recessive conditions can appear in children even when both parents appear unaffected because they may be carriers.
Dominant conditions
Dominant conditions show different patterns from recessive ones:
- Only one parent needs to have the allele
- The condition appears in every generation
- Affected people usually have at least one affected parent
Using probability calculations
You can work out the chance of children having certain traits using a Punnett square. This shows all possible combinations of alleles from both parents.
Worked Example: Probability Calculation
If both parents are carriers for cystic fibrosis (Ff × Ff):
- FF = normal ( chance)
- Ff = carrier ( chance)
- ff = affected ( chance)
So there's a 1 in 4 (25%) chance their child will have cystic fibrosis.
Understanding Punnett Squares: Punnett squares are grids that help visualise all possible genetic combinations from two parents. Each box in the grid represents an equally likely outcome for their offspring.
Worked example - PTC tasting
Worked Example: PTC Tasting Inheritance
PTC (phenylthiocarbamide) tasting is controlled by a dominant allele:
- People with the dominant allele can taste PTC (bitter)
- People with two recessive alleles cannot taste it
In the example family tree:
- Individuals 1 and 2 can taste PTC
- They have children who can't taste PTC (individuals 6 and 8)
- This proves PTC tasting must be controlled by a dominant allele
Key insight: The fact that two tasters can have non-taster children tells us that tasting must be the dominant trait, and both parents must be heterozygous.
Key Points to Remember:
- Family trees show how traits pass through generations using symbols
- Squares represent males, circles represent females
- Filled shapes show people with the condition
- Recessive conditions need two copies of the allele and can skip generations
- Dominant conditions need only one copy and appear in most generations
- Use Punnett squares to calculate probability of inheritance