Family trees (AQA GCSE Biology): Revision Notes
Family trees
What are family trees?
Family trees are diagrams that scientists use to track how traits pass from parents to children. They help us understand inheritance patterns and work out which alleles cause certain conditions.
Key point: Family trees show us how genetic traits move through generations of a family.
Reading family tree symbols
Understanding family tree symbols is essential for analysing inheritance patterns. Every family tree uses standardised symbols to make them universally readable across scientific studies.
Family Tree Symbol Guide:
- Squares = males
- Circles = females
- Filled-in shapes = person has the condition
- Empty shapes = person does not have the condition
Lines connect family members to show relationships between parents and children.
Cystic fibrosis example
Let's examine how cystic fibrosis passes through families. Cystic fibrosis is caused by a recessive allele, which means both parents must contribute a copy for the condition to appear.
Worked Example: Cystic Fibrosis Inheritance
In the example family tree:
- Arun and Beth are the parents (neither has cystic fibrosis)
- They have three children: Cal, Diane, and Ethan
- Only Ethan has cystic fibrosis
- Gill and Harry are also parents (neither has cystic fibrosis)
- They have two daughters: Laurie and Mia
- Only Mia has cystic fibrosis
Analysis:
- Cystic fibrosis needs two recessive alleles to show up
- Ethan and Mia must have inherited one recessive allele from each parent
- Their parents must be heterozygous (carry one recessive allele but don't show the condition)
PTC tasting example
PTC (phenylthiocarbamide) tasting ability demonstrates dominant inheritance. This means you only need one dominant allele to taste PTC, making it much easier to trace through family trees.
Worked Example: PTC Tasting Analysis
In the worked example:
- Individuals 1 and 2 can taste PTC
- Their children (6 and 8) cannot taste PTC
- This proves PTC tasting is controlled by a dominant allele
Key insight: If two people with a trait have children without that trait, the trait must be dominant.
Using probability with family trees
You can work out the chances of children having certain traits using Punnett squares. This mathematical approach helps predict inheritance patterns in future generations.
Worked Example: Calculating Cystic Fibrosis Probability
For cystic fibrosis (recessive condition):
- Both parents are heterozygous (Ff)
- Possible combinations: FF, Ff, Ff, ff
- Only ff children have cystic fibrosis
- Probability = or 25% or 1 in 4
Real families: The actual numbers might be different from predicted probability because families are usually small and probability only shows the expected ratio, not guaranteed outcomes.
Exam tips
Essential Exam Strategies:
When answering family tree questions:
- Use T for dominant alleles, t for recessive alleles
- Think about whether the condition needs one or two copies of an allele
- Work out parent genotypes first, then predict children
- Remember that carriers don't show recessive conditions but can pass them on
Remember!
Key Points to Remember:
- Squares = males, circles = females in family trees
- Filled shapes = affected by the condition
- Recessive conditions need two copies of the allele to show up
- Dominant conditions only need one copy of the allele
- Parents can be carriers without showing recessive conditions
- Punnett squares help calculate probability of inheritance