Reproduction (AQA GCSE Biology): Revision Notes
Inheritance
Inheritance is how certain characteristics pass from parents to their children through genes. Some medical conditions and even a person's biological sex are inherited in predictable ways.
Understanding inheritance patterns helps us predict the likelihood of certain traits appearing in offspring, which is particularly important in medical genetics and family planning.
What is polydactyly?
Polydactyly is a genetic condition where people are born with extra fingers or toes. The word comes from "poly" meaning many and "dactyl" meaning fingers.
This condition is caused by a dominant allele. This means:
- You only need one copy of the allele to have polydactyly
- If either parent has the condition, their children might inherit it
Key Point About Dominant Inheritance: With dominant alleles, the trait will appear even if you inherit just one copy from either parent. This makes dominant conditions more likely to appear in families compared to recessive conditions.
How polydactyly is passed on
When one parent has polydactyly:
- There's a 50% chance (1 in 2) each child will have extra digits
- Each child has an equal chance of inheriting the condition
When both parents have polydactyly:
- All children will have the condition
- This is because they'll definitely inherit at least one dominant allele
Worked Example: One Parent with Polydactyly
If one parent has polydactyly (Pp) and one parent is normal (pp):
- Parent 1 can give: P or p
- Parent 2 can give: p only
- Possible combinations: Pp (polydactyly) or pp (normal)
- Probability: 50% chance of polydactyly, 50% chance of normal
Important facts about polydactyly
- Usually doesn't cause serious health problems
- Extra fingers or toes can often be removed safely
- Other dominant genetic disorders can be more serious (like Huntington's disease)
What is cystic fibrosis?
Cystic fibrosis affects the cell membranes in the body, causing problems with breathing and digestion. It's caused by a recessive allele.
This means:
- You need two copies of the allele to have cystic fibrosis
- Both parents must pass on the faulty allele
Critical Difference from Dominant Conditions: Recessive conditions like cystic fibrosis are "hidden" unless you inherit two copies of the faulty allele - one from each parent. This is why two healthy parents can have a child with cystic fibrosis if they're both carriers.
How cystic fibrosis is passed on
When both parents are carriers:
- 25% chance (1 in 4) their child will have cystic fibrosis
- 50% chance (1 in 2) their child will be a carrier
- 25% chance (1 in 4) their child won't be affected at all
Worked Example: Two Carrier Parents
When both parents are carriers (Cc):
- Each parent can give: C (normal) or c (cystic fibrosis)
- Possible combinations: CC (normal), Cc (carrier), Cc (carrier), cc (cystic fibrosis)
- Results: 25% normal, 50% carriers, 25% with cystic fibrosis
Understanding carriers
A carrier is someone who:
- Has one copy of the faulty allele
- Doesn't have the condition themselves
- Can still pass the allele to their children
Carriers are particularly important in genetic counselling because they can pass on genetic conditions without showing any symptoms themselves. Many people don't know they're carriers until they have genetic testing.
How biological sex is inherited
Human body cells contain 23 pairs of chromosomes. One special pair determines biological sex.
Sex chromosomes
- Females have two X chromosomes (XX)
- Males have one X and one Y chromosome (XY)
Interesting Fact: The father's contribution determines the child's biological sex, since mothers can only contribute an X chromosome while fathers can contribute either X or Y.
How sex is determined
When parents have children:
- The mother always gives an X chromosome
- The father can give either X or Y
- If the father gives X → female child (XX)
- If the father gives Y → male child (XY)
Probability of having boys or girls
Each pregnancy has:
- 50% chance of having a girl
- 50% chance of having a boy
- Previous children don't affect the chances
Worked Example: Sex Determination
Mother (XX) × Father (XY):
- Mother can give: X only
- Father can give: X or Y
- Possible combinations: XX (female) or XY (male)
- Probability: 50% female, 50% male for each pregnancy
Using Punnett squares
Punnett squares are diagrams that help us predict inheritance patterns. They show:
- What alleles each parent can contribute
- All possible combinations in offspring
- The probability of different outcomes
Punnett squares are powerful tools because they work for any inherited characteristic - from simple traits like eye colour to complex medical conditions and even sex determination. They help geneticists and doctors provide accurate risk assessments to families.
These squares work for any inherited characteristic, from medical conditions to sex determination.
Key Points to Remember:
- Dominant conditions (like polydactyly) only need one copy of the allele to appear
- Recessive conditions (like cystic fibrosis) need two copies of the allele to appear
- Carriers have one copy of a recessive allele but don't show the condition
- Sex is determined by chromosomes - XX for females, XY for males
- Punnett squares help predict the chances of inheriting different characteristics