Inheritance (AQA GCSE Biology Combined Science): Revision Notes
Inheritance
What is inheritance?
Inheritance is how characteristics pass from parents to their children. Some medical conditions and whether you're male or female are inherited through genes.
Understanding inheritance helps us predict which traits and conditions might be passed from one generation to the next, and it's the foundation for understanding genetic disorders and family patterns.
Polydactyly
Polydactyly means having extra fingers or toes. This happens because of a dominant allele.
Key facts about polydactyly:
- You only need one copy of the faulty gene to have polydactyly
- If one parent has polydactyly, there's a 50% chance their child will have it too
- Most of the time, polydactyly doesn't cause health problems
- Other dominant genetic conditions can be more serious
With dominant conditions like polydactyly, you only need to inherit the faulty gene from ONE parent to have the condition. This makes dominant genetic disorders more likely to appear in families.
Understanding the genetics:
When one parent has polydactyly (let's call the allele P) and one parent doesn't (let's call this p):
- The parent with polydactyly has alleles Pp
- The parent without has alleles pp
- Their children have a 1 in 2 chance of getting polydactyly
Worked Example: Polydactyly Inheritance
Parent 1 (with polydactyly): Pp Parent 2 (normal): pp
Possible offspring:
- Pp (has polydactyly) - 50% chance
- pp (normal) - 50% chance
This shows why there's a 1 in 2 chance of inheritance for dominant conditions.
Cystic fibrosis
Cystic fibrosis affects the cell membranes in your body. This happens because of a recessive allele.
Key facts about cystic fibrosis:
- You need two copies of the faulty gene to have cystic fibrosis
- People with only one copy are called carriers
- Carriers don't have the condition but can pass it to their children
- Both parents must be carriers for a child to have cystic fibrosis
Carriers are crucial to understanding recessive inheritance. They don't show symptoms but can pass the condition to their children if they have children with another carrier.
Understanding the genetics:
When both parents are carriers (let's call the faulty allele f and normal allele F):
- Both parents have alleles Ff
- There's a 1 in 4 chance their child will have cystic fibrosis (ff)
- There's a 1 in 2 chance their child will be a carrier (Ff)
- There's a 1 in 4 chance their child won't have the allele at all (FF)
Worked Example: Cystic Fibrosis Inheritance
Parent 1 (carrier): Ff Parent 2 (carrier): Ff
Possible offspring:
- FF (normal) - 25% chance
- Ff (carrier) - 50% chance
- ff (has cystic fibrosis) - 25% chance
This 1:2:1 ratio is typical for recessive inheritance between two carriers.
Sex chromosomes
Your sex is determined by special chromosomes called sex chromosomes.
How sex inheritance works:
- Females have two X chromosomes (XX)
- Males have one X and one Y chromosome (XY)
- All egg cells carry an X chromosome
- Sperm cells carry either an X or a Y chromosome
The father's sperm determines the sex of the baby, since mothers can only contribute an X chromosome while fathers contribute either X (for a girl) or Y (for a boy).
Probability of having boys or girls:
- There's a 50% chance of having a girl
- There's a 50% chance of having a boy
- This is the same for every pregnancy
Worked Example: Sex Determination
Mother: XX (can only give X) Father: XY (can give X or Y)
Possible offspring:
- XX (female) - 50% chance
- XY (male) - 50% chance
Each pregnancy has equal probability regardless of previous children.
Using Punnett squares
Punnett squares help us work out the chances of inheriting different characteristics. They're powerful tools that show:
- What genetic combinations are possible
- The probability of each outcome
- Whether conditions are likely to be passed on
Punnett squares are like genetic calculators - they help us predict inheritance patterns by showing all possible combinations of alleles from parents.
Key Points to Remember:
- Dominant alleles only need one copy to show the characteristic
- Recessive alleles need two copies to show the characteristic
- Carriers have one copy of a recessive allele but don't show the condition
- Sex inheritance gives a 50:50 chance of male or female offspring
- Punnett squares help predict the probability of inheriting characteristics