Inherited Disorders and Genetic Diagrams

This lesson covers how genetic disorders are inherited, how to use genetic diagrams (including Punnett squares) to predict the probability of offspring inheriting particular alleles, and the key inherited disorders required by the AQA GCSE Combined Science Trilogy specification (8464).

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Key Genetic Terminology

Before working with genetic diagrams, you need to understand these terms:

Term	Definition
Gene	A section of DNA that codes for a particular protein
Allele	A version of a gene (e.g. the gene for eye colour has different alleles for brown, blue, etc.)
Dominant allele	An allele that is always expressed when present (shown with a capital letter, e.g. A)
Recessive allele	An allele that is only expressed when two copies are present (shown with a lower-case letter, e.g. a)
Homozygous	Having two identical alleles for a gene (e.g. AA or aa)
Heterozygous	Having two different alleles for a gene (e.g. Aa)
Genotype	The combination of alleles an organism has (e.g. Aa)
Phenotype	The physical characteristic expressed (e.g. brown eyes)
Carrier	A person who is heterozygous for a recessive disorder — they carry the allele but do not show symptoms

Exam Tip: Learn these definitions precisely. AQA often awards 1 mark for a correct definition. "Homozygous" does NOT mean "dominant" — it means two of the SAME allele.

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Punnett Squares

A Punnett square is a grid used to work out the possible genotypes and phenotypes of offspring from a genetic cross. It shows all possible combinations of alleles from two parents.

How to Draw a Punnett Square

Example: Two heterozygous parents (Aa x Aa)

	A	a
A	AA	Aa
a	Aa	aa

Results:

• Genotype ratio: 1 AA : 2 Aa : 1 aa
• Phenotype ratio: 3 dominant : 1 recessive

$\text{Probability of dominant phenotype} = \frac{3}{4} = 75\%$Probability of dominant phenotype=43​=75%

$\text{Probability of recessive phenotype} = \frac{1}{4} = 25\%$Probability of recessive phenotype=41​=25%

graph TD
    A["Parents: Aa × Aa"] --> B["Parent 1 gametes: A or a"]
    A --> C["Parent 2 gametes: A or a"]
    B --> D["Punnett Square"]
    C --> D
    D --> E["AA (25%)"]
    D --> F["Aa (25%)"]
    D --> G["Aa (25%)"]
    D --> H["aa (25%)"]
    E --> I["3 Dominant phenotype<br/>(75%)"]
    F --> I
    G --> I
    H --> J["1 Recessive phenotype<br/>(25%)"]
    style A fill:#bbdefb,stroke:#1565c0
    style D fill:#c8e6c9,stroke:#2e7d32
    style I fill:#fff9c4,stroke:#f9a825
    style J fill:#ffccbc,stroke:#d84315

Exam Tip: Always label your Punnett square clearly. Put one parent's alleles along the top and the other parent's alleles down the side. AQA awards marks for using the correct notation and showing the working, not just the final answer.

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Inherited Disorders: Polydactyly

Polydactyly is a condition where a person is born with extra fingers or toes. It is caused by a dominant allele, so only one copy is needed to express the condition.

Genotype	Phenotype
PP (homozygous dominant)	Polydactyly
Pp (heterozygous)	Polydactyly
pp (homozygous recessive)	Normal number of fingers/toes

Worked Example: One Parent With Polydactyly (Pp) x Normal Parent (pp)

	P	p
p	Pp	pp
p	Pp	pp

Results:

• 2 Pp (polydactyly) : 2 pp (normal)
• Probability of polydactyly = $\frac{2}{4} = \frac{1}{2} = 50\%$42​=21​=50%

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Inherited Disorders: Cystic Fibrosis

Cystic fibrosis (CF) is a genetic disorder caused by a recessive allele (f). A person must inherit two copies of the recessive allele (ff) to have the condition.

• FF — does not have CF, is not a carrier
• Ff — does not have CF, but is a carrier
• ff — has cystic fibrosis

Cystic fibrosis causes the body to produce thick, sticky mucus that affects the lungs, digestive system and other organs.

Worked Example: Two Carrier Parents (Ff x Ff)

	F	f
F	FF	Ff
f	Ff	ff

Results:

• 1 FF : 2 Ff : 1 ff
• Probability of having CF = $\frac{1}{4} = 25\%$41​=25%
• Probability of being a carrier = $\frac{2}{4} = 50\%$42​=50%
• Probability of being unaffected and not a carrier = $\frac{1}{4} = 25\%$41​=25%

Exam Tip: AQA frequently tests your ability to construct Punnett squares for cystic fibrosis. Always state the genotypes AND phenotypes, and give probabilities as fractions, percentages or ratios.

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Family Trees (Pedigree Diagrams)

Family tree diagrams (pedigree diagrams) show how a genetic condition is inherited through a family. You may be asked to interpret these in exams.

Key conventions:

• Squares represent males, circles represent females
• Filled (shaded) symbols mean the person has the condition
• Half-filled symbols sometimes indicate carriers (heterozygous)
• A horizontal line connecting two symbols represents a couple
• Vertical lines descending from a couple show their children

How to Work Out the Inheritance Pattern

1. If affected children can have unaffected parents, the allele is likely recessive
2. If every affected person has at least one affected parent, the allele is likely dominant

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Sex Determination

In humans, sex is determined by the 23rd pair of chromosomes — the sex chromosomes:

• XX = female
• XY = male

The mother always passes on an X chromosome. The father can pass on either an X or a Y:

	X	X
X	XX	XX
Y	XY	XY

$\text{Probability of male} = \frac{2}{4} = 50\% \quad \text{Probability of female} = \frac{2}{4} = 50\%$Probability of male=42​=50%Probability of female=42​=50%

Exam Tip: AQA (8464) may ask you to draw a Punnett square to show how sex is inherited. Remember: the gametes of the mother are X and X; the gametes of the father are X and Y.

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Screening and Ethical Issues

Genetic screening can identify whether a person carries alleles for genetic disorders. This raises ethical questions: