Genetic Inheritance

This lesson covers how characteristics are inherited from parents to offspring. You will learn about dominant and recessive alleles, homozygous and heterozygous genotypes, and how to use Punnett squares and genetic diagrams to predict the outcomes of genetic crosses. This is one of the most heavily examined parts of the AQA GCSE Biology specification.

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Key Genetic Terminology

Before studying inheritance, you must be confident with these key terms:

Term	Definition
Gene	A short section of DNA that codes for a specific protein (and therefore a characteristic)
Allele	A version of a gene; most genes have two or more alleles
Dominant allele	An allele that is always expressed when present (shown with a capital letter, e.g. B)
Recessive allele	An allele that is only expressed when two copies are present (shown with a lower-case letter, e.g. b)
Homozygous	Having two identical alleles for a gene (e.g. BB or bb)
Heterozygous	Having two different alleles for a gene (e.g. Bb)
Genotype	The combination of alleles an organism has for a particular gene (e.g. Bb)
Phenotype	The observable characteristic that results from the genotype (e.g. brown eyes)

Exam Tip: Learn these definitions precisely. AQA frequently tests them as 1-mark definition questions. A common mistake is confusing "gene" with "allele" — a gene is the section of DNA, while an allele is a specific version of that gene.

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Dominant and Recessive Alleles

For many characteristics, one allele is dominant and the other is recessive.

• If an organism has at least one dominant allele, the dominant phenotype is expressed
• The recessive phenotype is only expressed when the organism has two copies of the recessive allele (homozygous recessive)

Genotype	Description	Phenotype
BB	Homozygous dominant	Dominant trait shown
Bb	Heterozygous	Dominant trait shown (the recessive allele is "masked")
bb	Homozygous recessive	Recessive trait shown

Example: Fur colour in mice

• B = allele for brown fur (dominant)
• b = allele for white fur (recessive)

A mouse with genotype BB or Bb will have brown fur. Only a mouse with genotype bb will have white fur.

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Punnett Squares

A Punnett square is a simple diagram used to predict the possible genotypes and phenotypes of offspring from a genetic cross.

How to draw a Punnett square:

1. Write the genotypes of both parents
2. Separate the alleles of each parent
3. Place one parent's alleles across the top and the other's down the side
4. Fill in each box by combining the allele from the column and the row
5. Read off the possible genotypes and phenotypes

graph TD
    A[Identify parent genotypes] --> B[Separate alleles]
    B --> C[Place alleles on Punnett square]
    C --> D[Combine alleles in each box]
    D --> E[Read genotype ratios]
    E --> F[Determine phenotype ratios]

Example Cross: Two heterozygous parents (Bb x Bb)

Parent 1 genotype: Bb (brown fur, heterozygous) Parent 2 genotype: Bb (brown fur, heterozygous)

	B	b
B	BB	Bb
b	Bb	bb

Genotype ratio: 1 BB : 2 Bb : 1 bb Phenotype ratio: 3 brown : 1 white

This means there is a 75% (3 in 4) chance of brown fur and a 25% (1 in 4) chance of white fur for each offspring.

Exam Tip: Always show your working clearly in Punnett square questions. Write the parental genotypes, separate the alleles, draw the square, and state the ratio. Even if you get the final answer wrong, you can still pick up marks for method.

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Monohybrid Crosses

A monohybrid cross involves the inheritance of a single gene with two alleles. AQA expects you to be able to work through several types:

Cross 1: Homozygous dominant x Homozygous recessive (BB x bb)

	B	B
b	Bb	Bb
b	Bb	Bb

All offspring are Bb (heterozygous) — 100% show the dominant phenotype.

Cross 2: Heterozygous x Homozygous recessive (Bb x bb)

	B	b
b	Bb	bb
b	Bb	bb

Genotype ratio: 2 Bb : 2 bb (or 1:1) Phenotype ratio: 1 dominant : 1 recessive (50:50)

Cross 3: Heterozygous x Heterozygous (Bb x Bb)

As shown above: 3 dominant : 1 recessive

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Understanding Probability in Genetics

It is important to remember that the ratios from Punnett squares represent probabilities, not certainties.

• If two Bb parents have four children, we would predict 3 with brown fur and 1 with white fur
• But in reality, they could have 4 brown, or 4 white, or any combination — each child is an independent event
• The more offspring produced, the closer the actual results will be to the predicted ratio

This is similar to flipping a coin — each flip is independent, and you might get 4 heads in a row, even though the probability of heads is 50%.

Exam Tip: AQA sometimes asks why actual results differ from predicted ratios. The answer is that each fertilisation is a random, independent event, and ratios only become accurate with large sample sizes.

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Codominance [H]

In some cases, neither allele is completely dominant over the other. Instead, both alleles are expressed in the phenotype. This is called codominance.

A well-known example is blood type in cattle:

• Allele for red coat colour: C^R
• Allele for white coat colour: C^W
• Heterozygous (C^R C^W): roan coat (a mixture of red and white hairs)

In codominance, we use superscript letters rather than upper and lower case because neither allele is dominant.

Genotype	Phenotype
C^R C^R	Red
C^R C^W	Roan (codominant — both expressed)
C^W C^W	White

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Family Pedigree Diagrams