Inheritance and Punnett Squares

This lesson covers genetic inheritance and how to use Punnett squares to predict the outcomes of monohybrid crosses, as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand key genetic terms, construct Punnett squares and interpret the results.

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Key Genetic Terminology

Before studying inheritance, you must know these key terms:

Term	Definition
Gene	A section of DNA that codes for a specific protein
Allele	A version of a gene (e.g. the gene for eye colour has alleles for blue, brown, etc.)
Dominant	An allele that is always expressed when present (shown with a capital letter, e.g. B)
Recessive	An allele that is only expressed when two copies are present (shown with a lowercase 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 (e.g. Bb)
Phenotype	The observable characteristic (e.g. brown eyes)

Exam Tip: Genotype = the alleles you have (letters). Phenotype = what you look like (the physical trait). The dominant allele is always written first (Bb, not bB).

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How Inheritance Works

Inheritance follows these principles:

1. Each organism has two alleles for every gene — one inherited from each parent.
2. During meiosis, the alleles separate so each gamete carries only one allele.
3. At fertilisation, the gametes fuse, and the offspring receives one allele from each parent.
4. If the offspring has at least one dominant allele, the dominant phenotype is expressed.
5. The recessive phenotype is only expressed when the organism is homozygous recessive (two recessive alleles).

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Punnett Squares

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

Example 1: Two Heterozygous Parents (Bb x Bb)

Let B = brown fur (dominant) and b = white fur (recessive).

Both parents are heterozygous (Bb):

	B	b
B	BB	Bb
b	Bb	bb

Results:

Genotype	Ratio	Phenotype
BB	1	Brown fur
Bb	2	Brown fur
bb	1	White fur

• Genotypic ratio: 1 BB : 2 Bb : 1 bb
• Phenotypic ratio: 3 brown : 1 white
• Probability of white fur offspring: 1 in 4 = 25%

Exam Tip: The 3:1 ratio is the classic result of crossing two heterozygous parents. If you see this ratio in a question, it confirms both parents are heterozygous.

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Example 2: Heterozygous x Homozygous Recessive (Bb x bb)

This is called a test cross — it is used to determine whether an organism showing the dominant phenotype is BB or Bb.

	B	b
b	Bb	bb
b	Bb	bb

Results:

Genotype	Ratio	Phenotype
Bb	2	Brown fur
bb	2	White fur

• Phenotypic ratio: 1 brown : 1 white (or 50:50)

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Example 3: Homozygous Dominant x Homozygous Recessive (BB x bb)

	B	B
b	Bb	Bb
b	Bb	Bb

Results: All offspring are Bb (heterozygous) — all have brown fur.

• Phenotypic ratio: all brown
• All offspring are carriers of the recessive allele.

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Constructing a Punnett Square — Step by Step

1. Write the genotypes of both parents.
2. Determine the possible gametes each parent can produce.
3. Place one parent's gametes along the top and the other along the side.
4. Fill in the boxes by combining the alleles.
5. Identify the genotypic and phenotypic ratios.
6. Calculate probabilities (as fractions, percentages or ratios).

graph TD
    A["Identify parent genotypes"] --> B["Work out gametes"]
    B --> C["Draw Punnett square"]
    C --> D["Fill in offspring genotypes"]
    D --> E["Determine phenotypic ratio"]
    E --> F["Calculate probabilities"]

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Probability and Ratios

The ratios from a Punnett square tell you the probability (chance) of each outcome:

Ratio	As fraction	As percentage
1 in 4	1/4	25%
2 in 4	1/2	50%
3 in 4	3/4	75%
4 in 4	1	100%

Exam Tip: Ratios are theoretical probabilities. In reality, a family with four children will not necessarily have exactly three brown-furred and one white-furred. Each individual event is independent — like flipping a coin.

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Summary

• Alleles are different versions of a gene; they can be dominant or recessive.
• Homozygous = two identical alleles; heterozygous = two different alleles.
• Genotype = the alleles present; phenotype = the observable characteristic.
• A Punnett square predicts the genotypes and phenotypes of offspring.
• Crossing two heterozygous parents gives a 3:1 phenotypic ratio.
• The recessive phenotype only appears in homozygous recessive individuals.
• Ratios show theoretical probabilities; each fertilisation event is independent.

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Extended Worked Example: Pea Plant Height (Mendel's Classic Cross)

Gregor Mendel studied pea plants in the 1860s and established the principles of inheritance long before DNA was discovered. Let T represent the dominant allele for tall and t the recessive allele for short.

Cross 1 — Pure-breeding tall × pure-breeding short (TT × tt)

	T	T
t	Tt	Tt
t	Tt	Tt

All F1 offspring are Tt — all tall, all heterozygous, all carriers of the recessive allele. This was Mendel's first observation: offspring looked like one parent, not a blend.

Cross 2 — Two F1 heterozygotes (Tt × Tt)