Inheritance and Genetic Crosses

This lesson covers the principles of genetic inheritance, including Punnett squares and monohybrid crosses. This is essential content for Edexcel GCSE Biology (1BI0) Topic 3: Genetics.

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Key Genetic Terminology

Before studying inheritance, you must understand these key terms:

Term	Definition	Example
Gene	A section of DNA that codes for a specific protein.	The gene for eye colour.
Allele	A particular version of a gene.	The allele for brown eyes (B) or blue eyes (b).
Dominant	An allele that is expressed whenever it is present (even in heterozygous individuals). Written as a capital letter.	B (brown eyes).
Recessive	An allele that is only expressed when two copies are present (homozygous). Written as a lowercase letter.	b (blue eyes).
Homozygous	Having two identical alleles for a gene.	BB (homozygous dominant) or bb (homozygous recessive).
Heterozygous	Having two different alleles for a gene.	Bb (heterozygous — also called a "carrier" if the recessive allele is for a genetic condition).
Genotype	The combination of alleles an organism possesses.	BB, Bb, or bb.
Phenotype	The observable characteristic of an organism (what you can see or measure).	Brown eyes or blue eyes.

Exam Tip: In a heterozygous individual (e.g., Bb), the dominant allele (B) determines the phenotype. The recessive allele (b) is present but not expressed — it is "masked" by the dominant allele.

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Monohybrid Crosses

A monohybrid cross involves the inheritance of a single gene (one characteristic). We can use Punnett squares to predict the possible genotypes and phenotypes of offspring.

How to Set Up a Punnett Square

1. Determine the genotypes of both parents.
2. Work out the possible gametes (sex cells) each parent can produce. Each gamete carries one allele for the gene.
3. Draw a grid with one parent's gametes along the top and the other parent's gametes down the side.
4. Fill in the grid by combining the alleles from each gamete.
5. Read off the possible genotypes and phenotypes of the offspring.

graph TD
    A["Identify parent genotypes"] --> B["Determine possible gametes from each parent"]
    B --> C["Draw Punnett square grid"]
    C --> D["Combine alleles in each box"]
    D --> E["Read offspring genotypes"]
    E --> F["Determine phenotypes using dominance rules"]
    F --> G["Calculate genotype and phenotype ratios"]

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Worked Example 1: Heterozygous × Heterozygous (Bb × Bb)

Let B = the allele for brown eyes (dominant) and b = the allele for blue eyes (recessive).

Cross: A heterozygous brown-eyed parent (Bb) × a heterozygous brown-eyed parent (Bb).

Parental genotypes: Bb × Bb

Gametes:

• Parent 1 can produce gametes carrying B or b.
• Parent 2 can produce gametes carrying B or b.

Punnett Square:

	B	b
B	BB	Bb
b	Bb	bb

Offspring genotypes:

• 1 BB (homozygous dominant)
• 2 Bb (heterozygous)
• 1 bb (homozygous recessive)

Genotype ratio: 1 BB : 2 Bb : 1 bb

Offspring phenotypes:

• 3 brown eyes (BB and Bb both show brown eyes because B is dominant)
• 1 blue eyes (bb — only the homozygous recessive shows the recessive phenotype)

Phenotype ratio: 3 : 1 (3 brown : 1 blue)

Probability of blue eyes: 1 out of 4 = 25% or 1 in 4 chance.

Exam Tip: The 3:1 ratio is the classic result of crossing two heterozygous parents. Make sure you can explain why the ratio is 3:1 — three offspring have at least one dominant allele (and therefore show the dominant phenotype) while only one has two recessive alleles.

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Worked Example 2: Heterozygous × Homozygous Recessive (Bb × bb)

Cross: A heterozygous brown-eyed parent (Bb) × a homozygous recessive blue-eyed parent (bb).

Parental genotypes: Bb × bb

Gametes:

• Parent 1 (Bb) can produce gametes carrying B or b.
• Parent 2 (bb) can only produce gametes carrying b.

Punnett Square:

	B	b
b	Bb	bb
b	Bb	bb

Offspring genotypes:

• 2 Bb (heterozygous)
• 2 bb (homozygous recessive)

Genotype ratio: 1 Bb : 1 bb

Offspring phenotypes:

• 2 brown eyes (Bb — heterozygous, dominant allele expressed)
• 2 blue eyes (bb — homozygous recessive)

Phenotype ratio: 1 : 1 (1 brown : 1 blue)

Probability of blue eyes: 2 out of 4 = 50% or 1 in 2 chance.

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Worked Example 3: Homozygous Dominant × Homozygous Recessive (BB × bb)

Cross: A homozygous dominant brown-eyed parent (BB) × a homozygous recessive blue-eyed parent (bb).

Parental genotypes: BB × bb

Gametes:

• Parent 1 (BB) can only produce gametes carrying B.
• Parent 2 (bb) can only produce gametes carrying b.

Punnett Square:

	B	B
b	Bb	Bb
b	Bb	Bb

Offspring genotypes:

• All Bb (heterozygous)

Offspring phenotypes:

• All brown eyes (the dominant allele B is always present)

Phenotype ratio: All brown (100% brown eyes)

Exam Tip: This cross (BB × bb) produces all heterozygous offspring. All offspring show the dominant phenotype but they are all carriers of the recessive allele.

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Understanding Probability

Genetic crosses give us the probability (chance) of offspring having a particular genotype or phenotype. Important points:

• A 1 in 4 (25%) chance does not mean that out of 4 children, exactly 1 will have that phenotype. It means that each individual child has a 25% chance.
• Probabilities are theoretical — actual results may differ, especially with small numbers. With a very large number of offspring, results would approach the predicted ratio.
• Each pregnancy is an independent event — the outcome of one pregnancy does not affect the next.

Expressing Probability

You can express the chance of an outcome in several ways:

Expression	Example (1 in 4 chance)
Fraction	1/4
Percentage	25%
Decimal	0.25
Ratio	1 : 3 (affected : unaffected)
In words	One in four chance

Exam Tip: Make sure you can express probability in all of these formats. Exam questions may ask for a specific format (e.g., "Express the probability as a percentage"). Also remember that each child's outcome is independent — a couple who already have 3 brown-eyed children still have a 25% chance of their next child having blue eyes (if both parents are Bb).

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Using Genetic Diagrams

In the exam, you may be asked to draw a full genetic diagram. The standard format is: