So 75% show dominant phenotype, 25% show recessive phenotype.
EXAMPLE — Carrier × Normal (Bb × BB):
Offspring: BB, BB, Bb, Bb
All offspring show dominant phenotype — but 50% are carriers (Bb).
Probability in Genetics
Punnett squares give us PROBABILITIES — the likelihood of each outcome, not a guarantee.
If a cross gives a 1 in 4 (25%) chance of an affected offspring, this means:
Each pregnancy independently has a 25% chance.
It does NOT mean that exactly 1 in every 4 children will be affected.
Small sample sizes may not reflect the expected ratio.
Probability can be expressed as:
A fraction: 1/4.
A percentage: 25%.
A ratio: 1 in 4.
Genetic counsellors use Punnett squares to advise couples on the risk of inherited conditions in their children.
⚠️ Common Mistake
A CARRIER (Bb) has the recessive allele but does NOT show the condition — they appear completely normal. Students often say carriers 'have the disease mildly' — this is WRONG. A carrier is phenotypically normal but genotypically heterozygous. Two carriers (Bb × Bb) have a 25% chance of an affected (bb) child.
📌 Key Note
Dominant (capital) = expressed with one copy. Recessive (lowercase) = needs two copies. Genotype = alleles present. Phenotype = observable trait. Homozygous = both same. Heterozygous = different. Carrier = Bb, appears normal.
🎯 Matching Activity — Genetics Vocabulary
Match each genetics term to its correct definition. — drag the symbols on the right to match the component names on the left.
Genotype
Drop here
Phenotype
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Homozygous
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Heterozygous
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Dominant allele
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Carrier
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Both alleles are the same — e.g. BB or bb
The observable characteristic — e.g. brown eyes
The alleles present in an organism — e.g. Bb
Alleles are different — e.g. Bb
Expressed in the phenotype even if only one copy is present
Heterozygous individual with one recessive allele — appears normal but can pass it on
⚽ FIFA Worked Examples
Punnett Square — Heterozygous Cross
Two parents are both heterozygous for brown eyes (Bb). Brown (B) is dominant over blue (b). Calculate the probability of a child having blue eyes.
F
Draw a Punnett square: Bb × Bb
I
Offspring: BB, Bb, Bb, bb → genotype ratio 1:2:1
F
Only bb gives blue eyes (recessive phenotype) = 1 out of 4
A
Probability of blue eyes = 1/4 = 25%
⭐ Higher Tier Only
Codominance: both alleles expressed in the phenotype. Example: blood groups — IA and IB are codominant; IO is recessive. Blood group AB genotype = IAIB (both A and B antigens expressed). Students should be able to construct Punnett squares for crosses involving codominant alleles and predict the phenotype ratios for codominant inheritance.
🎯 Test Yourself
Question 1 of 3
1. In a Bb × Bb cross, what fraction of offspring will show the RECESSIVE phenotype?
2. A person is described as a 'carrier' of cystic fibrosis. What does this mean?
3. If a Punnett square shows a 1 in 4 chance of having an affected child, what does this mean for a couple?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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I'll use FIFA for calculations and flag Higher/Triple content clearly.