What is Monohybrid Inheritance in the context of Cambridge IGCSE Biology?

Monohybrid Inheritance refers to the study of the inheritance of a single characteristic controlled by one pair of alleles. In Cambridge IGCSE Biology, it involves understanding how traits are passed from parents to offspring through dominant and recessive alleles, typically illustrated using a Punnett square.

How do you use a Punnett square to predict Monohybrid Inheritance outcomes?

A Punnett square is a grid used to predict the genotypes of offspring in monohybrid inheritance. By placing the alleles of one parent on the top and the alleles of the other parent on the side, you can fill in the squares to show all possible combinations of alleles. This helps visualize the probability of offspring inheriting particular traits.

What is the difference between dominant and recessive alleles in Monohybrid Inheritance?

In Monohybrid Inheritance, a dominant allele is one that expresses its trait even if only one copy is present, while a recessive allele requires two copies to express its trait. For example, if 'A' is a dominant allele and 'a' is a recessive allele, the genotype 'Aa' will express the dominant trait.

Can you explain the concept of homozygous and heterozygous genotypes in Monohybrid Inheritance?

In Monohybrid Inheritance, a homozygous genotype consists of two identical alleles for a trait, such as 'AA' or 'aa'. A heterozygous genotype has two different alleles, such as 'Aa'. Homozygous individuals will express the trait of the alleles they possess, while heterozygous individuals will express the dominant trait.

Why is Monohybrid Inheritance important in understanding genetics?

Monohybrid Inheritance is fundamental in genetics as it provides a simple model to understand how traits are passed from one generation to the next. It helps explain the basic principles of heredity, such as dominant and recessive traits, and is a stepping stone to more complex genetic concepts covered in the Cambridge IGCSE Biology curriculum.

Why is "Saying you ALWAYS get 3:1 from Tt × Tt." a common mistake in Monohybrid Inheritance?

Why it happens: It's the predicted ratio. How to avoid it: 3:1 is the EXPECTED ratio over many offspring. Small numbers can vary by chance — flipping a coin doesn't always give 50:50.

Why is "Saying only males can have sex-linked traits." a common mistake in Monohybrid Inheritance?

Why it happens: Mostly male in colour blindness. How to avoid it: Females CAN be affected (homozygous recessive on both X chromosomes) — just less common.

InheritanceCambridge IGCSE Biology (0610)

Monohybrid Inheritance

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Monohybrid Inheritance — Cambridge IGCSE 0610 Biology Extended (2026)

Tracking ONE gene's pattern. Punnett squares predict offspring ratios. Sex-linked traits show different patterns in males and females.

What you’ll learn

Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).

17.5 — Solve monohybrid inheritance problems using Punnett squares.
17.5 — Predict offspring genotype and phenotype ratios.
17.5 — Describe the use of test crosses.
17.5 — Explain sex-linked inheritance with X-linked recessive examples.

How to do a Punnett square

A grid that crosses parent gametes. Predicts offspring proportions.

Step 1. Identify the alleles + which is dominant. Example: T = tall (dominant), t = short (recessive).

Step 2. Identify parent genotypes. Example: Tt × Tt (heterozygous parents).

Step 3. Determine GAMETE types each parent can produce. Tt parent makes gametes T or t (each ½ chance).

Step 4. Draw a 2×2 grid; gametes of one parent on top, other on side.

        T       t
   +-------+-------+
T  |  TT   |  Tt   |
   +-------+-------+
t  |  Tt   |  tt   |
   +-------+-------+

Step 5. Count GENOTYPES: 1 TT, 2 Tt, 1 tt = ratio 1:2:1.

Step 6. Count PHENOTYPES: 3 tall (TT and Tt) : 1 short (tt) = 3:1.

Step 7. Express as percentages or fractions:

25% TT, 50% Tt, 25% tt.
75% tall, 25% short.

Other crosses:

Cross	Genotype ratio	Phenotype ratio
TT × tt	All Tt	All tall
Tt × tt	1 Tt : 1 tt	1 tall : 1 short
Tt × Tt	1 TT : 2 Tt : 1 tt	3 tall : 1 short
TT × Tt	1 TT : 1 Tt	All tall

Worked qualitative. A heterozygous brown-eyed parent (Bb) crosses with a blue-eyed parent (bb). What proportion of children will be blue-eyed?

Bb × bb cross.
Punnett square: ½ Bb (brown) and ½ bb (blue).
50% chance each child is blue-eyed.

Cambridge tip. Always show the Punnett square clearly. Label the alleles. Cambridge marks for working as well as the answer.

Identify alleles + dominance.
Set up Punnett square.
Read off genotypes + phenotypes.
Express as ratio or %.

Test crosses — figuring out unknowns

Cross with homozygous recessive. Outcome reveals the unknown genotype.

Sometimes we have an organism showing the dominant phenotype but don't know if it's homozygous (TT) or heterozygous (Tt). Both look the same.

The test cross: cross the unknown with a HOMOZYGOUS RECESSIVE (tt).

If unknown is TT:

TT × tt
All offspring: Tt → all show DOMINANT phenotype.

If unknown is Tt:

Tt × tt
Half offspring: Tt (dominant phenotype).
Half: tt (recessive phenotype).
Mix of phenotypes.

Reading the result.

ALL OFFSPRING DOMINANT → unknown was TT.
MIX → unknown was Tt.

Worked qualitative. A farmer has a black bull (B = black, b = brown; B dominant). Wants to know if the bull is BB or Bb. How does he find out?

Test cross: mate the bull with a brown cow (bb).
If all calves are black → bull is BB.
If some calves are brown → bull is Bb.
A practical use of genetics in animal breeding.

Cambridge tip. When asked how to find an unknown genotype, the answer is ALWAYS 'test cross with a homozygous recessive'.

Cross unknown with homozygous recessive.
All dominant offspring → unknown homozygous dominant.
Mixed → unknown heterozygous.
Used in agriculture / breeding.

Sex-linked inheritance

Genes on X chromosome show different inheritance in males vs females.

Some traits are determined by genes on the X chromosome (sex chromosomes).

Why it matters.

Males have only ONE X (XY).
Females have TWO Xs (XX).
A recessive allele on X affects males with just ONE copy (no other X to mask it).
Females need TWO copies to be affected → much rarer.

Common examples.

Red-green colour blindness.
Haemophilia (impaired blood clotting).
Duchenne muscular dystrophy.
All X-linked recessive traits — much more common in males.

Notation.

Use X^B for normal allele (dominant), X^b for affected allele (recessive).
Y has no allele (it's not the homologous gene).

Genotypes:

Genotype	Phenotype	Sex
X^B X^B	Normal	Female
X^B X^b	Carrier (normal but can pass to sons)	Female
X^b X^b	Affected	Female (rare)
X^B Y	Normal	Male
X^b Y	Affected	Male

A typical example: carrier mother + normal father.

Mother X^B X^b × Father X^B Y:

Daughters: ½ X^B X^B (normal), ½ X^B X^b (carrier).
Sons: ½ X^B Y (normal), ½ X^b Y (AFFECTED).

So 50% of sons of a carrier mother will be affected.

Worked qualitative. Why is haemophilia called the 'Royal Disease'?

Queen Victoria of England was a haemophilia carrier.
She passed the allele through her descendants — sons affected, daughters carriers.
Spread to royal families across Europe (Spain, Russia, Germany).
Most famous: Tsarevich Alexei of Russia.
A famous historical example of X-linked recessive inheritance.

Cambridge tip. Always include the X notation for sex-linked questions. Cambridge often gives a pedigree and asks students to track inheritance.

Genes on X chromosome.
Males XY → 1 copy → affected if recessive.
Females XX → 2 copies → usually masked.
Examples: colour blindness, haemophilia.

How it’s examined

Monohybrid inheritance is a Paper 4 staple (8-12 marks: Punnett squares, ratios, sex linkage). Examiner reports flag students forgetting to label alleles and predicting incorrect ratios.

Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (17.5); 0610/42 May/Jun 2024 — Q20 (monohybrid); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.

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Step-by-step worked examples — Monohybrid Inheritance

Step-by-step solutions to past-paper-style questions on monohybrid inheritance, written exactly the way a tutor would explain them at the board.

1A monohybrid cross
Core• Punnett square
Question
In pea plants, T = tall (dominant), t = short. Cross Tt × Tt. Predict the offspring genotypes and phenotypes.
Step-by-step solution
1. Step 1
  Punnett square:
  $\begin{array}{c|cc} & T & t \\ \hline T & TT & Tt \\ t & Tt & tt \end{array}$
2. Step 2
  Genotypes: 1 TT : 2 Tt : 1 tt = 1:2:1.
3. Step 3
  Phenotypes (T dominant): 3 tall : 1 short = 3:1.
4. Step 4
  Each offspring has 25% chance of being TT, 50% of Tt, 25% of tt.
Answer
Genotypes: 1 TT : 2 Tt : 1 tt. Phenotypes: 3 tall : 1 short.
Examiner tip
Cambridge marks the Punnett square AND the resulting ratios. Show working.
2Test cross — finding an unknown genotype
Extended• Adapted from 0610/42 May/Jun 2024 Q20• test cross
Question
A tall pea plant could be TT or Tt. Describe a TEST CROSS to find out which.
Step-by-step solution
1. Step 1
  Cross the tall plant with a HOMOZYGOUS RECESSIVE (tt, short).
2. Step 2
  If unknown plant is TT: TT × tt → all offspring Tt → ALL TALL.
3. Step 3
  If unknown plant is Tt: Tt × tt → ½ Tt (tall) + ½ tt (short) → mixed offspring.
4. Step 4
  Conclusion: all-tall offspring = TT. Mix of tall + short = Tt.
Answer
Cross with homozygous recessive (tt). All-tall offspring → TT. Mixed offspring → Tt.
3Sex-linked inheritance — colour blindness
Extended• sex-linked
Question
Why is RED-GREEN COLOUR BLINDNESS more common in males than females?
Step-by-step solution
1. Step 1
  Colour blindness is caused by a recessive allele on the X CHROMOSOME (sex-linked).
2. Step 2
  Males have only ONE X (XY) — if the allele is on it, they're affected.
3. Step 3
  Females have TWO Xs (XX) — usually only ONE has the recessive allele → masked by the dominant on the other X.
4. Step 4
  Female only affected if BOTH X chromosomes have the recessive allele (rare).
5. Step 5
  Females can be CARRIERS (one allele, unaffected, can pass to sons).
Answer
Males XY: one X → if recessive, affected. Females XX: usually masked. Females rarely affected; males commonly.

Key Definitions and Keywords — Monohybrid Inheritance

Definitions to memorise and the exact keywords mark schemes credit for monohybrid inheritance answers — sharpened from recent examiner reports for the 2026 0610 sitting.

Punnett square
Examiner keyword
Grid used to predict the genotypes (and phenotypes) of offspring from a genetic cross.
Test cross
Crossing an organism with a HOMOZYGOUS RECESSIVE individual to determine the unknown genotype.
Sex-linked inheritance
Examiner keyword
Inheritance of a gene located on a SEX CHROMOSOME (usually X). Pattern of inheritance differs between males and females.
Codominance
Both alleles in a heterozygous individual are EXPRESSED (neither is dominant). Example: blood type AB.

Common Mistakes and Misconceptions — Monohybrid Inheritance

The traps other students keep falling into on monohybrid inheritance questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.

✕Saying you ALWAYS get 3:1 from Tt × Tt.
Why it happens
It's the predicted ratio.
How to avoid it
3:1 is the EXPECTED ratio over many offspring. Small numbers can vary by chance — flipping a coin doesn't always give 50:50.
✕Saying only males can have sex-linked traits.
Why it happens
Mostly male in colour blindness.
How to avoid it
Females CAN be affected (homozygous recessive on both X chromosomes) — just less common.

Practice questions

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Past paper style quiz

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Monohybrid Inheritance — frequently asked questions

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Monohybrid Inheritance

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1A monohybrid cross

2Test cross — finding an unknown genotype

3Sex-linked inheritance — colour blindness

Punnett square

Test cross

Sex-linked inheritance

Codominance

✕Saying you ALWAYS get 3:1 from Tt × Tt.

✕Saying only males can have sex-linked traits.

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Monohybrid Inheritance — frequently asked questions