What is Monohybrid Inheritance in the context of Cambridge IGCSE Coordinated Science?

Monohybrid Inheritance refers to the study of how a single pair of contrasting traits is passed from parents to offspring. In the Cambridge IGCSE Coordinated Science curriculum, it involves understanding how alleles for a single gene are inherited and how they determine specific traits in organisms.

How do Punnett squares help in understanding Monohybrid Inheritance?

Punnett squares are a useful tool in Monohybrid Inheritance as they allow students to predict the probability of offspring inheriting particular alleles from their parents. By organizing the possible combinations of parental alleles, students can visualize and calculate the expected genotypic and phenotypic ratios in the offspring.

What is the difference between genotype and phenotype in Monohybrid Inheritance?

In Monohybrid Inheritance, the genotype refers to the genetic makeup of an organism, specifically the alleles it possesses for a particular trait. The phenotype, on the other hand, is the observable characteristic or trait that results from the genotype. For example, if the trait is flower color, the genotype might be represented as 'RR' or 'Rr', while the phenotype would be the actual color of the flower.

Why is Gregor Mendel important in the study of Monohybrid Inheritance?

Gregor Mendel is considered the father of genetics due to his pioneering work with pea plants, which laid the foundation for the study of Monohybrid Inheritance. His experiments demonstrated how traits are inherited through discrete units, now known as genes, and established the basic principles of heredity, including the concepts of dominant and recessive alleles.

What are dominant and recessive alleles in Monohybrid Inheritance?

In Monohybrid Inheritance, dominant alleles are those that express their trait even if only one copy is present in the genotype. Recessive alleles, however, require two copies to express their trait. For example, if 'R' is a dominant allele for red flower color and 'r' is a recessive allele for white flower color, a plant with the genotype 'Rr' will have red flowers, as the dominant allele masks the effect of the recessive allele.

Why is "Stating that a 3:1 ratio means exactly 3 tall and 1 short in every 4 offspring" a common mistake in Monohybrid Inheritance?

Why it happens: Students confuse expected probability ratios with certain outcomes. How to avoid it: A 3:1 ratio is a **probability** based on large numbers of offspring. In a small sample (e.g. 4 offspring), any combination is possible. Always write 'expected ratio' not 'definite outcome'.

Why is "Confusing codominance with incomplete dominance" a common mistake in Monohybrid Inheritance?

Why it happens: Both involve heterozygotes with intermediate-looking phenotypes in some cases. How to avoid it: Codominance: **both alleles fully expressed** (e.g. AB blood group: both A and B antigens). Incomplete dominance: a **blended intermediate** phenotype (e.g. red × white = pink flowers). Different concepts.

Why is "Stating that daughters cannot be colour blind" a common mistake in Monohybrid Inheritance?

Why it happens: Students know colour blindness is more common in males and incorrectly conclude daughters cannot be affected. How to avoid it: Daughters CAN be colour blind if they are homozygous recessive (X^b X^b). This requires a colour-blind father (X^b Y) AND a carrier or colour-blind mother.

Why is "Forgetting that blood group O is homozygous recessive (ii)" a common mistake in Monohybrid Inheritance?

Why it happens: Students know A, B, AB, O blood groups but confuse the genetics of the i (O) allele. How to avoid it: Alleles: I^A (codominant), I^B (codominant), i (recessive). Blood group O = genotype ii. Blood group A = I^A I^A or I^A i. Blood group B = I^B I^B or I^B i.

Cambridge IGCSE Coordinated Science (0654)

Monohybrid Inheritance

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Short Notes - Monohybrid Inheritance

Detailed Study Notes

Detailed notes on Inheritance for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.

Monohybrid Inheritance — Cambridge IGCSE Coordinated Science 0654

Monohybrid crosses track one gene at a time using Punnett squares. Cambridge tests crosses for dominant/recessive traits, codominance, and sex-linked conditions. Always show gametes and full Punnett squares.

At a glance

Monohybrid cross: crosses tracking ONE gene with TWO alleles.
Punnett square: gametes on axes, combine to show all possible offspring genotypes.
Bb × Bb cross → 3:1 phenotype ratio (1 BB : 2 Bb : 1 bb genotype ratio).
Sex determination: XX = female, XY = male. 50% chance of each sex in every pregnancy.
Codominance: both alleles expressed — e.g., blood groups (IA, IB, IO). AB = IAIb (both expressed).
Sex-linked traits: gene on X chromosome — males (XY) more likely to express recessive X-linked conditions.

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

B22.1 — Use Punnett squares to predict outcomes of monohybrid crosses.
B22.2 — Explain codominance with an example.
B22.3 — Explain sex determination in humans.
B22.4 — Explain sex-linked inheritance (e.g. colour blindness, haemophilia).

The Punnett Square

A Punnett square shows all possible genotype combinations from a cross. It predicts probabilities, not certainties.

How to draw a Punnett square:

Write the cross (e.g. Bb × Bb)
Identify gametes for each parent (B or b)
Place gametes along the axes of the square
Fill in each box by combining the gametes
State genotype and phenotype ratios

Example — Monohybrid cross: Tall (Bb) × Tall (Bb)

T = tall (dominant); t = short (recessive)

Gametes of parent 1: T, t Gametes of parent 2: T, t

	T	t
T	TT	Tt
t	Tt	tt

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

Crossing two heterozygotes (Tt × Tt) gives the classic 3 : 1 dominant : recessive phenotype ratio.

Test cross: To determine if a dominant phenotype individual is homozygous (TT) or heterozygous (Tt), cross with a homozygous recessive (tt).

All offspring tall → parent was TT
50% tall, 50% short → parent was Tt

Punnett square: gametes on axes → combine to show offspring genotypes.
Bb × Bb → 3:1 phenotype ratio (three dominant : one recessive).
Test cross with homozygous recessive reveals if dominant parent is TT or Tt.

Codominance

When both alleles are expressed equally in the phenotype — neither is dominant over the other.

Codominance: when both alleles of a gene are expressed simultaneously in the phenotype of a heterozygote — neither masks the other.

ABO Blood Groups (human example):

Three alleles: IA (produces A antigen), IB (produces B antigen), IO (no antigen — recessive to both IA and IB)
IA and IB are codominant with each other

Genotype	Blood group (phenotype)
IAIA or IAIO	A
IBIB or IBIO	B
IAIB	AB (both antigens expressed — codominance)
IOIO	O

Another example: Snapdragon flower colour

CR (red) × CW (white) → CRCW (pink) — intermediate phenotype from both alleles expressed

Notation for codominant alleles: Both written as superscripts of the same letter (IA, IB, IO for blood groups; CR, CW for flowers).

Codominance: both alleles expressed in heterozygote phenotype.
Blood group AB = IAIB — both A and B antigens present.
Codominant alleles use equal capital letters (not capital/lowercase).

Sex Determination

Sex is determined by the sex chromosomes: XX = female, XY = male. The Y chromosome from sperm determines the sex of the offspring.

Humans have 23 pairs of chromosomes:

22 pairs = autosomes (non-sex chromosomes)
1 pair = sex chromosomes: XX (female) or XY (male)

Sex determination cross:

Father: XY (gametes: X or Y) Mother: XX (gametes: X only)

	X	Y
X	XX (female)	XY (male)
X	XX (female)	XY (male)

Result: 50% XX (female) : 50% XY (male) — in every pregnancy, 50% chance of each sex.

The sperm determines sex: Eggs always carry X. Sperm carry either X or Y. Which sperm fertilises the egg determines the sex.

XX = female; XY = male. Y chromosome from sperm determines sex.
Every pregnancy: 50% chance boy, 50% chance girl.
Father contributes X or Y sperm — determines offspring sex.

Sex-Linked Inheritance

Genes on the X chromosome are sex-linked. Males (XY) with one copy of a recessive X-linked allele will show the condition — they have no second X to mask it.

Sex-linked genes: genes located on the X chromosome (very few on Y). Since males have only ONE X chromosome, any allele on that X will be expressed — even if recessive.

Colour blindness (red-green) — recessive, X-linked:

Let XC = normal vision (dominant); Xc = colour blind (recessive)
Males: XcY = colour blind (only one copy of allele, no second X to mask it)
Females: XCXc = carrier (normal vision, but carries the allele); XcXc = colour blind (rare — needs two copies)

Cross: Carrier female × Normal male: Mother: XCXc × Father: XCY

	XC	Y
XC	XCXC (normal female)	XCY (normal male)
Xc	XCXc (carrier female)	XcY (colour blind male)

Phenotype ratio: 25% normal female : 25% carrier female : 25% normal male : 25% colour blind male

Key features of X-linked recessive inheritance:

More common in males (only one X)
Females can be carriers (heterozygous — not affected)
Colour blind father cannot pass the allele to his sons (Y from father; X from mother)
All daughters of a colour-blind father will be at least carriers

X-linked: gene on X chromosome. Males (XY) show recessive X-linked conditions more easily.
Carrier female: XCXc — normal phenotype but carries recessive allele.
Colour blind father (XcY) → all daughters carry allele; sons unaffected (get Y from father).

Quick recap

Punnett square: gametes on axes → offspring genotypes. Bb × Bb → 3:1 phenotype ratio.
Codominance: both alleles expressed (e.g. AB blood group — IAIB).
Sex: XX = female, XY = male. Sperm (X or Y) determines sex.
X-linked recessive: males (XY) more likely affected. Females can be carriers.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

Punnett square — grid used to predict offspring genotypes from a genetic cross.
Codominance — condition where both alleles are expressed in the phenotype of a heterozygote.
Carrier — an organism with one dominant and one recessive allele for an X-linked gene; phenotypically normal but can pass the recessive allele to offspring.
Sex-linked gene — a gene located on a sex chromosome (usually X).
Test cross — crossing an organism showing the dominant phenotype with a homozygous recessive individual to determine the unknown genotype.

How it’s examined

Paper 4: Always requires a complete Punnett square — write out gametes, draw the grid, fill in all four boxes, state genotype AND phenotype ratios. 'Explain why haemophilia is more common in males' (3 marks — X-linked, one allele expressed, males have no second X to mask it). 'A man has blood group O and a woman has blood group AB. State the possible blood groups of their children and show your working' (requires Punnett square). Mark schemes require the Punnett square to be shown — answers without it lose marks even if correct.

Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (B22); 0654/42 May/Jun 2024 — Q11 (genetics crosses); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.

Take this whole topic with you

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.

1Punnett square cross of two heterozygous tall plants
Core• Punnett square, monohybrid cross, ratio
Question
Two tall pea plants (Tt) are crossed. Show, using a Punnett square, the expected ratio of tall to short plants in the offspring.
Step-by-step solution
1. Step 1
  Set up the Punnett square. Parents: Tt × Tt. Gametes: T or t (from each parent).
  $\begin{array}{c|cc} & T & t \\ \hline T & TT & Tt \\ t & Tt & tt \end{array}$
2. Step 2
  Offspring genotypes: TT (1) : Tt (2) : tt (1). Since T is dominant, TT and Tt are tall; tt is short.
3. Step 3
  Phenotype ratio: 3 tall : 1 short.
Answer
Expected ratio: 3 tall : 1 short (genotype ratio 1 TT : 2 Tt : 1 tt).
2Sex-linked inheritance of colour blindness — sons' genotypes
Extended• sex-linked, colour blindness, X chromosome
Question
A woman is a carrier for colour blindness ( $X^N X^b$ ). Her husband has normal vision ( $X^N Y$ ). Show the possible genotypes and phenotypes of their sons.
Step-by-step solution
1. Step 1
  Sons inherit the Y chromosome from their father and either $X^N$ or $X^b$ from their mother.
  $\begin{array}{c|cc} & X^N & X^b \\ \hline X^N & X^N X^N & X^N X^b \\ Y & X^N Y & X^b Y \end{array}$
2. Step 2
  Sons' possible genotypes: $X^N Y$ (normal vision) or $X^b Y$ (colour blind). Since males are hemizygous (only one X chromosome), a single $X^b$ allele causes colour blindness.
3. Step 3
  Expected: 50% of sons have normal vision; 50% of sons are colour blind.
Answer
Sons: $X^N Y$ (normal vision) or $X^b Y$ (colour blind). 50% probability of colour blindness in sons because males have only one X chromosome; a single $X^b$ allele is expressed.
3Explain why blood group AB is an example of codominance
Extended• codominance, ABO blood groups, genotype
Question
A person has blood group AB. State their genotype and explain why this is an example of codominance.
Step-by-step solution
1. Step 1
  The genotype of a blood group AB individual is $I^A I^B$ .
2. Step 2
  In codominance, neither allele is dominant over the other. Both $I^A$ and $I^B$ alleles are expressed simultaneously in the heterozygote.
3. Step 3
  The person produces both type A and type B antigens on their red blood cells. Neither allele is masked — both phenotypes are expressed → this is codominance.
Answer
Genotype: $I^A I^B$ . Codominance: neither allele is dominant; both $I^A$ and $I^B$ are fully expressed; the person shows both A and B characteristics (AB blood group).
4Why sex-linked conditions are more common in males
Extended• sex-linked, X chromosome, hemizygous
Question
Explain why sex-linked conditions such as colour blindness are more common in males than females.
Step-by-step solution
1. Step 1
  The gene for colour blindness is located on the X chromosome (X-linked). Males have only one X chromosome (XY) — they are hemizygous for X-linked genes.
2. Step 2
  If a male inherits even a single recessive allele ( $X^b$ ) on his only X chromosome, there is no second X chromosome to carry a dominant allele to mask it. The condition is expressed.
3. Step 3
  Females (XX) need two copies of the recessive allele ( $X^b X^b$ ) to show colour blindness. A female with one $X^b$ is only a carrier — the normal $X^N$ allele masks the effect. This is statistically less likely.
Answer
Males have only one X chromosome (hemizygous); a single recessive X-linked allele ( $X^b$ ) causes the condition. Females need two copies of the recessive allele to be affected; one copy makes them a carrier with normal vision. Hence colour blindness is more common in males.

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 0654 sitting.

Monohybrid cross
Examiner keyword
A genetic cross between two individuals for a single gene, showing the expected ratios of offspring genotypes and phenotypes using a Punnett square.
Codominance
Examiner keyword
A situation in which both alleles of a gene are fully expressed in the phenotype of a heterozygote. Neither allele is dominant over the other.
Example
ABO blood groups: $I^A I^B$ → blood group AB (both A and B antigens expressed).
Sex-linked gene
Examiner keyword
A gene located on one of the sex chromosomes (usually the X chromosome). Sex-linked conditions such as colour blindness and haemophilia are more common in males.
Carrier
Examiner keyword
A heterozygous individual who carries one copy of a recessive allele (including sex-linked recessive) but does not show the associated phenotype.
Hemizygous
Examiner keyword
Having only one allele of a gene, as in males for X-linked genes (XY individuals have only one X chromosome and therefore one allele for each X-linked gene).
Punnett square
Examiner keyword
A grid used to predict the expected genotypic and phenotypic ratios of offspring from a genetic cross by systematically combining parental gametes.

Common Mistakes and Misconceptions — Monohybrid Inheritance

The traps other students keep falling into on monohybrid inheritance questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.

✕Stating that a 3:1 ratio means exactly 3 tall and 1 short in every 4 offspring
Why it happens
Students confuse expected probability ratios with certain outcomes.
How to avoid it
A 3:1 ratio is a probability based on large numbers of offspring. In a small sample (e.g. 4 offspring), any combination is possible. Always write 'expected ratio' not 'definite outcome'.
✕Confusing codominance with incomplete dominance
Why it happens
Both involve heterozygotes with intermediate-looking phenotypes in some cases.
How to avoid it
Codominance: both alleles fully expressed (e.g. AB blood group: both A and B antigens). Incomplete dominance: a blended intermediate phenotype (e.g. red × white = pink flowers). Different concepts.
✕Stating that daughters cannot be colour blind
Why it happens
Students know colour blindness is more common in males and incorrectly conclude daughters cannot be affected.
How to avoid it
Daughters CAN be colour blind if they are homozygous recessive ( $X^b X^b$ ). This requires a colour-blind father ( $X^b Y$ ) AND a carrier or colour-blind mother.
✕Forgetting that blood group O is homozygous recessive (ii)
Why it happens
Students know A, B, AB, O blood groups but confuse the genetics of the i (O) allele.
How to avoid it
Alleles: $I^A$ (codominant), $I^B$ (codominant), $i$ (recessive). Blood group O = genotype ii. Blood group A = $I^A I^A$ or $I^A i$ . Blood group B = $I^B I^B$ or $I^B i$ .

Monohybrid Inheritance — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Genotype

Blood group (phenotype)

IAIA or IAIO

IBIB or IBIO

IAIB

AB (both antigens expressed — codominance)

IOIO

XX (female)

XY (male)

XX (female)

XY (male)

XCXC (normal female)

XCY (normal male)

XCXc (carrier female)

XcY (colour blind male)

Monohybrid Inheritance

Short Notes - Monohybrid Inheritance

Detailed Study Notes

At a glance

What you’ll learn

The Punnett Square

Codominance

Sex Determination

Sex-Linked Inheritance

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Punnett square cross of two heterozygous tall plants

2Sex-linked inheritance of colour blindness — sons' genotypes

3Explain why blood group AB is an example of codominance

4Why sex-linked conditions are more common in males

Monohybrid cross

Codominance

Sex-linked gene

Carrier

Hemizygous

Punnett square

✕Stating that a 3:1 ratio means exactly 3 tall and 1 short in every 4 offspring

✕Confusing codominance with incomplete dominance

✕Stating that daughters cannot be colour blind

✕Forgetting that blood group O is homozygous recessive (ii)

Monohybrid Inheritance — frequently asked questions

Monohybrid Inheritance

Short Notes - Monohybrid Inheritance

Detailed Study Notes

At a glance

What you’ll learn

The Punnett Square

Codominance

Sex Determination

Sex-Linked Inheritance

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Punnett square cross of two heterozygous tall plants

2Sex-linked inheritance of colour blindness — sons' genotypes

3Explain why blood group AB is an example of codominance

4Why sex-linked conditions are more common in males

Monohybrid cross

Codominance

Sex-linked gene

Carrier

Hemizygous

Punnett square

✕Stating that a 3:1 ratio means exactly 3 tall and 1 short in every 4 offspring

✕Confusing codominance with incomplete dominance

✕Stating that daughters cannot be colour blind

✕Forgetting that blood group O is homozygous recessive (ii)

Monohybrid Inheritance — frequently asked questions