What you'll learn
Monohybrid inheritance forms the foundation of genetics in CXC CSEC Biology and appears regularly in Section B (Structured Questions) and Section C (Extended Response) on exam papers. This topic covers how single traits pass from parents to offspring through alleles, and how to predict offspring ratios using Punnett squares. Mastering the terminology, genetic crosses, and probability calculations is essential for securing marks in both calculation-based and explanation questions.
Key terms and definitions
Gene — a section of DNA that codes for a specific characteristic or trait (e.g., flower colour, height).
Allele — alternative forms of the same gene (e.g., the tall allele T and the short allele t for plant height).
Dominant allele — an allele that expresses its phenotype even when only one copy is present; represented by a capital letter (e.g., T).
Recessive allele — an allele that only expresses its phenotype when two copies are present; represented by a lowercase letter (e.g., t).
Genotype — the genetic makeup of an organism for a particular trait (e.g., TT, Tt, or tt).
Phenotype — the observable characteristic expressed by the genotype (e.g., tall or short plant).
Homozygous — having two identical alleles for a gene (e.g., TT or tt); also called pure-breeding.
Heterozygous — having two different alleles for a gene (e.g., Tt); also called hybrid.
Core concepts
Mendel's principles of inheritance
Gregor Mendel, an Austrian monk, established the foundation of genetics through experiments with pea plants (Pisum sativum) in the 1860s. His work identified patterns of inheritance that apply to all sexually reproducing organisms, including Caribbean crop plants like tomatoes and peppers grown in Trinidad and Jamaica.
Mendel's Law of Segregation states that:
- Each parent carries two alleles for each trait
- These alleles separate during gamete formation (meiosis)
- Each gamete receives only one allele for each trait
- During fertilization, offspring receive one allele from each parent
This principle explains why offspring show predictable ratios when parents with known genotypes are crossed.
Monohybrid crosses explained
A monohybrid cross examines the inheritance of a single characteristic controlled by one gene with two alleles. The cross tracks how this trait passes from the parental generation (P) through the first filial generation (F₁) and second filial generation (F₂).
Example trait: Seed shape in peas
- Round seeds (R) — dominant
- Wrinkled seeds (r) — recessive
When Mendel crossed pure-breeding round-seeded plants (RR) with pure-breeding wrinkled-seeded plants (rr):
- P generation: RR × rr
- F₁ generation: All offspring were Rr (100% round seeds)
- F₂ generation: Crossing Rr × Rr produced a 3:1 phenotypic ratio (3 round : 1 wrinkled)
Constructing and interpreting Punnett squares
The Punnett square is a diagram used to predict the genotypes and phenotypes of offspring from a genetic cross. CXC CSEC Biology exams frequently require you to construct these squares and calculate ratios.
Steps to construct a Punnett square:
Identify parent genotypes — Determine the alleles each parent carries (e.g., Tt × Tt)
Determine possible gametes — Write the alleles each parent can pass on
- Tt parent can produce T or t gametes
- Place one parent's gametes across the top
- Place the other parent's gametes down the left side
Fill in the squares — Combine alleles from each gamete to show all possible offspring genotypes
Count genotypes and phenotypes — Determine ratios from the completed square
Example: Heterozygous cross (Tt × Tt)
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Genotypic ratio: 1 TT : 2 Tt : 1 tt
Phenotypic ratio: 3 tall : 1 short (because both TT and Tt express the dominant phenotype)
Types of monohybrid crosses
1. Homozygous dominant × Homozygous recessive (TT × tt)
- All F₁ offspring are heterozygous (Tt)
- 100% show the dominant phenotype
- Genotypic ratio: 4 Tt
- Phenotypic ratio: 4 dominant : 0 recessive
2. Heterozygous × Heterozygous (Tt × Tt)
- The classic 3:1 ratio cross
- Genotypic ratio: 1 TT : 2 Tt : 1 tt
- Phenotypic ratio: 3 dominant : 1 recessive
- 25% chance of homozygous recessive offspring
3. Heterozygous × Homozygous recessive (Tt × tt)
- Called a test cross or back cross
- Used to determine if an organism showing the dominant phenotype is TT or Tt
- Genotypic ratio: 1 Tt : 1 tt
- Phenotypic ratio: 1 dominant : 1 recessive (50:50 ratio)
4. Homozygous dominant × Heterozygous (TT × Tt)
- All offspring show dominant phenotype
- Genotypic ratio: 1 TT : 1 Tt
- Phenotypic ratio: 2 dominant : 0 recessive
Caribbean agricultural applications
Caribbean farmers apply principles of monohybrid inheritance when breeding crops and livestock:
Scotch Bonnet peppers (Jamaica): Farmers may cross plants to select for heat intensity, size, or disease resistance. If disease resistance (R) is dominant over susceptibility (r), crossing RR × Rr plants ensures all offspring carry at least one resistance allele.
Cattle breeding (Trinidad): When breeding cattle for beef production, farmers track traits like coat colour. If black coat (B) is dominant over red (b), understanding genotypes helps predict offspring appearance and maintain breed standards.
Cocoa cultivation (Grenada, Trinidad): Pod colour and disease resistance follow Mendelian patterns. Breeding programmes use test crosses to identify homozygous dominant trees that consistently produce desired traits.
Calculating probabilities
CXC CSEC Biology exams test your ability to calculate the probability of specific offspring genotypes or phenotypes.
Probability formula: Probability = (Number of desired outcomes) ÷ (Total number of possible outcomes)
Example: In a Tt × Tt cross, what is the probability of producing a homozygous recessive (tt) offspring?
- Desired outcome: 1 (tt)
- Total outcomes: 4
- Probability = 1/4 = 0.25 = 25%
For multiple offspring: Use the multiplication rule when calculating the probability of two independent events occurring together.
Example: What is the probability that two offspring from Tt × Tt will both be tt?
- Probability of first offspring being tt = 1/4
- Probability of second offspring being tt = 1/4
- Combined probability = 1/4 × 1/4 = 1/16 = 6.25%
Worked examples
Example 1: Guinea pig fur colour
In guinea pigs, black fur (B) is dominant over white fur (b). A black male guinea pig is crossed with a white female, producing 6 black offspring and 5 white offspring.
(a) State the genotype of:
- (i) the white female [1 mark]
- (ii) the black male [1 mark]
(b) Using a Punnett square, show the genetic cross between these parents. [3 marks]
(c) State the expected phenotypic ratio from this cross. [1 mark]
SOLUTION:
(a)(i) bb ✓
(White fur is recessive, so the female must be homozygous recessive)
(a)(ii) Bb ✓
(The male produces both black and white offspring, so must be heterozygous)
(b)
| B | b | |
|---|---|---|
| b | Bb | bb |
| b | Bb | bb |
✓ (correct gametes)
✓ (correct offspring genotypes)
✓ (proper Punnett square format)
(c) 1 black : 1 white OR 50% black : 50% white ✓
Example 2: Tomato plant height
Tall tomato plants (T) are dominant over short plants (t). A farmer in St. Lucia crosses two tall plants and obtains 84 tall plants and 28 short plants in the F₁ generation.
(a) Determine the genotypes of the parent plants. Explain your reasoning. [3 marks]
(b) If the farmer selects two of the tall F₁ offspring and crosses them, what is the probability they will produce a short plant? Show your working. [3 marks]
SOLUTION:
(a) Both parents are Tt ✓
The appearance of short (tt) offspring indicates both parents carry the recessive allele ✓
The 84:28 ratio approximates 3:1 (dominant:recessive), which confirms heterozygous × heterozygous cross ✓
(b)
The tall F₁ plants have genotypes in ratio 1 TT : 2 Tt ✓
Probability first plant is Tt = 2/3
Probability second plant is Tt = 2/3
From Tt × Tt, probability of tt = 1/4
Overall probability = 2/3 × 2/3 × 1/4 = 4/36 = 1/9 ✓✓
Example 3: Test cross application
A chicken farmer in Barbados has a rooster showing the dominant feather pattern but wants to know if it is pure-breeding (FF) or hybrid (Ff). The farmer performs a test cross with a hen showing the recessive pattern (ff).
(a) Explain what results would indicate the rooster is:
- (i) Pure-breeding [2 marks]
- (ii) Hybrid [2 marks]
SOLUTION:
(a)(i) If the rooster is FF, all offspring will be Ff ✓
All offspring will show the dominant feather pattern (100% dominant phenotype) ✓
(a)(ii) If the rooster is Ff, offspring will be 50% Ff and 50% ff ✓
Approximately half the offspring will show the recessive pattern ✓
Common mistakes and how to avoid them
Mistake 1: Confusing genotype and phenotype
Students write "the phenotype is Tt" or "the genotype is tall." Genotype refers to alleles (TT, Tt, tt); phenotype refers to observable traits (tall, short). Always match the term to what you're describing.
Mistake 2: Using the same letter for different alleles
Writing T for tall and S for short is incorrect because they're versions of the same gene. Use T for tall (dominant) and t for short (recessive). The same letter in different cases shows they're alternative forms of one gene.
Mistake 3: Incorrectly labelling gametes
Writing both alleles in a gamete (e.g., Tt in a gamete cell) violates Mendel's Law of Segregation. Each gamete receives only ONE allele per gene. A Tt parent produces either T or t gametes, never Tt.
Mistake 4: Calculating ratios incorrectly from Punnett squares
Students count TT once but forget Tt appears twice in the square. When determining phenotypic ratios for Tt × Tt, count all squares: 1 TT + 2 Tt = 3 showing dominant trait, plus 1 tt = 3:1 ratio.
Mistake 5: Forgetting that ratios are probabilities, not guarantees
A 3:1 ratio doesn't mean exactly 3 and 1 offspring in every family of 4. It's the expected average over many offspring. Small sample sizes (like 4 offspring) often deviate from theoretical ratios. Larger samples approximate predicted ratios more closely.
Mistake 6: Poor Punnett square presentation
Examiners deduct marks for unclear squares. Always draw a proper grid, label gametes outside the square (not inside), and write genotypes clearly inside each box. Show your working even if the question doesn't explicitly require a Punnett square—it helps avoid errors and can earn method marks.
Exam technique for Mendelian Genetics: Monohybrid Inheritance
Command word awareness:
"State" requires a brief answer without explanation (1 mark). "Explain" or "Account for" requires reasoning (2-3 marks). "Using a Punnett square" means you must show the square for full marks—written answers alone won't suffice. "Calculate" requires numerical working shown step-by-step.
Structuring genetic cross answers:
Always follow this sequence: (1) State parental genotypes clearly; (2) Show gametes produced by each parent; (3) Construct Punnett square with proper labels; (4) State genotypic ratio, then phenotypic ratio; (5) If asked to explain, link genotypes to phenotypes using dominance. This structure matches mark scheme expectations and ensures you don't omit steps.
Mark allocation patterns:
Punnett square questions typically award: 1 mark for correct gametes, 1 mark for correct offspring genotypes, 1 mark for proper square format. Ratio questions award 1 mark for genotypic ratio, 1 mark for phenotypic ratio. Probability calculations earn 1 mark for method, 1 mark for correct answer. Never skip showing working—partial credit is available even if your final answer is incorrect.
Using genetic terminology precisely:
CXC mark schemes require specific terms. Write "homozygous dominant" not "pure dominant"; "heterozygous" not "mixed" or "hybrid" (unless the question uses "hybrid"); "allele" not "gene" when referring to T or t. Precision in terminology demonstrates understanding and secures marks that vague language loses.
Quick revision summary
Monohybrid inheritance tracks one gene with two alleles through generations. Dominant alleles (capitals) mask recessive alleles (lowercase) in heterozygotes. Construct Punnett squares by placing gametes (one allele each) outside the grid and combining them inside. Key ratios: TT × tt gives 100% Tt; Tt × Tt gives 3:1 phenotypic ratio and 1:2:1 genotypic ratio; Tt × tt gives 1:1 ratio. Always show working, use correct terminology (genotype vs phenotype), and remember ratios are probabilities. Test crosses (Tt × tt) reveal unknown genotypes by analyzing offspring ratios.