CSEC BIOLOGY · SECTION C · LESSON C3

Inheritance: Genetic Crosses

Dominant, recessive, codominance, genotype, phenotype, homozygous, heterozygous — and how to set out a genetic cross that scores full marks.

Continuity and VariationSO C 2.7SO C 2.8SO C 2.9≈ 40 min🎮 3 games
Before you start

By the end of this lesson you can…

1WatchExplainer video 2ReadNotes & worked examples 3PractisePaper 1 style 4ApplyPaper 2 style
Step 1 · Watch

Explainer video

6 narrated scenes. Press play, listen and read along — the points appear as they are explained. Use ‹ Prev / Next › or the scene bar to jump to any scene.

LESSON C3 · EXPLAINER

Inheritance: Genetic Crosses

Two parents with normal skin pigment have an albino child. How is that possible?

Narration uses your device’s built-in voice. No sound? Switch to “Captions only” — the captions advance on their own.

Step 2 · Read

Key notes

Copy these into your notebook in your own words — that is how they stick.

Genotype / phenotype

Genotype = alleles present (e.g. Tt). Phenotype = observable feature (e.g. tall).

Homozygous / heterozygous

Two identical alleles / two different alleles.

Dominant / recessive

Dominant expressed in heterozygote; recessive only expressed when homozygous.

Codominance

Both alleles expressed in heterozygote (pink snapdragons; AB blood group).

Monohybrid ratios

TT × tt → all Tt. Tt × Tt → 3:1. Tt × tt → 1:1.

Test cross

Cross with homozygous recessive to reveal an unknown genotype.

Genetic diagram

Parents (phenotype, genotype) → gametes → offspring genotypes → phenotypes and ratio.

EXAM TIPS & COMMON MISTAKES

  • Use ONE letter for a gene: capital for dominant, lower case for recessive (T/t, not T/S).
  • Circle the gametes and show each only once per type.
  • For codominance use superscripts: Cᴿ, Cᵂ. White = CᵂCᵂ, pink = CᴿCᵂ, red = CᴿCᴿ.
  • Heterozygous × heterozygous: 50% of offspring are homozygous (25% dominant + 25% recessive).
Step 2 · Learn from examples

Worked examples & study resources

The kind of support a teacher gives after the lesson: worked examples to attempt first, a board illustration, key terms, common misconceptions and memory aids.

Worked example 1

A full genetic diagram — and a test cross

In guinea pigs, black fur (B) is dominant to brown fur (b). A breeder crosses a black guinea pig with a brown one, and gets 5 black and 4 brown offspring. Use a genetic diagram to explain the result.
Bb (black) × bb (brown)BbbbBbbbBbbb2 Bb : 2 bb1 black : 1 brown

Try it in your notebook first, then reveal the solution one step at a time.

  1. The brown parent shows the recessive phenotype, so it must be homozygous: bb.
  2. Some offspring are brown (bb), so they each received a b allele from the black parent too. The black parent must be Bb (heterozygous).
  3. Set out the cross: parents’ phenotypes black × brown; genotypes Bb × bb; gametes B, b and b, b.
  4. Offspring genotypes from the Punnett square: 2 Bb : 2 bb, i.e. 1 Bb : 1 bb.
  5. Phenotypes: 1 black : 1 brown — close to the observed 5 : 4. This is a test cross: crossing an unknown dominant with a homozygous recessive reveals its genotype.
Answer. Black parent Bb × brown parent bb → gametes (B, b) and (b) → offspring 1 Bb (black) : 1 bb (brown). The 5 : 4 result fits the expected 1 : 1 ratio.
Examiner’s tip. Label every line of the diagram: parental phenotypes, parental genotypes, gametes (circled), offspring genotypes, offspring phenotypes, ratio. Each line is usually a mark.
Worked example 2

Codominance in four o’clock flowers

In four o’clock plants (common in Caribbean gardens), the alleles for red (Cᴿ) and white (Cᵂ) flowers are codominant: CᴿCᵂ plants have pink flowers. Predict the offspring of a pink × red cross.
red (columns) × pink (rows)CᴿCᴿCᴿCᵂCᴿCᴿCᴿCᴿCᴿCᵂCᴿCᵂ2 CᴿCᴿ : 2 CᴿCᵂ1 red : 1 pink

Try it in your notebook first, then reveal the solution one step at a time.

  1. Pink = CᴿCᵂ; red = CᴿCᴿ.
  2. Gametes: pink parent gives Cᴿ or Cᵂ; red parent gives only Cᴿ.
  3. Combine: Cᴿ + Cᴿ = CᴿCᴿ; Cᵂ + Cᴿ = CᴿCᵂ (twice each in the Punnett square).
  4. Ratio: 1 CᴿCᴿ : 1 CᴿCᵂ → 1 red : 1 pink. No white flowers can appear, because every offspring gets a Cᴿ from the red parent.
Answer. 1 red (CᴿCᴿ) : 1 pink (CᴿCᵂ). No white offspring.
Examiner’s tip. With codominance, use a capital letter plus superscripts (Cᴿ, Cᵂ) — never a capital and a lower-case letter, which would mean one allele is dominant.
On the board · How to set out a genetic cross for full marks
Parental phenotypesblack × brown↓Parental genotypesBb × bb↓GametesB, b × b, b↓Offspring genotypesPunnett square: Bb, Bb, bb, bb↓Offspring phenotypesblack, black, brown, brown↓Ratio1 black : 1 brown

Six labelled lines, every time. Circle the gametes.

Key terms

Genotype
The alleles an organism has, e.g. Bb.
Phenotype
The observable characteristic, e.g. black fur.
Dominant
An allele expressed whenever it is present.
Recessive
An allele expressed only when two copies are present.
Homozygous
Two identical alleles, e.g. BB or bb.
Heterozygous
Two different alleles, e.g. Bb.
Codominance
Both alleles are expressed in the heterozygote.
Test cross
Crossing with a homozygous recessive to find an unknown genotype.

Common misconceptions

  • ✗ The dominant allele is the most common one.✓ Dominance is about which allele shows in a heterozygote, not how common the allele is.
  • ✗ A 3 : 1 ratio means exactly 3 of every 4 offspring will show the dominant trait.✓ Ratios are probabilities. With small numbers, real results often differ from the expected ratio.
  • ✗ Recessive traits always skip a generation.✓ A recessive trait appears whenever a child inherits two recessive alleles — that may or may not skip a generation.

Memory aid

Genotype = Genes (the letters). Phenotype = Physical (what you can see).

Real-life link

Sickle cell anaemia is caused by a recessive allele. If two parents are both carriers (heterozygous), each child has a 1 in 4 chance of having the disease — which is why genetic counselling and screening are offered in many Caribbean countries.

Try this

Model a Tt × Tt cross with two coins (heads = T, tails = t). Toss both 40 times and record TT, Tt and tt. How close are you to 1 : 2 : 1?

Play & practise

Games

3 quick games on this lesson. Earn XP and badges as you play — your best score for each game is saved. Games are for practice and do not change your exam readiness.

Step 3 · Practise

Paper 1 practice — multiple choice

8 items modelled on real CSEC Paper 1 questions. Choose one answer for each, then check. Aim for 80% or better.

Adapted from CSEC P1 May/June 2024, Item 54

1. If a diploid organism has two different alleles for the same gene, it is described as

Answer: D. Different alleles = heterozygous.
Adapted from CSEC P1 2022 Item 55; 2024 Item 55

2. Albinism is caused by a recessive allele. Two normal parents produce an albino child because

Answer: B. Aa × Aa can give aa.
Adapted from CSEC P1 May/June 2024, Item 56

3. Two goats heterozygous for fast growth are crossed. What percentage of offspring are homozygous?

Answer: C. GG (25%) + gg (25%) = 50%.
Adapted from CSEC P1 May/June 2022, Item 54

4. Which term refers to the visible characteristics of an organism?

Answer: D. Phenotype = observable features.
Adapted from CSEC P1 May/June 2023, Item 55

5. Purple (P) is dominant to white (p) in peas. What percentage of offspring are WHITE in (i) PP × pp and (ii) Pp × pp?

Answer: A. PP × pp → all Pp; Pp × pp → 1 Pp : 1 pp.
Based on CSEC P2 May/June 2026, Q5(b)–(c)

6. A red snapdragon (CᴿCᴿ) is crossed with a white one (CᵂCᵂ). The offspring are

Answer: A. All CᴿCᵂ — codominance gives pink.
EduTech practice item (SO C2.9)

7. A tall plant crossed with a short plant gives 52 tall and 48 short offspring. The tall parent’s genotype is

Answer: B. A 1:1 ratio means the tall parent was heterozygous.
Based on CSEC P2 May/June 2021, Q5(c)

8. Parents with blood groups A (Iᴬi) and B (Iᴮi) can have a child with blood group

Answer: C. Gametes Iᴬ, i × Iᴮ, i give IᴬIᴮ, Iᴬi, Iᴮi, ii.
Step 4 · Apply

Paper 2 practice — structured questions

Write full answers, then reveal the mark scheme and tick the points you earned (22 marks in total). Your answers are saved on this device.

Based on CSEC P2 January 2024, Question 5(e)

Question 1 [7 marks]

Height in pea plants is controlled by one gene with two alleles; tall (T) is dominant to short (t). A tall pure-breeding plant is crossed with another pea plant and all the offspring have the same phenotype.

(a) Using a full genetic diagram, show the cross and state the phenotype of the offspring. (Consider the cross TT × tt.) [7]

MARK SCHEME — (a) (max 7)

Based on CSEC P2 May/June 2026, Question 5

Question 2 [15 marks]

Mr Banks grows snapdragons. Allele Cᵂ gives white flowers and Cᴿ gives red; they are codominant.

(a) Define “codominance” and “artificial selection”. [3]

MARK SCHEME — (a) (max 3)

(b) State the genotype of white, pink and red snapdragons. [3]

MARK SCHEME — (b) (max 3)

(c) Using a genetic diagram, explain how Mr Banks can continuously produce pink snapdragons; give the phenotype percentages. [6]

MARK SCHEME — (c) (max 6)

(d) Some yellow snapdragons appeared among white ones. Suggest a biological explanation. [3]

MARK SCHEME — (d) (max 3)

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Sources

Where this topic appears in CSEC papers

Find these in your own past-paper booklets for extra timed practice. The practice above is adapted from them.