# Monohybrid Inheritance; Codominance, ABO and Sex Linkage

> CIE IGCSE Biology · 0610 2023-2028
> Source: https://www.owlsprep.com/study/cie-0610-u12-monohybrid-inheritance-codominance-abo-and/

This guide covers all Core and Extended content for monohybrid inheritance, codominance, ABO blood group inheritance and sex linkage aligned to CIE IGCSE Biology 0610 syllabuses 2023–2028, with worked examples and exam-focused tips.

**Prerequisites:** [Understanding of genes, alleles and basic inheritance terminology](https://www.owlsprep.com/study/cie-0610-u12-introduction-to-inheritance/); [Ability to interpret and construct basic Punnett square diagrams](https://www.owlsprep.com/study/cie-0610-u12-genetic-diagrams-basics/)

## Learning objectives

- Define monohybrid inheritance and construct genetic diagrams for standard monohybrid crosses
- Explain codominance and predict outcomes of codominant monohybrid crosses
- Apply knowledge of ABO blood group inheritance to solve Extended-tier genetics problems
- Describe sex linkage and interpret sex-linked genetic cross diagrams for Extended tier
- Distinguish between Core and Extended content to target exam preparation effectively

## Core: Monohybrid Inheritance Basics

**Monohybrid Inheritance** — The inheritance of a single characteristic controlled by one pair of alleles.

For standard monohybrid crosses with a dominant and recessive allele, a cross between two heterozygous parents will produce a 3:1 phenotypic ratio of dominant to recessive traits in offspring. This is one of the most commonly tested Core cross patterns.

**Worked example:** In pea plants, tall stem (T) is dominant over short stem (t). Cross two heterozygous tall pea plants, and give the phenotypic ratio of the offspring.

1. Step 1: Identify parent genotypes: both parents are heterozygous, so Tt.
2. Step 2: Identify possible gametes: each parent can pass either a T or t allele to offspring.
3. | Gametes | T | t |
| --- | --- | --- |
| T | TT | Tt |
| t | Tt | tt |
4. Step 3: Count phenotypes: TT and Tt = tall (3), tt = short (1).
5. Final phenotypic ratio: 3 tall : 1 short.

> **Exam tip:** Always label gametes clearly in Punnett squares, as examiners award marks for this even if your final ratio is incorrect.

## Core: Codominance Fundamentals

**Codominance** — An inheritance pattern where both alleles of a gene are fully expressed in the phenotype of a heterozygous individual, with no allele being recessive to the other.

> **warning**
>
> In codominance both alleles are fully expressed, so a heterozygote shows both traits separately (e.g. roan cattle have both red and white hairs). The two traits do not blend into a single intermediate colour.

**Worked example:** In cattle, red coat (R) and white coat (W) are codominant. Heterozygous individuals have a roan coat (both red and white hairs present). Cross a red cow and a roan bull, and give the phenotypic ratio of the offspring.

1. Step 1: Identify parent genotypes: red cow = RR (homozygous dominant), roan bull = RW (heterozygous codominant).
2. Step 2: Identify possible gametes: cow only passes R, bull can pass R or W.
3. | Gametes | R |
| --- | --- |
| R | RR |
| W | RW |
4. Step 3: Count phenotypes: RR = red, RW = roan.
5. Final phenotypic ratio: 1 red : 1 roan.

## Extended: ABO Blood Group Inheritance

ABO blood groups are controlled by three alleles: I<sup>A</sup>, I<sup>B</sup> (both dominant, codominant to each other) and I<sup>O</sup> (recessive). The four possible blood groups are A (I<sup>A</sup>I<sup>A</sup> / I<sup>A</sup>I<sup>O</sup>), B (I<sup>B</sup>I<sup>B</sup> / I<sup>B</sup>I<sup>O</sup>), AB (I<sup>A</sup>I<sup>B</sup>) and O (I<sup>O</sup>I<sup>O</sup>).

**Worked example:** A parent with blood group AB has a child with a parent with blood group O. What are the possible blood groups of their child?

1. Step 1: Identify parent genotypes: AB parent = I<sup>A</sup>I<sup>B</sup>, O parent = I<sup>O</sup>I<sup>O</sup>.
2. Step 2: Identify possible gametes: AB parent can pass I<sup>A</sup> or I<sup>B</sup>, O parent only passes I<sup>O</sup>.
3. | Gametes | I<sup>O</sup> |
| --- | --- |
| I<sup>A</sup> | I<sup>A</sup>I<sup>O</sup> |
| I<sup>B</sup> | I<sup>B</sup>I<sup>O</sup> |
4. Step 3: Interpret genotypes: I<sup>A</sup>I<sup>O</sup> = blood group A, I<sup>B</sup>I<sup>O</sup> = blood group B.
5. Final result: 50% chance of group A, 50% chance of group B, no AB or O possible.

> **Exam tip:** Always use the allele notation provided in the exam question for blood groups, even if you have seen alternative symbols elsewhere, to avoid losing marks.

## Extended: Sex Linkage

**Sex-linked gene** — A gene located on a sex chromosome (almost always the X chromosome in humans for IGCSE questions). Males (XY) only have one copy of X-linked genes, so are more likely to express recessive sex-linked traits than females (XX), who need two copies of the recessive allele.

**Worked example:** Haemophilia is a recessive sex-linked condition (allele X<sup>h</sup>). A carrier female (X<sup>H</sup>X<sup>h</sup>) has children with a normal male (X<sup>H</sup>Y). What is the probability their son will have haemophilia?

1. Step 1: Identify parent genotypes as given in the question.
2. Step 2: Identify possible gametes: female can pass X<sup>H</sup> or X<sup>h</sup>, male can pass X<sup>H</sup> or Y.
3. | Gametes | X<sup>H</sup> | Y |
| --- | --- | --- |
| X<sup>H</sup> | X<sup>H</sup>X<sup>H</sup> (normal female) | X<sup>H</sup>Y (normal male) |
| X<sup>h</sup> | X<sup>H</sup>X<sup>h</sup> (carrier female) | X<sup>h</sup>Y (haemophiliac male) |
4. Step 3: Count male offspring outcomes: 1 normal, 1 haemophiliac.
5. Final probability: 50% (1 in 2) chance their son has haemophilia.

> **Exam tip:** Always write X and Y sex chromosomes alongside alleles for sex-linked crosses, and clearly label the sex of offspring in your answers to get full marks.

## Exam Framework for Genetic Cross Answers

1. Identify allele relationships (dominant/recessive, codominant, sex-linked) from the question
2. Write clear, labeled parent genotypes using the notation provided in the question
3. List all possible gametes each parent can produce, labeled clearly
4. Complete a fully labeled Punnett square
5. State both genotypic and phenotypic ratios as requested by the question
6. Double-check your answer matches the question requirements (e.g. probability, ratio, specific phenotype)

> **tip**
>
> Always show all working for genetic cross questions, as partial marks are awarded for correct steps even if your final answer is wrong.

## Common pitfalls

- **Wrong:** Assuming all heterozygous crosses produce a 3:1 phenotypic ratio
  - Why it fails: The 3:1 ratio only applies to dominant/recessive monohybrid crosses; codominant and sex-linked crosses produce different ratios
  - Correct: First confirm the allele inheritance pattern (dominant/recessive, codominant, sex-linked) before predicting offspring ratios
- **Wrong:** Describing codominant heterozygous phenotypes as 'blended'
  - Why it fails: In codominance both alleles are fully expressed, so the heterozygote shows both traits separately rather than a single blended intermediate colour
  - Correct: Describe codominant heterozygotes as showing both traits, e.g. roan cattle have both red and white hairs, not pink hairs
- **Wrong:** Forgetting that males inherit X chromosomes only from their mother in sex-linked crosses
  - Why it fails: The Y chromosome has no corresponding allele for most X-linked traits, so recessive X-linked alleles are always expressed in males
  - Correct: Track X chromosome inheritance separately for male and female offspring when solving sex-linked problems
- **Wrong:** Using unapproved allele notation for blood groups or sex-linked traits
  - Why it fails: Examiners only award marks for notation specified in the question, even if alternative valid notation exists
  - Correct: Always copy the allele symbols directly from the exam question when constructing your answers
- **Wrong:** Failing to label gametes and Punnett square axes in genetic diagrams
  - Why it fails: Examiners award specific marks for correct labeling, even if your final cross result is incorrect
  - Correct: Label parent genotypes, gametes, and Punnett square rows/columns clearly in all cross answers

## Cheatsheet

| Concept | Core Requirement | Extended Requirement |
| --- | --- | --- |
| Monohybrid Cross | Construct Punnett squares for dominant/recessive crosses, predict 3:1 ratio | Same as Core, plus apply to codominant, ABO and sex-linked crosses |
| Codominance | Define codominance, recognise heterozygous phenotype shows both traits | Solve codominant cross problems, apply to ABO blood groups |
| ABO Blood Groups | Not assessed for Core | Know 3 alleles (I<sup>A</sup>, I<sup>B</sup>, I<sup>O</sup>), 4 blood groups, solve cross problems |
| Sex Linkage | Not assessed for Core | Explain X-linked inheritance, solve sex-linked cross problems, explain higher male incidence of recessive traits |

## What's next

Now that you have mastered monohybrid inheritance, codominance, ABO blood groups and sex linkage for CIE IGCSE Biology 0610, you can apply these skills to exam-style structured questions and move on to learning about inherited disorders and genetic screening. This topic is tested almost every year across all papers, so practicing full structured cross answers will help you maximise marks. Be sure to focus only on content relevant to your tier (Core or Extended) to avoid wasting study time on unnecessary material.

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