Study Guide

Monohybrid Inheritance; Codominance, ABO and Sex Linkage

CIE IGCSE BiologyΒ· 17.4Β· 45 min read

1. Core: Monohybrid Inheritance Basicsβ˜…β˜…β˜†β˜†β˜†β± 10 min

πŸ“˜ Definition

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.

πŸ“ 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. 1

    Step 1: Identify parent genotypes: both parents are heterozygous, so Tt.

  2. 2

    Step 2: Identify possible gametes: each parent can pass either a T or t allele to offspring.

  3. 3
    GametesTt
    TTTTt
    tTttt
  4. 4

    Step 3: Count phenotypes: TT and Tt = tall (3), tt = short (1).

  5. 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.

2. Core: Codominance Fundamentalsβ˜…β˜…β˜…β˜†β˜†β± 10 min

πŸ“˜ Definition

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.

πŸ“ 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. 1

    Step 1: Identify parent genotypes: red cow = RR (homozygous dominant), roan bull = RW (heterozygous codominant).

  2. 2

    Step 2: Identify possible gametes: cow only passes R, bull can pass R or W.

  3. 3
    GametesR
    RRR
    WRW
  4. 4

    Step 3: Count phenotypes: RR = red, RW = roan.

  5. 5

    Final phenotypic ratio: 1 red : 1 roan.

3. Extended: ABO Blood Group Inheritanceβ˜…β˜…β˜…β˜…β˜†Extended only⏱ 12 min

ABO blood groups are controlled by three alleles: IA, IB (both dominant, codominant to each other) and IO (recessive). The four possible blood groups are A (IAIA / IAIO), B (IBIB / IBIO), AB (IAIB) and O (IOIO).

πŸ“ 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. 1

    Step 1: Identify parent genotypes: AB parent = IAIB, O parent = IOIO.

  2. 2

    Step 2: Identify possible gametes: AB parent can pass IA or IB, O parent only passes IO.

  3. 3
    GametesIO
    IAIAIO
    IBIBIO
  4. 4

    Step 3: Interpret genotypes: IAIO = blood group A, IBIO = blood group B.

  5. 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.

4. Extended: Sex Linkageβ˜…β˜…β˜…β˜…β˜†Extended only⏱ 12 min

πŸ“˜ Definition

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.

πŸ“ Worked Example

Haemophilia is a recessive sex-linked condition (allele Xh). A carrier female (XHXh) has children with a normal male (XHY). What is the probability their son will have haemophilia?

  1. 1

    Step 1: Identify parent genotypes as given in the question.

  2. 2

    Step 2: Identify possible gametes: female can pass XH or Xh, male can pass XH or Y.

  3. 3
    GametesXHY
    XHXHXH (normal female)XHY (normal male)
    XhXHXh (carrier female)XhY (haemophiliac male)
  4. 4

    Step 3: Count male offspring outcomes: 1 normal, 1 haemophiliac.

  5. 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.

5. Exam Framework for Genetic Cross Answersβ˜…β˜…β˜…β˜†β˜†β± 8 min

  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)

6. Common Pitfalls

Wrong move:

Assuming all heterozygous crosses produce a 3:1 phenotypic ratio

Why:

The 3:1 ratio only applies to dominant/recessive monohybrid crosses; codominant and sex-linked crosses produce different ratios

Correct move:

First confirm the allele inheritance pattern (dominant/recessive, codominant, sex-linked) before predicting offspring ratios

Wrong move:

Describing codominant heterozygous phenotypes as 'blended'

Why:

In codominance both alleles are fully expressed, so the heterozygote shows both traits separately rather than a single blended intermediate colour

Correct move:

Describe codominant heterozygotes as showing both traits, e.g. roan cattle have both red and white hairs, not pink hairs

Wrong move:

Forgetting that males inherit X chromosomes only from their mother in sex-linked crosses

Why:

The Y chromosome has no corresponding allele for most X-linked traits, so recessive X-linked alleles are always expressed in males

Correct move:

Track X chromosome inheritance separately for male and female offspring when solving sex-linked problems

Wrong move:

Using unapproved allele notation for blood groups or sex-linked traits

Why:

Examiners only award marks for notation specified in the question, even if alternative valid notation exists

Correct move:

Always copy the allele symbols directly from the exam question when constructing your answers

Wrong move:

Failing to label gametes and Punnett square axes in genetic diagrams

Why:

Examiners award specific marks for correct labeling, even if your final cross result is incorrect

Correct move:

Label parent genotypes, gametes, and Punnett square rows/columns clearly in all cross answers

7. Quick Reference 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 (IA, IB, IO), 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

Going deeper

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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