What Is Monohybrid Inheritance? A Visual Guide for IGCSE Biology Students

One gene, two alleles, one clear pattern

Monohybrid inheritance is the study of how a single characteristic is passed from parents to offspring when that characteristic is controlled by one gene with two possible forms (alleles). It is the simplest model of inheritance and the one Cambridge IGCSE Biology tests most frequently. If you can set up a genetic diagram, complete a Punnett square and read off the expected ratios, you hold the key to a reliable block of marks in almost every exam session.

The logic is mathematical. Once you understand the rules, every genetics question becomes a problem you can solve step by step, with no ambiguity about what the examiner expects.

Key facts at a glance

• Gene: a section of DNA that codes for a specific protein, and therefore a specific characteristic.
• Allele: one version of a gene. For monohybrid inheritance, there are exactly two alleles to consider.
• Dominant allele: the allele whose effect on the phenotype is shown even when only one copy is present. Represented by a capital letter (e.g. R).
• Recessive allele: the allele whose effect on the phenotype is only shown when two copies are present (no dominant allele to mask it). Represented by a lowercase letter (e.g. r).
• Homozygous: both alleles for a gene are the same (RR or rr).
• Heterozygous: the two alleles for a gene are different (Rr).
• Genotype: the combination of alleles an organism has (e.g. Rr).
• Phenotype: the observable characteristic that results from the genotype (e.g. round seeds).

Understanding the terminology through an example

Gregor Mendel's pea experiments remain the clearest illustration. In peas, seed shape is controlled by a single gene with two alleles:

• R (dominant) produces round seeds.
• r (recessive) produces wrinkled seeds.

Three genotypes are possible:

Notice that RR and Rr produce the same phenotype. This is precisely why the recessive allele can "hide" for generations: a heterozygous parent looks identical to a homozygous dominant parent, yet carries a recessive allele it can pass on.

The Punnett square method: step by step

A Punnett square is a grid that maps every possible combination of alleles from two parents. It is the standard tool Cambridge expects you to use, and it must be set out correctly to earn full marks.

Worked example: Cross two heterozygous pea plants (Rr x Rr).

Step 1 - State the parental phenotypes and genotypes:

• Parent 1: Round seeds (Rr)
• Parent 2: Round seeds (Rr)

Step 2 - Identify the gametes each parent can produce:

Each parent produces two types of gamete: one carrying R and one carrying r.

Step 3 - Draw the Punnett square:

Step 4 - Read off the ratios:

• Genotype ratio: 1 RR : 2 Rr : 1 rr
• Phenotype ratio: 3 round : 1 wrinkled

The 3:1 phenotype ratio is the signature result of a heterozygous cross in monohybrid inheritance. Whenever you see 3:1 in a question, think: both parents were heterozygous.

Other cross types and their expected ratios

Different parental combinations produce different ratios. Knowing these by heart saves time in the exam, because you can predict the outcome before drawing the grid and then use the Punnett square to confirm.

The second row (Rr x rr producing a 1:1 ratio) is particularly important because it forms the basis of the test cross, described below.

The test cross: determining an unknown genotype

If an organism shows the dominant phenotype, it could be either homozygous dominant (RR) or heterozygous (Rr). You cannot tell just by looking. The test cross solves this problem systematically.

Method: Cross the organism with an individual that is homozygous recessive (rr).

• If all offspring show the dominant phenotype, the unknown parent is most likely RR (because RR x rr produces all Rr).
• If roughly half the offspring show the recessive phenotype, the unknown parent is Rr (because Rr x rr produces 1 Rr : 1 rr).

The logic is clean: the homozygous recessive parent can only contribute the recessive allele, so any recessive offspring must have received a recessive allele from the unknown parent as well. If recessive offspring appear, the unknown parent must carry at least one recessive allele and is therefore heterozygous.

Writing a full genetic diagram for the exam

Cambridge mark schemes award marks at each stage of a genetic diagram, not just for the final answer. Skipping a step means losing marks even if your ratio is correct. Here is the full sequence you should write every time:

1. Parental phenotypes: Round seeds x Wrinkled seeds
2. Parental genotypes: Rr x rr
3. Gametes: Circle each allele separately. Parent 1 produces R and r. Parent 2 produces r and r.
4. Punnett square or genetic diagram: Show every possible combination.
5. Offspring genotypes: List them (Rr, Rr, rr, rr).
6. Offspring genotype ratio: 1 Rr : 1 rr
7. Offspring phenotype ratio: 1 round : 1 wrinkled

Each numbered step above typically corresponds to one mark. A six-mark genetics question that asks you to "complete a genetic diagram" is testing whether you can produce this entire chain, not just whether you know the answer is 1:1.

Co-dominance (Extended/Supplement)

In standard dominance, one allele completely masks the other. Co-dominance is different: both alleles contribute to the phenotype, and the heterozygous individual shows a blended or combined trait rather than the dominant one alone.

The classic IGCSE Biology example is flower colour in snapdragons:

• C^RC^R produces red flowers.
• C^WC^W produces white flowers.
• C^RC^W produces pink flowers (neither allele dominates).

Notice the notation: because neither allele is dominant, you cannot use uppercase and lowercase of the same letter. Instead, both alleles are written as superscripts on a shared base letter.

Crossing two pink flowers (C^RC^W x C^RC^W) gives a phenotype ratio of 1 red : 2 pink : 1 white. The genotype ratio is 1 C^RC^R : 2 C^RC^W : 1 C^WC^W. In co-dominance, the genotype ratio and phenotype ratio are the same (1:2:1), because every genotype produces a distinct phenotype.

Common IGCSE exam mistakes

1. Using different letters for alleles of the same gene. Writing "R" for round and "w" for wrinkled is wrong. Both alleles must use the same letter: R and r.
2. Forgetting to circle gametes. The mark scheme often has a specific mark for showing gametes. Circle each one individually.
3. Confusing genotype with phenotype. Genotype is the allele combination (Rr). Phenotype is the visible trait (round seeds). Questions that ask for one and receive the other score zero.
4. Stating ratios as fractions of the total. "1 in 4 are wrinkled" is less precise than "1 wrinkled : 3 round." Use ratio notation.
5. Omitting the parental cross details. Jumping straight to the Punnett square without stating parental genotypes and gametes loses the early marks.
6. Treating predicted ratios as guaranteed outcomes. A 3:1 ratio is a probability, not a certainty. In a real cross of four offspring, you might get 4:0 by chance. If a question asks "explain why the actual results may differ from the predicted ratio," the answer involves the random nature of fertilisation and small sample size.

Self-check questions

1. Define the terms "allele," "genotype" and "phenotype."
2. A pea plant with genotype Rr is crossed with a pea plant with genotype rr. Draw a Punnett square and state the expected phenotype ratio of the offspring.
3. Two heterozygous brown mice (Bb) are crossed. What is the probability that any one offspring will be white (bb)?
4. Explain why a test cross uses a homozygous recessive individual.
5. In snapdragons, red (C^RC^R) and white (C^WC^W) show co-dominance. What phenotype ratio would you expect from crossing two pink flowers?
6. A student writes the following genetic diagram: "Parents: Tall x Short. Genotypes: T x s." Identify two errors in the student's notation.