Inheritance: dominant, recessive and codominant alleles

What you'll learn

This guide covers how characteristics are inherited through different types of alleles. You'll learn to predict inheritance patterns using genetic diagrams, understand how dominant and recessive alleles determine phenotypes, and explain codominance where both alleles are expressed. These concepts are essential for Paper 2 of AQA GCSE Biology and frequently appear in 4-6 mark questions.

Key terms and definitions

Allele — a version of a gene; for each gene you inherit two alleles, one from each parent

Dominant allele — an allele that is expressed in the phenotype even when only one copy is present; represented by a capital letter (e.g. B)

Recessive allele — an allele that is only expressed in the phenotype when two copies are present (homozygous); represented by a lowercase letter (e.g. b)

Homozygous — having two identical alleles for a particular gene (e.g. BB or bb)

Heterozygous — having two different alleles for a particular gene (e.g. Bb)

Phenotype — the observable characteristics of an organism resulting from its genotype and environment

Genotype — the genetic makeup of an organism; the combination of alleles present

Codominant alleles — two different alleles that are both expressed in the phenotype of a heterozygous organism; neither is recessive

Core concepts

Understanding dominant and recessive inheritance

When organisms reproduce sexually, offspring inherit genetic information from both parents. For each characteristic, you receive one allele from your mother and one from your father.

A dominant allele controls the development of a characteristic even if only one copy is present. If you inherit at least one dominant allele, that characteristic will appear in your phenotype. For example, the allele for brown eyes (B) is dominant.

A recessive allele only controls the characteristic when two copies are present. If you inherit one dominant and one recessive allele, the dominant characteristic appears and the recessive one remains hidden. The recessive characteristic only shows when an individual is homozygous recessive. For example, blue eyes (b) is recessive—you need two b alleles to have blue eyes.

Key inheritance patterns:

• BB (homozygous dominant) = brown eyes
• Bb (heterozygous) = brown eyes (dominant allele expressed)
• bb (homozygous recessive) = blue eyes

The genotype Bb is called a carrier for the recessive characteristic—the person doesn't show blue eyes but can pass the allele to offspring.

Genetic diagrams and Punnett squares

Genetic diagrams predict the probability of offspring inheriting particular characteristics. At GCSE level, you must construct and interpret Punnett squares.

Steps to construct a Punnett square:

1. Identify the phenotypes of both parents
2. Work out the genotypes of both parents
3. Determine the possible gametes each parent can produce (remember: gametes contain only one allele from each pair)
4. Draw a 2×2 or 4×4 grid (depending on whether examining one or two characteristics)
5. Place one parent's gametes across the top, the other parent's down the side
6. Fill in the grid by combining alleles
7. State the ratio of genotypes and phenotypes in the offspring

For a monohybrid cross (examining one characteristic), if both parents are heterozygous (Bb × Bb):

• Possible gametes from each parent: B or b
• Offspring genotypes: 1 BB : 2 Bb : 1 bb
• Offspring phenotypes: 3 brown eyes : 1 blue eyes
• Probability of blue eyes = 1/4 or 25%

This 3:1 ratio is characteristic of two heterozygous parents crossing for a single gene with complete dominance.

Family pedigree analysis

Pedigree diagrams show inheritance patterns across generations. You need to interpret these and deduce genotypes.

Key symbols:

• Circles = females; squares = males
• Shaded shapes = affected individuals (showing the characteristic)
• Unshaded shapes = unaffected individuals
• Horizontal line between shapes = partners/parents
• Vertical lines downward = offspring

When analysing pedigrees:

• If two unaffected parents have an affected child, the characteristic is recessive (both parents are heterozygous carriers)
• If an affected parent has unaffected children, the characteristic is dominant
• If a characteristic skips generations, it's typically recessive

Codominance

Codominance occurs when both alleles in a heterozygous organism are equally expressed in the phenotype. Neither allele is dominant or recessive to the other.

The classic GCSE example is coat colour in cattle:

• Red coat = CRCR
• White coat = CWCW
• Roan coat (mixture of red and white hairs) = CRCW

Notice different notation: both alleles use capital letters because both are expressed. Superscript letters (CR and CW) distinguish the different alleles of the same gene.

In codominance:

• Heterozygotes show a distinct phenotype different from either homozygote
• Both alleles contribute to the phenotype
• No allele is "masked" or hidden

Another example relevant to GCSE is blood groups (ABO system):

• IA and IB are codominant to each other
• Both IA and IB are dominant to IO (recessive)
• Genotype IAIB produces blood group AB (both antigens present)

Predicting inheritance ratios

Understanding expected ratios helps interpret genetic crosses:

Dominant/recessive inheritance:

• Homozygous dominant × homozygous recessive (BB × bb) = 100% heterozygous offspring (Bb), all showing dominant phenotype
• Heterozygous × heterozygous (Bb × Bb) = 3:1 phenotype ratio (3 dominant : 1 recessive)
• Heterozygous × homozygous recessive (Bb × bb) = 1:1 phenotype ratio (test cross to identify carriers)

Codominant inheritance:

• Homozygote × homozygote (CRCR × CWCW) = 100% heterozygous (CRCW), all showing codominant phenotype
• Heterozygote × heterozygote (CRCW × CRCW) = 1:2:1 phenotype ratio (1 red : 2 roan : 1 white)
• Homozygote × heterozygote (CRCR × CRCW) = 1:1 ratio (1 red : 1 roan)

These ratios are probabilities, not guarantees. With small numbers of offspring, actual ratios may differ from expected ratios.

Sex determination and sex-linked characteristics

Humans have 23 pairs of chromosomes. The 23rd pair are sex chromosomes:

• Females: XX
• Males: XY

The Y chromosome is smaller and carries fewer genes than the X chromosome. Some characteristics are determined by genes on the X chromosome—these are sex-linked characteristics.

Common GCSE examples include:

• Colour blindness
• Haemophilia

Males are more likely to show sex-linked recessive conditions because:

• Males have only one X chromosome
• If that X carries the recessive allele, there's no corresponding allele on the Y chromosome to mask it
• Females need two copies of the recessive allele to be affected (much rarer)

Notation for sex-linked inheritance uses:

• XB = X chromosome with dominant allele
• Xb = X chromosome with recessive allele
• Y = Y chromosome (no corresponding allele)

A carrier female has genotype XBXb (heterozygous—not affected but can pass the condition to offspring).

Worked examples

Example 1: Dominant/recessive monohybrid cross

Question: In pea plants, tall stem (T) is dominant to short stem (t). Two heterozygous tall pea plants are crossed. Complete a Punnett square to show the possible genotypes and phenotypes of the offspring. Calculate the probability that an offspring will be short. (4 marks)

Mark scheme answer:

Parent genotypes: Tt × Tt

Gametes: T and t from each parent

Punnett square:

Genotype ratio: 1 TT : 2 Tt : 1 tt ✓

Phenotype ratio: 3 tall : 1 short ✓

Probability of short offspring = 1/4 or 0.25 or 25% ✓

Examiner note: Award marks for correct gametes (1), correct Punnett square completion (1), genotype ratio (1), and correct probability (1).

Example 2: Codominance

Question: In snapdragon flowers, red (CRCR) and white (CWCW) are codominant alleles. When a red snapdragon is crossed with a white snapdragon, all offspring have pink flowers. Explain why the offspring are pink, and predict the outcome if two pink snapdragons are crossed. (5 marks)

Mark scheme answer:

Offspring from red × white are heterozygous / CRCW ✓

Both alleles are expressed / codominant ✓

So red and white both contribute to phenotype / pink flowers result ✓

Pink × pink cross: CRCW × CRCW

Offspring ratio: 1 CRCR : 2 CRCW : 1 CWCW ✓

Phenotype ratio: 1 red : 2 pink : 1 white ✓

Examiner note: Must explain codominance correctly and show understanding that heterozygotes display both alleles' effects. Punnett square not required but ratio must be correct.

Example 3: Pedigree analysis

Question: The pedigree below shows the inheritance of cystic fibrosis in a family. Cystic fibrosis is caused by a recessive allele (f). The dominant allele is F.

       ○—□          Key: ○ = unaffected female
        |                □ = unaffected male  
    ————————             ● = affected female
    |      |             ■ = affected male
    ○      ■

Individual 1 (mother) and Individual 2 (father) are unaffected. Individual 4 (son) has cystic fibrosis. What are the genotypes of individuals 1, 2 and 4? Explain your reasoning. (4 marks)

Mark scheme answer:

Individual 4 (affected son) must be ff / homozygous recessive ✓

Individual 4 inherited one f allele from each parent ✓

Therefore individuals 1 and 2 must both be Ff / heterozygous / carriers ✓

Two unaffected parents can only have affected child if both are heterozygous ✓

Examiner note: Must deduce that unaffected parents of affected child are carriers. Clear genetic reasoning required.

Common mistakes and how to avoid them

• Confusing genotype with phenotype — Genotype refers to alleles present (e.g. Bb), phenotype refers to observable characteristics (e.g. brown eyes). Always use correct terminology in explanations.

• Writing gametes incorrectly — Gametes contain only ONE allele from each pair, not both. For genotype Bb, gametes are B or b separately, never Bb together.

• Incorrect notation for codominance — Don't use upper and lowercase letters (Bb) for codominant alleles. Use superscripts (CRCW) or different capital letters to show both are expressed equally.

• Misunderstanding probability — A 25% probability means 1 in 4 chance, not a guarantee. With small numbers of offspring, actual results often differ from expected ratios. Don't say "there will definitely be" when describing outcomes.

• Forgetting to show working — In genetic diagram questions, always show parent genotypes, gametes, and Punnett square. Answers without working rarely earn full marks, even if the final answer is correct.

• Misreading pedigree symbols — Check the key carefully. Don't assume shaded always means affected—sometimes questions reverse this. Read every pedigree key before answering.

Exam technique for "Inheritance: dominant, recessive and codominant alleles"

• Command words matter: "Explain" requires reasoning (e.g. "because both parents are Ff, they can each pass on the f allele"), while "State" or "Give" just needs the fact. "Predict" requires you to construct a genetic diagram and state outcomes.

• Show genetic diagrams systematically: Use the standard format—parent phenotypes, parent genotypes, gametes, Punnett square, offspring genotypes, offspring phenotypes, ratio. This structure ensures you don't miss steps that carry marks.

• For 6-mark questions: These assess your ability to link ideas logically. Use correct terminology throughout, explain causal relationships (use "because," "therefore," "this results in"), and ensure every statement relates to the question. Quality of written communication is assessed.

• Ratios can be expressed differently: 3:1, 3/4 and 1/4, or 75% and 25% are all acceptable for the same ratio. Choose the format the question requests or use the clearest format if no preference is stated.

Quick revision summary

Alleles are different versions of genes. Dominant alleles (capital letters) are expressed in heterozygotes; recessive alleles (lowercase) require two copies to be expressed. Codominant alleles are both expressed in heterozygotes, creating a distinct phenotype. Use Punnett squares to predict inheritance patterns: heterozygous crosses produce 3:1 ratios for dominant/recessive traits and 1:2:1 for codominant traits. Analyse pedigrees by working out whether traits skip generations (recessive) or appear in every generation (dominant). Always show working in genetic diagrams.