IB Biology SL D3.2 Inheritance Questions

Practise IB Biology SL D3.2 by predicting inheritance from crosses and pedigrees while linking alleles, genotypes and phenotype.

91 questions in this syllabus scope, with question parts counted together.

Exam analysis

SL · 2010–2025

Papers with this topic70 / 141in the analysed sample

Latest appearanceNovember 2025

Tagged question parts110

Paper types5

Most tested objectives

D3.2.9—ABO blood groups20 papers
D3.2.5—Dominant and recessive alleles19 papers
D3.2.13—Pedigree charts15 papers
D3.2.10—Incomplete dominance and codominance8 papers
D3.2.12—Haemophilia8 papers

Common question formats

Definition or recall
Structured response
Diagram interpretation
Data analysis
Calculation
Graph interpretation
Comparison

Recent exam appearances

November 2025Paper 1B SL · TZ3Q4(c), 4(d)[ 4 ]

November 2025Paper 1A SL · TZ1Q20[ 1 ]

May 2025Paper 2 SL · TZ3Q1(d), 1(e), 4(a), 4(b)(i), 4(b)(ii)[ 6 ]

141 papers analysed · Tagging incomplete

Exam points

Identify haploid gametes and distinguish genotype, phenotype and allele combinations.
Use flowering-plant or autosomal Punnett grids to predict genotypes, phenotypes and ratios.
Infer dominance and unknown genotypes from test-cross results or offspring evidence.
Deduce ABO cross outcomes from IA, IB and i, including codominance and recessive alleles.
Distinguish codominance from incomplete dominance and predict or interpret offspring phenotype ratios.
Calculate XX/XY offspring probabilities; explain how Y-chromosome SRY/TDF initiates testis development.
Use X-linked haemophilia grids to calculate carrier and affected outcomes by sex.
Infer dominant, recessive or X-linked inheritance and genotype risks from pedigrees.
Explain recessive PKU from PAH deficiency to phenylalanine build-up and dietary treatment.
Explain how environment alters gene expression and phenotype without changing genotype.
Distinguish alleles by an SNP's single-base difference.
Read medians, quartiles, range and outliers from box plots to compare continuous data.

Covered syllabus points

D3.2.1—Haploid gametes + fusion = diploid zygote D3.2.2—Genetic crosses in flowering plants D3.2.3—Genotype D3.2.4—Phenotype D3.2.5—Dominant and recessive alleles D3.2.6—Phenotypic plasticity D3.2.7—Phenylketonuria (PKU)D3.2.8—SNPs and multiple alleles D3.2.9—ABO blood groups D3.2.10—Incomplete dominance and codominance D3.2.11—Sex determination D3.2.12—Haemophilia D3.2.13—Pedigree charts D3.2.14—Continuous variation D3.2.15—Box-and-whisker plots

Exam points

Identify a normal gamete genotype as haploid, with one chromosome set and one allele of each gene.

[Maximum number: 1]

Which genotype would be normally found in a gamete?

rStt

TUt

Exam points

Use a Punnett grid to trace parental, F1 and F2 flowering-plant crosses and derive offspring ratios.
Explain why observed F2 flower-colour counts may differ from the expected ratio through chance.
Predict the flower-colour ratio when an F1 pea plant is crossed with a white-flowered plant.

[Maximum number: 4]

The dolichos bean (Lablab purpureus) is cultivated as a food crop in tropical countries. Leaf cells in L. purpureus have 24 chromosomes.

(a)

L. purpureus can have purple or white flowers. Two pure-breeding varieties were crossed: HA 4 with white flowers and GL 424 with purple flowers. All of the $F_{1}$ plants had purple flowers. The $F_{1}$ plants were self-pollinated to produce an $F_{2}$ generation. There were 97 plants with purple flowers and 38 plants with white flowers in the $F_{2}$ generation.

Using a Punnett grid, explain the results of this cross.

[ 3 ]

Male and female gamete genotypes/alleles shown as P and p, or other letters following convention with a suitable key, in a Punnett grid.
$\mathrm{F}_{2}$ genotypes shown as $\mathrm{PP}$ , $\mathrm{Pp}$ , $\mathrm{pP}$ and $\mathrm{pp}$ .
$\mathrm{F}_{2}$ phenotypes indicated for each genotype on the Punnett grid / 3 purple to 1 white ratio indicated.

Official Punnett-grid answer image from the markscheme.

Accept other upper-case and lower-case letters for the alleles.
No punnett grid = 2 MAX.
Allow ECF if alleles incorrect.

(b)

Suggest a reason for the $F_{2}$ generation not corresponding exactly to the expected ratio of purple-flowered and white-flowered plants.

[ 1 ]

Exam points

Define genotype as the combination of alleles inherited or carried by an organism.

[Maximum number: 1]

Define the term genotype.

Exam points

Define phenotype as an observable characteristic or trait and distinguish it from genotype as the inherited combination of alleles.

[Maximum number: 1]

The graph shows a sigmoid population growth curve.

The table summarizes the genome size of several organisms.

Distinguish between the terms genotype and phenotype.

Exam points

Explain that a dominant allele is expressed in a heterozygote, while a recessive allele is expressed when no dominant allele masks it.
Construct or interpret an autosomal Punnett grid to calculate genotype and recessive-condition probabilities from two heterozygous parents.
Calculate or identify dominant/recessive offspring phenotypes and ratios, including crosses with lethal dominant homozygotes.
Infer an unknown genotype or dominance relationship from a test-cross or observed offspring phenotype result.

[Maximum number: 3]

Watson and Crick's discovery of the double helical structure of DNA gave rise to our current understanding of genes.

From experiments, it was concluded that pea (Pisum sativum) plants have a recessive allele ( t ) for short stems.

Explain the reason that recessive alleles are not always expressed in the phenotype.

One additional mark is available for the construction of your answer.

Exam points

Explain phenotypic plasticity as an environment-driven change in phenotype without a change in genotype, linking the environmental condition to altered gene expression.

No public sample questions are available for this syllabus point yet.

Exam points

Explain the autosomal-recessive PKU cause and link deficient phenylalanine hydroxylase to phenylalanine accumulation and harmful effects.
Explain how low-phenylalanine dietary treatment and early diagnosis or screening reduce the harmful effects of PKU.

[Maximum number: 3]

Doctors investigated the effect of diets with different protein content on appetite for food in overweight males. One group followed a diet with normal protein (NP group) and a second group followed a diet with high protein (HP group). Participants were asked to record their perceptions of appetite every hour during 15 waking hours.

Explain the reason that a protein-rich diet would not be recommended to obese adults suffering from phenylketonuria (PKU).

Exam points

Define an allele as an alternative form of a gene and identify how a difference in DNA base sequence distinguishes two alleles.

[Maximum number: 1]

Which statement defines alleles?

They are the different forms of a gene that have the same effect on the phenotype.

They are the similar forms of a gene in different positions of a chromosome.

They are the various forms of a gene with slight differences in their base sequences.

They are the different forms of a gene coding for identical polypeptide chains.

Exam points

Explain that ABO blood groups use three alleles (IA, IB and i), with IA and IB codominant and i recessive, producing four phenotypes.
Deduce parental genotypes and predict offspring ABO genotypes, phenotypes or probabilities from a stated family or Punnett-grid cross.

[Maximum number: 1]

A couple have four children whose blood groups are A, B and A B. What is the likely combination of the parents' genotypes?

$\mathrm{I}^{\mathrm{A}} \mathrm{i}$ and $\mathrm{I}^{\mathrm{B}} \mathrm{i}$

$I^{A} i$ and $I^{B} I^{B}$

$\mid \mathrm{I}^{\mathrm{A}} \mathrm{I}^{\mathrm{B}}$ and ii

$\mid \mathrm{I}^{\mathrm{A}} \mathrm{I}^{\mathrm{A}}$ and $\mathrm{I}^{\mathrm{B}} \mathrm{I}^{\mathrm{B}}$

Exam points

Explain codominance as expression of both alleles in a heterozygote, as in AB blood type, and distinguish it from the intermediate heterozygote phenotype in incomplete dominance.
Predict offspring phenotypes and ratios in incomplete-dominance crosses, including the intermediate heterozygote, as in red, pink and white flowers.
Use codominant crosses, such as speckled chicken feathers, to identify heterozygote expression and predict phenotypic ratios.

[Maximum number: 1]

Pink flowers arise from the incomplete dominance between red and white flowers in the four o'clock flower (Mirabilis jalapa). A pink-flowering variety is crossed with a white-flowering variety.

What could be the possible phenotypes of the offspring?

White flowers only

Pink and white flowers

Red and white flowers

Pink flowers only

Exam points

Calculate the probability of a female offspring from a sex-determination question, including when previous births are stated.
Explain how a gene on the Y chromosome triggers testis development and how embryonic testes lead to male development.

[Maximum number: 1]

If a woman has three male offspring, what is the probability that her next child will be female?

25 %

50 %

75 %

100 %

Exam points

Explain haemophilia as an X-linked recessive disorder, using X-linked allele notation to identify carrier females and affected males.
Use an X-linked Punnett grid to predict offspring genotypes, phenotypes and sex-specific haemophilia risks.

[Maximum number: 4]

The pedigree chart shows the incidence of hemophilia in some of the descendants of Queen Victoria.

(a)

State the probability that Edward had hemophilia.

[ 1 ]

(b)

Explain the reasons for none of the females in the pedigree chart having hemophilia.

[ 3 ]

Exam points

Read pedigree symbols and family generations to identify autosomal dominant, autosomal recessive or X-linked inheritance patterns.
Infer an individual genotype or calculate an inherited-disorder probability from the relationships and phenotypes shown in a pedigree.

[Maximum number: 1]

The pedigree chart shows the incidence of colour blindness in three generations of a family. Colour blindness is caused by a sex-linked recessive allele.

What could the genotype of individual Z be?

$X^{B} X^{B}$ only

$X^{B} X^{b}$ only

$X^{B} X^{B}$ or $X^{B} X^{b}$

$X^{b} X^{b}$ or $X^{B} X^{b}$

Exam points

Read a continuous human-trait distribution and identify the modal range, recognizing that many intermediate phenotypes can occur.

[Maximum number: 1]

Both the genome and the environment of an organism affect its growth and development. Variation between individuals of a species can be continuous or discrete (discontinuous). Mitosis produces identical cells for growth and meiosis allows for variation.

The micrographs show onion and whitefish cells in mitosis.

The National Health and Nutrition Examination Survey in the USA measured heights of individuals aged 21-79. Height is an example of continuous variation.

State the range of heights that is the mode.

Exam points

Read medians, quartiles, ranges and outliers from box-and-whisker plots of biological measurements.
Calculate or compare a stated difference between medians using the plotted scale and units.

[Maximum number: 4]

(a)

Samples of feces were collected from 40 individuals and were immediately frozen to preserve them. The numbers of bacteria in the feces (cell counts $/ 10^{11}$ cells $\mathrm{g}^{-1}$ ) were later measured and the enterotype was determined. The box plot shows this data. Each data point shows the cell count from one fecal sample.

Estimate the median number of bacterial cells per gram of feces in the R enterotype.

[ 1 ]

$1.9 \times 10^{11}$ / 190000 million / 190 billion (cells per gram);

Cells per gram not needed as in stem. Accept $1.80 \times 10^{11}$ to $1.95 \times 10^{11}$ .

(b)

Distinguish between the cell counts in the R and B2 enterotypes.

[ 2 ]

(c)

Comment on the data for the P enterotype.

[ 1 ]

[Maximum number: 4]

(a)

Define the term genotype.

[ 1 ]

(b)

PKD (polycystic kidney disease) is an inherited disorder that causes the kidneys to become enlarged and lose function over time. PKD is caused by a dominant allele, D, which is not sex-linked.

The pedigree chart shows the inheritance of PKD in a family.

[ 3 ]

(i)

Calculate the probability of individuals 1 and 2 having a child with PKD.

[ 1 ]

0.5 / 50% / $\frac{1}{2}$ ;

(ii)

Explain how the pedigree chart shows that the dominant allele causing PKD is not on the X chromosome.

[ 2 ]

(If on the X chromosome)

ALTERNATIVE 1 Father evidenced route:

a. 1 has only one dominant allele on X (and not on the Y ) / would be $\mathrm{X}^{\mathrm{D}} \mathrm{Y}$ / OWTTE;

b. 1 passed his X chromosome/dominant allele to 3/7 I OWTTE OR

son/8 could not inherit the disease;

c. (so) all daughters would be affected / not possible for 3/7 to be healthy / OWTTE;

ALTERNATIVE 2 Mother evidenced route:

d. 2 does not have the dominant allele / is homozygous recessive / would be $\mathrm{X}^{\mathrm{d}} \mathrm{X}^{\mathrm{d}}$ IOWTTE;

e. 2 passed her X chromosome/ X ${ }^{d}$ /recessive allele(s) to 6/8;
OR

6/8/sons would receive the Y chromosome with no (dominant) allele / OWTTE;

f. (so) all sons/8 would be healthy / not possible for 8 to be affected / OWTTE;

Marking guidance:

Accept Punnett grids, providing they are
clearly annotated and identify specific
individuals.

All question bank results loaded

a	hemophilia is X-linked/sex-linked/carried on the X chromosome;
b	females have two X chromosomes OR males only have one X chromosome;
c	hemophilia is caused by a recessive allele;
d	(trait) must be on both alleles to be expressed OR females would require the allele on both X chromosomes to have the disease OR females can be carriers when allele is only on one chromosome;

a	only one sample/count/data point;
b	only analysed feces from one person (with this enterotype);
c	not a big enough sample;

Exam analysis

Most tested objectives

Common question formats

Recent exam appearances

Covered syllabus points

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