IB Biology HL D3.2 Inheritance Questions

Practise IB Biology HL D3.2 with inheritance diagrams and data questions on alleles, pedigrees, linkage, variation, probability and genetic disorders.

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

Exam analysis

HL · 2010–2025

Papers with this topic71 / 117in the analysed sample

Latest appearanceNovember 2025

Tagged question parts138

Paper types5

Most tested objectives

D3.2.19 (HL)—Autosomal gene linkage17 papers
D3.2.14—Continuous variation15 papers
D3.2.17 (HL)—Dihybrid crosses15 papers
D3.2.9—ABO blood groups13 papers
D3.2.13—Pedigree charts11 papers

Common question formats

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

Recent exam appearances

November 2025Paper 2 HL · TZ3Q6(a), 6(b)[ 5 ]

November 2025Paper 1B HL · TZ1Q2(a)(ii), 2(b)[ 2 ]

November 2025Paper 1A HL · TZ3Q22, 32[ 2 ]

117 papers analysed · Tagging incomplete

Exam points

Identify haploid gametes, genotype, phenotype and homozygous or heterozygous alleles.
Solve monohybrid crosses with Punnett grids; infer genotypes and dominant or recessive outcomes.
Explain recessive PKU from PAH deficiency to phenylalanine accumulation and treatment.
Explain environment-driven gene-expression changes that alter phenotype, not genotype.
Explain SNPs as single-base variants and identify multiple-allele crosses from a Punnett grid.
Deduce ABO inheritance and transfusion compatibility from IA, IB, i, antigens and antibodies.
Explain codominance and incomplete dominance and interpret their phenotype ratios in genetic crosses.
Explain XX/XY inheritance and the Y-chromosome SRY/TDF signal in testis development.
Infer X-linked or autosomal inheritance, genotypes and risks from haemophilia and pedigree data.
Interpret continuous height data using polygenic effects, environment and trait distributions.
Compare box plots by median and IQR; identify outliers with the 1.5 × IQR rule.
Explain allele segregation and independent assortment through meiosis I and random bivalent orientation.
Use 9:3:3:1 or 1:1:1:1 ratios to solve and interpret dihybrid crosses.
Infer linkage from parental and recombinant offspring; relate recombination to crossing over.
Use expected counts, degrees of freedom and p-values to test genetic ratios with χ².

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 D3.2.16 (HL)—Segregation and independent assortment D3.2.17 (HL)—Dihybrid crosses D3.2.18 (HL)—Human gene loci D3.2.19 (HL)—Autosomal gene linkage D3.2.20 (HL)—Recombinants D3.2.21 (HL)—Chi-squared test

Exam points

Select a gamete genotype with one allele for each gene to reflect its haploid set.
Identify the haploid chromosome set contributed by the mother in sexual reproduction.

[Maximum number: 1]

What is always passed to the next generation as a result of sexual reproduction?

Homologous chromosomes from the mother

A chromatid from every chromosome of the father

A haploid set of chromosomes from the mother

All alleles from each parent

Exam points

Predict the flower-colour ratio when an F1 pea plant is crossed with a white-flowered plant.

[Maximum number: 1]

What are the genotypes of the parental generation?

Parental generation

Male is homozygous dominant and female is homozygous recessive.

Male is heterozygous and female is homozygous dominant.

Male is homozygous recessive and female is homozygous dominant.

Male is homozygous recessive and female is heterozygous.

Exam points

Identify each soybean parent’s two-gene genotype as HhFf.

[Maximum number: 1]

In a strain of soybeans, high oil content (H) in seeds is dominant to low oil content (h) and four seeds in a pod (F) is dominant to two seeds in a pod (f). A farmer crosses two soybean plants, both with high oil content and four seeds in a pod. The offspring have a phenotypic ratio of 9: 3: 3: 1.

Identify the genotypes of the soybean plants with high oil content and four seeds in a pod that were used in the cross.

$\mathrm{HhFf} \times \mathrm{HhFf} /($ both ) HhFf;

Exam points

Identify the observable phenotype represented by each part of a stated phenotypic ratio, matching each ratio outcome to its named trait.

[Maximum number: 2]

Identify the phenotypes of each part of the phenotypic ratio.

ratio	phenotypes
9	high oil four seeds;
3	high oil two seeds;
3	low oil four seeds;
1	low oil two seeds;

Exam points

Explain that a dominant allele is expressed in a heterozygote, while a recessive allele is expressed when no dominant allele masks it.
Calculate dominant/recessive genotype or phenotype probabilities from heterozygous parents, including lethal-dominant outcomes and inherited disorders.
Infer an unknown genotype or dominance relationship from a test-cross or observed offspring phenotype result.

[Maximum number: 1]

Huntington's disease can develop in middle age and leads to death of brain cells. It is carried by an autosomal dominant gene. Assume that the disease-causing allele was inherited through the family and that no new mutations occurred. What can be deduced about a man who has the disease?

Half of his children will get the disease.

Neither of his parents had the disease, but one must have been a carrier.

At least one of his grandparents carried the disease-causing allele.

His sons are more likely to have the disease than his daughters.

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.

[Maximum number: 2]

Common shrews (Sorex araneus) are small mammals found in Northern Europe. Their diet includes insects, slugs, spiders, worms and amphibians. They do not hibernate in winter because their bodies are too small to store sufficient fat reserves.

To study brain size in shrews, researchers anesthetize them, X-ray their skulls and measure the height of the braincase (BCH) where the brain is located. The graph shows the relationship between BCH and the brain mass of individual adult shrews.

Compare and contrast the results for winter and spring.

Exam points

Identify the autosomal-recessive PKU genotype and connect PAH deficiency with phenylalanine accumulation and the resulting condition.
Evaluate treatment evidence for PKU by weighing phenylalanine-level changes and limitations, and compare THB with protein-restricted treatment.

[Maximum number: 1]

Phenylketonuria is caused by a mutation to a gene.

Which genotype would result in the condition and what is the consequence?

Genotype

Consequence

AA only

Tyrosine is converted to phenylalanine

aa only

Tyrosine is not converted to phenylalanine

AA and Aa

Phenylalanine is converted to tyrosine

aa only

Phenylalanine is not converted to tyrosine

Exam points

Explain that an SNP is a single-base difference that can produce a distinct allele or genome within a species.
Interpret a Punnett grid as evidence that a population has multiple alleles while each diploid individual inherits at most two.

[Maximum number: 1]

What is always a consequence of single-nucleotide polymorphisms (SNPs) within a species?

Different proteomes

Different genomes

Deletion mutations

Insertion mutations

Exam points

Explain how IA and IB are codominant while i is recessive, linking the allele combinations to ABO phenotypes A, B, AB and O.
Deduce possible parental ABO genotypes from the blood-group phenotypes of their children.
Use an ABO cross or Punnett grid to predict offspring genotypes, phenotypes or their probabilities.

[Maximum number: 1]

Blood groups depend on the presence and absence of antigens on the surface of red blood cells. Antibodies in blood plasma react against foreign antigens, causing agglutination. The table shows the antigens and antibodies present in each blood group.

What deductions can be made?

A person of blood group O can receive blood of any blood type.

A person of blood group B can receive O blood.

A child whose parents are blood groups AB and O will have two types of antibodies.

A child whose parents are both blood group A can only produce anti-B antibodies.

Exam points

Explain codominance as the expression of both alleles in a heterozygote and incomplete dominance as an intermediate heterozygote phenotype.
Use an incomplete-dominance cross to predict offspring phenotypes and ratios, including the intermediate phenotype.
Predict phenotype ratios from a codominant cross in which the heterozygote expresses both allele-associated traits.

[Maximum number: 1]

The curly hair of the coat of Selkirk Rex cats is due to the presence of the allele $S^{C}$ . These cats can either have tight curls or be moderately curly, whereas the coat of other cats is usually made of straight hair with no curls because of the allele $S^{s}$ . Circles indicate female cats and squares indicate males.

$\mathbf{S}^{\mathbf{s}} \mathbf{s}^{\mathbf{s}}$

$\mathbf{S}^{\mathbf{s}} \mathbf{s}^{\mathbf{c}}$

tight curls

no curls

tight curls

moderate curls

no curls

tight curls

Exam points

Identify activation of the Y-chromosome SRY gene as the trigger for embryonic testis development.
Distinguish autosomes from XX/XY sex chromosomes and compare the genes carried by X and Y.
Select the X and Y chromosome plus testosterone combination in a fetal-development table.

[Maximum number: 1]

What is a characteristic of the human Y chromosome?

It is made of DNA and histones covered by phospholipids.

It contains some genes that are not present on the X chromosome.

It is the largest chromosome in the human karyotype.

It has a condensed length of approximately $100 \mu \mathrm{~m}$ .

Exam points

Explain haemophilia as an X-linked recessive disorder, using allele notation to distinguish carrier females from affected males.
Use parental X-linked genotypes and a Punnett grid to trace haemophilia through a family and predict affected sons or carrier daughters.

[Maximum number: 1]

The pedigree chart shows the inheritance of hemophilia in some of the descendants of Queen Victoria. Which letter points to a family member certain to be heterozygous?

Exam points

Infer autosomal dominant, autosomal recessive or X-linked inheritance from affected individuals, unaffected parents and sex-specific transmission in a pedigree.
Deduce a specified individual genotype or offspring probability from a pedigree and its inheritance pattern.

[Maximum number: 1]

The pedigree chart shows the incidence of Becker muscular dystrophy, a sex-linked condition.

What conclusion can be drawn?

The parents in the first generation are both carriers of the allele for Becker muscular dystrophy.

People with Becker muscular dystrophy die before they are old enough to have children.

Any of the females in the fourth generation could be carriers.

The chance of daughters inheriting Becker muscular dystrophy is 1 in 4 .

Exam points

Explain continuous variation as the result of polygenic inheritance and environmental effects, using height, skin colour or body mass as a named example.
Distinguish continuous from discrete variation and identify polygenic inheritance from a stated trait distribution.
Interpret a graph or data distribution for a continuously varying trait, including a modal range or relationship between measurements.

[Maximum number: 1]

Research into eye colour in humans shows a range from very light blue or grey to dark brown or black. In Europe eye colour shows a strong correlation with latitude, with lighter colours being more frequent in northern latitudes and darker eyes more frequent in the south.

What do these facts suggest about the genetic basis for eye colour in humans?

It shows discrete variation and is determined by a pair of alleles.

It is a polygenic trait controlled by multiple genes.

It is environmentally determined and is not controlled by genes.

It shows discrete variation and is independent of environmental factors.

Exam points

Read and compare medians, quartiles, ranges and outliers in box-and-whisker plots, including a labelled biological context.
Explain a plotted outlier using the 1.5 × interquartile-range rule and use distribution overlap to justify a comparison or deduction.

[Maximum number: 3]

Male and female human skeletons obtained from different archaeological sites in Greece were studied in order to find trends in human height evolution in Europe. The box-and-whisker plots show heights of skeletons dating from two historical periods: Neolithic (6500-3000 BC) and Roman (146-324 AD).

(a)

Using the graph,

[ 3 ]

(i)

Using the box-and-whisker plot, estimate the median height of females in the Roman period.

[ 1 ]

(ii)

Using the box-and-whisker plot, estimate the maximum height of males in the Neolithic period.

[ 1 ]

(iii)

State the reason that some of the data points were plotted outside the whiskers.

[ 1 ]

Exam points

Explain allele segregation as homologous chromosomes separate during meiosis I, producing one allele of each pair in each gamete.
Explain independent assortment of unlinked genes from the random orientation of bivalents at metaphase I.

[Maximum number: 1]

Which is a statement of Mendel's law of independent assortment?

Allele pairs separate during gamete formation and recombine during fertilization.

Allele pairs for different genes separate independently during gamete formation.

Unlinked alleles are assorted with a 9: 3: 3: 1 ratio in a dihybrid cross.

Allele pairs for the same gene are assorted independently during gamete formation.

Exam points

Construct and interpret a dihybrid cross for two unlinked autosomal genes, giving gamete genotypes and offspring phenotypes.
Use 9:3:3:1 F2 or 1:1:1:1 test-cross ratios to calculate or evaluate two-gene inheritance outcomes.

[Maximum number: 3]

Scientists determined the loci of three genes on chromosomes 1 and 4 of the thale cress plant (Arabidopsis thaliana). All three genes are involved in influencing the characteristics (width, symmetry and colour) of the plant's leaves.

The diagrams show the names and symbols of the alleles of these genes.

The expected ratio of phenotypes in the offspring of a cross between a plant with narrow, yellow leaves and a plant heterozygous for the genes for leaf width and colour is 1: 1: 1: 1.

Justify this expected ratio using a Punnett grid or other diagram.

bbgg gamete / BbGg gamete	BG	Bg	bG	bg
bg	BbGg	Bbgg	bbGg	bbgg

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

Exam points

Identify linked autosomal genes as loci on the same non-sex chromosome and explain why they tend to be inherited together.
Use crossing-over between linked loci to explain recombinant offspring and departures from expected Mendelian or 1:1:1:1 ratios.

[Maximum number: 1]

A heterozygous fruit fly (Drosophila melanogaster) with straight wings and red eyes was crossed with a curly-winged, sepia-eyed homozygous fruit fly. The offspring produced are shown in the table.

What conclusions can be drawn from this experiment?

The wing shape and eye colour genes are on the same chromosome.

The alleles of both genes are codominant.

Individuals with straight wings and sepia eyes must be the result of crossing over.

The genes for wing shape and eye colour are not linked.

Exam points

Identify recombinant gametes or offspring by allele combinations that differ from the parental chromosome combinations.
Explain that crossing over between linked loci creates recombinant types, which are fewer than the parental types in offspring.

[Maximum number: 1]

An individual is heterozygous for two linked genes $\frac{\mathrm{AB}}{\overline{\mathrm{ab}}}$ .

To investigate the frequency of crossing over, a test cross is carried out between the individual and another that is homozygous recessive for both genes. What are the possible recombinants in the offspring of this cross?

$\frac{\mathrm{Ab}}{\mathrm{ab}}$ and $\frac{\mathrm{Ab}}{\mathrm{ab}}$

$\frac{\mathrm{AB}}{\mathrm{ab}}$ and $\frac{\mathrm{Ab}}{\mathrm{aB}}$

$\frac{\mathrm{Ab}}{\mathrm{ab}}$ and $\frac{\mathrm{aB}}{\mathrm{ab}}$

$\frac{\mathrm{AA}}{\mathrm{aa}}$ and $\frac{\mathrm{BB}}{\mathrm{bb}}$

Exam points

Use observed and expected genetic counts to interpret a chi-squared result against the expected inheritance ratio.
Apply degrees of freedom, the p = 0.05 critical value or p value to decide whether deviation from the expected ratio is statistically significant.

[Maximum number: 1]

A dihybrid cross was carried out between two plants to determine whether the genes for seed shape and colour are linked. If the genes are unlinked, the expected ratio of 9: 3: 3: 1 should occur. A chi-squared test was carried out on the observed results of the cross. The critical value for chi squared at the 5 % level of significance in this test was 7.82 . The calculated value for chi squared was 6.25 . What can be concluded from this data?

The results prove that the genes are linked.

The results prove that the genes are unlinked.

There is significant evidence that the genes are linked.

There is significant evidence that the genes are unlinked.

[Maximum number: 5]

(a)

Identify the genotypes of the soybean plants with high oil content and four seeds in a pod that were used in the cross.

[ 1 ]

$\mathrm{HhFf} \times \mathrm{HhFf} /($ both ) HhFf;

(b)

For the soybean cross HhFf × HhFf, determine the genotypes of the gametes and offspring using a Punnett grid.

[ 2 ]

gametes	HF	Hf	hF	hf
HF	HHFF	HHFf	HhFF	HhFf
Hf	HHFf	HHff	HhFf	Hhff
hF	HhFF	HhFf	hhFF	hhFf
hf	HhFf	Hhff	hhFf	hhff

(c)

Identify the phenotypes of each part of the phenotypic ratio.

[ 2 ]

ratio	phenotypes
9	high oil four seeds;
3	high oil two seeds;
3	low oil four seeds;
1	low oil two seeds;

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Common question formats

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Covered syllabus points

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