Biology  by Bradford
IB Biology 2025 · Theme D · Continuity and change

D3.2 Inheritance. Practice questions with markscheme.

76 original IB-style questions on D3.2, written from the 2025 guide: 37 multiple-choice, 23 short-answer, 10 data-based, 3 extended-response part, 2 drawing, 1 labelling. Below is a 20-mark standard-level practice paper built from them, ready to hand out as a class quiz or homework, or to sit yourself and mark against the scheme. Print it, project it, or build a fresh one on the same topic.

What the guide asks for

15 statements at SL and HL, 6 additional higher level.

  1. D3.2.1SL / HL Production of haploid gametes in parents and their fusion to form a diploid zygote as the means of inheritance
  2. D3.2.2SL / HL Methods for conducting genetic crosses in flowering plants
  3. D3.2.3SL / HL Genotype as the combination of alleles inherited by an organism
  4. D3.2.4SL / HL Phenotype as the observable traits of an organism resulting from genotype and environmental factors
  5. D3.2.5SL / HL Effects of dominant and recessive alleles on phenotype
  6. D3.2.6SL / HL Phenotypic plasticity as the capacity to develop traits suited to the environment experienced by an organism, by varying patterns of gene expression
  7. D3.2.7SL / HL Phenylketonuria as an example of a human disease due to a recessive allele
  8. D3.2.8SL / HL Single-nucleotide polymorphisms and multiple alleles in gene pools
  9. D3.2.9SL / HL ABO blood groups as an example of multiple alleles
  10. D3.2.10SL / HL Incomplete dominance and codominance
  11. D3.2.11SL / HL Sex determination in humans and inheritance of genes on sex chromosomes
  12. D3.2.12SL / HL Haemophilia as an example of a sex-linked genetic disorder
  13. D3.2.13SL / HL Pedigree charts to deduce patterns of inheritance of genetic disorders
  14. D3.2.14SL / HL Continuous variation due to polygenic inheritance and/or environmental factors
  15. D3.2.15SL / HL Box-and-whisker plots to represent data for a continuous variable such as student height
  16. D3.2.16HL Segregation and independent assortment of unlinked genes in meiosis
  17. D3.2.17HL Punnett grids for predicting genotypic and phenotypic ratios in dihybrid crosses involving pairs of unlinked autosomal genes
  18. D3.2.18HL Loci of human genes and their polypeptide products
  19. D3.2.19HL Autosomal gene linkage
  20. D3.2.20HL Recombinants in crosses involving two linked or unlinked genes
  21. D3.2.21HL Use of a chi-squared test on data from dihybrid crosses

In the bank for D3.2

  • 37 multiple-choice
  • 23 short-answer
  • 10 data-based
  • 3 extended-response part
  • 2 drawing
  • 1 labelling
  • 18 higher level only

Every question is original and tagged to a guide statement. See the whole bank →

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The practice paper

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Biology · topic quiz
Standard level · topic practice, not an exam format
30 minutes20 marks

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Covers D3.2 Inheritance
Name:
1.

In the pedigree, I-1 (XᴴY) and I-2 (XᴴXʰ, a known carrier) have an unaffected daughter, II-1. What is the probability that II-1 is a carrier of the haemophilia allele?

[1]
I II 12 1342 5 affected male known carrier female
  1. 0
  2. ¼
  3. ½
  4. 1
2.

In four o'clock plants (Mirabilis jalapa), flower colour shows incomplete dominance: red-flowered (CRCR) crossed with white-flowered (CWCW) produces pink-flowered (CRCW) offspring. What phenotypic ratio is expected among the offspring of a cross between two pink-flowered plants?

[1]
  1. 1 red : 2 pink : 1 white
  2. 3 pink : 1 white
  3. 1 red : 1 white
  4. All offspring pink
3.

Phenylketonuria (PKU) is caused by a recessive allele of the gene for the enzyme that converts phenylalanine to tyrosine. Babies diagnosed at birth and given a special diet develop normally. What does this show?

[1]
  1. PKU is not really a genetic disease at all
  2. Phenotype depends on diet as well as genotype
  3. The dietary treatment corrects the allele
  4. One functional allele cannot prevent disease
4.

The ABO blood group gene has three alleles, IA, IB and i, but any one person carries only two of them. A man with genotype IAi and a woman with genotype IBi have a child. What is the probability that the child has blood group O?

[1]
  1. 0
  2. ¼
  3. ½
  4. 1
5.

In sexual reproduction, how does an offspring inherit its genes?

[1]
  1. It receives all of the alleles of both parents
  2. It receives half of one parent's genome only
  3. Chromosomes pass straight from the grandparents
  4. One allele of each gene comes from each parent
6.

The pedigree chart shows the inheritance of haemophilia, an X-linked recessive condition. What is the genotype of individual I-1, an unaffected male?

[1]
I II 12 1342 5 affected male known carrier female
  1. XᴴY
  2. XʰY
  3. XᴴXʰ
  4. XᴴXᴴ
7.

In cats, coat colour is X-linked with codominant alleles Xᴼ (orange) and Xᴮ (black); heterozygous females (XᴼXᴮ) are tortoiseshell. A tortoiseshell female is crossed with an orange male. Predict the coat colours and their proportions among the male and the female offspring.

[4]
8.

Outline what is meant by phenotypic plasticity, using an example.

[2]
9.

Describe how a controlled genetic cross between two varieties of pea plant is carried out.

[3]
10.

"An organism's genes determine its phenotype." Discuss this claim, using examples of single-gene, polygenic and environmentally influenced characteristics.

[5]

Original practice questions © Biology by Bradford · CC BY-NC-SA 4.0 · Not affiliated with or endorsed by the International Baccalaureate Organization.
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Markscheme BbB-EAAAAAAACBQAO8xB

One mark per point; / separates alternative wording within a point, OR separates alternative answers, words in brackets are not required, underlined words are essential. OWTTE = or words to that effect.

1. D3.2.12, D3.2.13 [1]
  • C: II-1 always receives Xᴴ from her father; from her mother she receives Xᴴ or Xʰ with equal probability, so there is a ½ chance she is XᴴXʰ;
2. D3.2.10 [1]
  • A — CRCW × CRCW gives genotypes CRCR : CRCW : CWCW in a 1:2:1 ratio, and because heterozygotes are pink (intermediate), the phenotypic ratio is 1 red : 2 pink : 1 white;
3. D3.2.7 [1]
  • B: the genotype is unchanged, but keeping dietary phenylalanine low prevents the harmful phenotype, a genotype–environment interaction (heterozygotes with one functional allele are healthy);
4. D3.2.9 [1]
  • B — cross IAi × IBi gives offspring genotypes IAIB, IAi, IBi, ii in a 1:1:1:1 ratio; only ii (¼ of offspring) has blood group O;
5. D3.2.1 [1]
  • D: haploid gametes fuse to form the diploid zygote, which has two alleles of every gene, one maternal and one paternal;
6. D3.2.11, D3.2.12 [1]
  • A: males are hemizygous, so an unaffected male carries a single copy of the dominant allele on his one X chromosome;
7. D3.2.11 [4]
  • mother XᴼXᴮ produces eggs carrying Xᴼ or Xᴮ; father XᴼY produces sperm carrying Xᴼ or Y;
  • daughters all receive Xᴼ from the father, so ½ are XᴼXᴼ (orange) and ½ are XᴼXᴮ (tortoiseshell);
  • sons receive Y from the father and one X from the mother, so ½ are XᴼY (orange) and ½ are XᴮY (black);
  • no sons are tortoiseshell, because tortoiseshell requires two different X alleles (heterozygosity), which a male's single X cannot provide, OWTTE;

Heterozygous (tortoiseshell) female correctly XᴼXᴮ.

8. D3.2.6 [2 max]
  • the capacity of an organism to develop/change traits suited to its environment;
  • without any change in genotype (and the changes may be reversible);
  • suitable example: skin darkening with sun exposure / increased red blood cell count at altitude / muscle growth with exercise / sun and shade leaves differing in shape, OWTTE;
9. D3.2.2 [3 max]
  • true-breeding parental (P) varieties are chosen (for the trait studied);
  • anthers are removed from the flowers of one parent before they shed pollen, preventing self-pollination;
  • pollen from the other (chosen) parent is transferred to the stigma (with a brush) and the flower is covered/bagged to exclude any other pollen;
  • the seeds are grown as the F1 generation and the phenotypes of large numbers of offspring are counted (the F1 can then be crossed/self-pollinated to give an F2), OWTTE;
10. D3.2.14 [5 max]
  • for: some characteristics are determined (almost) entirely by genotype, e.g. ABO blood group, where environment has no (practical) influence;
  • many characteristics are polygenic: many genes each contribute a small (additive) effect, producing continuous variation (e.g. height, skin colour);
  • against: the environment also affects phenotype, e.g. nutrition influences height / sunlight affects skin colour / temperature or soil conditions affect plant growth;
  • (so) phenotype results from the interaction of genotype and environment, the genotype sets a potential range and the environment determines what is expressed within it;
  • evidence: genetically identical individuals (identical twins, clones) can differ in phenotype, differences that can only be environmental;
  • (judgement) the claim is an over-simplification: accurate for some discrete single-gene traits, but incomplete for most characteristics, OWTTE;

Discuss: require at least one point supporting and one challenging the claim, plus a judgement.

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