IB Biology · Theme D: Continuity and change · SL and HL

D3.2 Inheritance

A one-page summary of D3.2 Inheritance, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.

Open the D3.2 revision slides Practise D3.2 in the app

Guiding questions

What patterns of inheritance exist in plants and animals?

What is the molecular basis of inheritance patterns?

What D3.2 covers

From gametes to phenotype

D3.2.1Haploid gametes fuse to make a diploid zygote
D3.2.2A genetic cross needs pollination
D3.2.2P, F1, F2 and the Punnett grid
D3.2.3Genotype is the combination of alleles an organism inherits
D3.2.4Phenotype comes from genotype and environment
D3.2.5One dominant allele is enough to show the dominant phenotype
D3.2.6Phenotypic plasticity: gene expression changes, the DNA does not

Patterns of inheritance

D3.2.7PKU: a recessive condition caused by a faulty enzyme
D3.2.8Many alleles can exist in a gene pool, but a person has two
D3.2.9ABO blood groups: three alleles, four groups
D3.2.10Incomplete dominance gives an intermediate phenotype
D3.2.10Codominance gives a dual phenotype

Sex linkage, pedigrees and variation

D3.2.11The sperm’s sex chromosome decides the zygote’s sex
D3.2.12Haemophilia: a sex-linked recessive disorder
D3.2.13A pedigree chart follows a disorder through generations
D3.2.13Induction finds the pattern; deduction finds the genotype
D3.2.14Continuous variation comes from many genes and the environment
D3.2.15A box-and-whisker plot shows six features of the data

HL — dihybrid crosses, linkage and statistics

D3.2.16 · HLChromosomes in meiosis explain the ratios of a dihybrid cross
D3.2.17 · HLA dihybrid cross gives 9: 3: 3: 1
D3.2.17 · HLA testcross gives 1: 1: 1: 1, and Mendel’s law has exceptions
D3.2.18 · HLEvery gene has a locus and a polypeptide product
D3.2.19 · HLLinked genes tend to be inherited together
D3.2.20 · HLRecombinants show that crossing over happened
D3.2.21 · HLThe chi-squared test asks whether the differences are due to chance

D3.2 Inheritance: summary

Gametes and crosses

Haploid gametes fuse to a diploid zygote: one allele from each parent. Cross: P, F1, F2, Punnett grid. Monohybrid F2: 3 : 1 phenotypes, 1 : 2 : 1 genotypes.

Genotype and phenotype

Genotype = alleles (homozygous or heterozygous). Phenotype = genotype + environment. Dominant: one copy is enough. Plasticity changes gene expression, not DNA, and can be reversible.

Patterns of inheritance

PKU: recessive, faulty enzyme. Multiple alleles: many in the gene pool, two per person. ABO: I^A, I^B, i. Codominance: dual (AB). Incomplete dominance: intermediate (pink Mirabilis).

Sex linkage, pedigrees, variation

Sperm X or Y decides sex, 1 : 1. Haemophilia: X^h, males need one copy. Pedigree: induce the pattern, deduce genotypes. Continuous = polygenic + environment. Box plot: min, Q1, median, Q3, max, outliers (1.5 × IQR).

HL · Meiosis and dihybrid crosses

Segregation and independent assortment. RrYy × RrYy gives 9 : 3 : 3 : 1; testcross gives 1 : 1 : 1 : 1. Mendel's second law fails for linked genes.

HL · Linkage and statistics

Every gene has a locus. Recombinants come from crossing over: 50% if unlinked, fewer if linked. χ² = Σ (O − E)² ÷ E; compare with the critical value at p = 0.05; H₀ is not proved.

Key terms

P, F1 and F2: The parental generation, the first filial generation (their offspring) and the second (the offspring of two F1 individuals).
Punnett grid: A table that combines all the possible gametes of two parents to predict offspring genotypes and phenotypes.
Genotype: The combination of alleles that an organism has inherited for a gene.
Phenotype: The observable traits of an organism, resulting from genotype and environmental factors.
Homozygous: Having two identical alleles of a gene.
Heterozygous: Having two different alleles of a gene.
Dominant allele: An allele with the same effect on the phenotype in a homozygote and a heterozygote.
Recessive allele: An allele that has an effect on the phenotype only in a homozygote.
Phenotypic plasticity: The capacity to develop traits suited to the environment, by varying patterns of gene expression.
SNP: A single-nucleotide polymorphism: a difference of one base in the DNA sequence between individuals.
Gene pool: All the genes and their different alleles in a population.
Codominance: The heterozygote has a dual phenotype, with both alleles expressed at once.
Incomplete dominance: The heterozygote has an intermediate phenotype between those of the two homozygotes.
Sex-linked gene: A gene on a sex chromosome; most are on the X chromosome.
Carrier: A heterozygous individual who has a recessive allele but shows no sign of the condition.
Pedigree chart: A family tree that follows a genetic condition through generations, using standard symbols.
Continuous variation: Variation with a smooth range of values, due to polygenic inheritance and/or the environment.
Interquartile range: Q3 minus Q1: the spread of the middle half of the data, used to identify outliers.
Segregation: The separation of the two alleles of a gene into different gametes in meiosis.
Independent assortment: The random orientation of each pair of homologous chromosomes, so unlinked genes are sorted independently.
Dihybrid cross: A cross that follows two genes at once.
Locus: The position of a gene on a chromosome.
Linked genes: Genes close together on the same chromosome, which tend to be inherited together.
Recombinant: A gamete, genotype or phenotype with a combination of alleles that differs from the parental combinations.
Null hypothesis: The statement that there is no significant difference between observed and expected results.
Chi-squared test: A statistical test comparing observed results with expected results, using χ² = Σ (O − E)² ÷ E.
Degrees of freedom: The number of categories minus 1, used to find the critical value.

Sample exam questions

Three of the 79 multiple-choice questions for D3.2. Try each one before opening the answer.

Question 1. In humans, a diploid somatic cell contains two copies of each autosomal gene. What is the direct origin of these two copies?

Both copies from the mother's egg only
One from the haploid gamete of each parent, fused at fertilization
One copy created by mitosis after fertilization
Both copies from a single gamete that failed to reduce chromosome number

Show the answer

Answer: B. Meiosis produces haploid gametes; fertilization fuses one from each parent, restoring the diploid number with two copies of each autosomal gene.

Question 2. Which statement best describes why the diploid-to-haploid-to-diploid pattern of inheritance is described as common to eukaryotes with a sexual life cycle?

It only applies to animals, not plants or fungi
It applies only to organisms with an XY sex-determination system
It applies only when self-fertilization occurs
Meiosis (halving) and fertilization (restoring) alternate in every generation of any sexually reproducing eukaryote

Show the answer

Answer: D. The alternation of meiosis and fertilization is the shared underlying mechanism of inheritance across all sexually reproducing eukaryotes, regardless of taxon.

Question 3. Two parents with blood groups A (genotype I^A i) and B (genotype I^B i) have a child. Which blood groups are possible for the child?

A and B only
AB only
A, B, AB and O
A, B and AB only

Show the answer

Answer: C. I^A i × I^B i can give I^A I^B (group AB), I^A i (group A), I^B i (group B) or ii (group O), each with a probability of 1/4.

Linking questions

Questions that connect D3.2 to other parts of the course, the kind that come up in Paper 2.

Meiosis is a reduction division and a source of variation (D2.1.9, D2.1.11). Explain how meiosis and the fusion of gametes together give a diploid zygote one allele of each autosomal gene from each parent, and name two biological processes in a sexual life cycle that involve halving and doubling the chromosome number. (see D2.1)
Single-nucleotide polymorphisms result from base substitution mutations (D1.3.2), and a gene pool consists of all the genes and their different alleles present in a population (D4.1.9). Explain how a base substitution can create a new allele, and why an individual can carry only two of the alleles present in a gene pool. (see D1.3, D4.1)
Phenotypic plasticity is due to varying patterns of gene expression, not to a change of genotype (D3.2.6). Using an example of an environmental effect on gene expression (D2.2.8), explain how one genotype can produce different phenotypes and why the change may be reversible. (see D2.2)
What are the principles of effective sampling in biological research? Discuss how the size and randomness of a sample affect how well it represents a population (C4.1.2), using the offspring counted from a genetic cross as an example. (see C4.1)

Practise D3.2

79 quiz questions8 data questions7 exam questionsmarkschemes included

Study notes, every question and full markschemes for D3.2 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.

Practise D3.2 in the app Revision slides