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.
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: IA, IB, i. Codominance: dual (AB). Incomplete dominance: intermediate (pink Mirabilis).
Sex linkage, pedigrees, variation
- Sperm X or Y decides sex, 1 : 1. Haemophilia: Xh, 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. A human egg is haploid. How many alleles of each autosomal gene does a zygote receive from the egg?
- None
- One
- Two
- Four
Show the answer
Answer: B. Meiosis gives a gamete one copy of each autosomal gene, so the zygote receives one allele from the egg and one from the sperm.
Question 2. Which statement explains how the diploid number of chromosomes stays the same from one generation to the next?
- Mitosis halves the number, and meiosis doubles it.
- Gametes are diploid, and the zygote is tetraploid.
- Fertilization halves the number of chromosomes.
- Meiosis halves the number; fertilization restores it.
Show the answer
Answer: D. Meiosis produces haploid gametes, and the fusion of two gametes at fertilization restores the diploid number in the zygote.
Question 3. A woman with genotype Aa and a man with genotype aa have a child. Which statement about the zygote is correct?
- It has one allele from each parent, so it is Aa or aa.
- It has two alleles from the mother, so it is AA or Aa.
- It has one allele in total, so it is either A or a.
- It has two alleles from the father, so it must be aa.
Show the answer
Answer: A. The egg carries either A or a and the sperm carries a, so the zygote receives one allele from each parent and is Aa or aa.
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)
- 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 questions6 data questions5 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.