D3.2 Inheritance
Inheritance explains how alleles, chromosomes, meiosis, pedigrees, linkage, variation and statistical tests predict genetic outcomes across generations in inheritance problems.
- Syllabus
- First assessment 2025
- Topic
- D3.2
- Level
- SL
Exam analysis
Chance of appearing48%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1
Typical marks1–2
Most tested objectives
Common question formats
- Definition or recall
- Structured response
- Diagram interpretation
- Data analysis
- Calculation
- Graph interpretation
- Comparison
Recent exam appearances
November 2025Paper1B ["SL"] · TZ34(d)[ 3 ]D3.2.9—ABO blood groups
November 2025Paper1A ["SL"] · TZ120[ 1 ]D3.2.13—Pedigree charts
May 2025Paper2 ["SL"] · TZ15(c)[ 3 ]D3.2.5—Dominant and recessive alleles
May 2025Paper2 ["SL"] · TZ34(b)(ii)[ 2 ]D3.2.13—Pedigree charts
May 2025Paper2 ["SL"] · TZ34(b)(i)[ 1 ]D3.2.13—Pedigree charts
Practice this topic
Coverage 2010–2025 · Updated 16 Jul 2026
Objective notes
15 learning objectives• Haploid gametes fuse during fertilization to form a diploid zygote
• Diploid organisms usually carry two alleles for each autosomal gene
• Genetic crosses track parental, F1, and F2 generations using Punnett grids
• Flowering plant crosses control pollen transfer to study inheritance ratios
• Genotype is the allele combination inherited for a gene or genes
• Homozygous genotypes have matching alleles; heterozygous genotypes have different alleles
• Phenotype is the observable characteristic or trait
• Phenotype can be determined by genotype, environment, and their interaction
• Dominant alleles are expressed in heterozygotes
• Recessive alleles are expressed when no dominant allele masks them
• Phenotypic plasticity is environment-driven phenotype change without genotype change
• It depends on altered gene expression and can be adaptive
• PKU is an autosomal recessive disorder affecting phenylalanine metabolism
• Low-phenylalanine diet and newborn screening reduce harmful effects
• SNPs are single-base differences that can create different alleles
• Populations can have multiple alleles, but diploid individuals carry at most two
• ABO blood group is controlled by IA, IB, and i alleles
• IA and IB are codominant; i is recessive, producing four blood phenotypes
• Codominance expresses both heterozygous alleles, as in AB blood type
• Incomplete dominance gives an intermediate heterozygote phenotype
• Human chromosomal sex is usually determined by XX or XY chromosome combination
• The SRY/TDF region on the Y chromosome directs testis development
• Haemophilia is an X-linked recessive blood-clotting disorder
• Carrier females and affected males are represented with X-linked allele notation
• Pedigree charts show family inheritance across generations
• Patterns help infer autosomal dominant, autosomal recessive, or sex-linked inheritance
• Continuous variation often results from polygenic inheritance plus environment
• Human skin colour, height, and body mass show many intermediate phenotypes
• Box-and-whisker plots summarize non-normal continuous data
• They show median, quartiles, interquartile range, maximum/minimum, and outliers