Inheritance — IB Diploma Biology SL
IB Biology D3.2: genetic crosses, dominance, PKU, ABO blood groups, sex linkage, pedigrees and box plots.
Revision notes
7 short notes, in the order of the specification. Each one in short:
Inheritance in all eukaryotes with a sexual life cycle works by parents producing haploid gametes that fuse to form a diploid zygote, which has two copies of each autosomal gene. Genetic crosses in flowering plants transfer pollen, which contains the male gametes, to the stigma of another plant, and follow the P, F1 and F2 generations using Punnett grids.
Genotype is the combination of alleles inherited by an organism, and phenotype is the observable traits resulting from genotype and environmental factors. A dominant allele has the same effect in one or two copies, so homozygous-dominant and heterozygous individuals look alike. Phenotypic plasticity is the capacity to develop traits suited to the environment by varying gene expression.
Phenylketonuria (PKU) is a recessive genetic condition caused by mutation in an autosomal gene for the enzyme that converts phenylalanine to tyrosine. Single-nucleotide polymorphisms create many alleles of a gene in a gene pool, but each individual inherits only two. ABO blood groups are an example of multiple alleles: Iᴬ, Iᴮ and i.
In incomplete dominance, heterozygotes have an intermediate phenotype, such as pink four o'clock flowers from red and white parents. In codominance, heterozygotes have a dual phenotype, with both alleles fully expressed, such as blood group AB from Iᴬ and Iᴮ. Both differ from complete dominance, where heterozygotes look like the homozygous dominant.
In humans the sex chromosome in the sperm, X or Y, determines whether a zygote develops male-typical or female-typical characteristics. The X chromosome carries far more genes than the Y, so males have only one copy of most X-linked genes. Haemophilia is a sex-linked recessive disorder, so it is much more common in males.
A pedigree chart shows the inheritance of a trait through several generations of a family, with squares for males, circles for females and shading for affected individuals. Patterns in the chart allow deduction of whether a disorder is dominant, recessive or sex-linked, and then the genotypes of particular individuals.
Continuous variation is variation in which a trait can take any value in a range, due to polygenic inheritance and/or environmental factors, such as skin colour or height. Discrete variables, such as ABO blood group, fall into separate classes. A box-and-whisker plot shows a continuous variable's minimum, quartiles, median, maximum and outliers.
Exam questions
5 exam-style questions (16 marks), each with its mark scheme.
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22 cards: flip them, mark what you knew, and practise the rest.
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Quick check questions
Why do AA and Aa individuals have the same phenotype?
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One dominant allele produces enough functional protein for the trait to appear.
Give the genotype of a person with blood group O.
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ii.
What phenotype does a heterozygote show in incomplete dominance?
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An intermediate phenotype, e.g. pink four o'clock flowers.
State the outlier rule for a box-and-whisker plot.
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More than 1.5 × IQR above Q3 or below Q1.
Frequently asked questions
What is the difference between genotype and phenotype?
Genotype is the combination of alleles an organism inherits, for example Aa. Phenotype is the observable traits of the organism, which result from its genotype and environmental factors. Some traits depend on genotype only, such as ABO blood group; others, such as height, depend on genes interacting with the environment.
What is the difference between codominance and incomplete dominance?
In codominance the heterozygote has a dual phenotype, with both alleles fully expressed, as in blood group AB with A and B antigens. In incomplete dominance the heterozygote has an intermediate phenotype, as in pink four o'clock flowers from red and white parents. Both give a 1:2:1 phenotype ratio in the F2.
Why are sex-linked disorders more common in males?
Sex-linked disorders such as haemophilia are caused by recessive alleles on the X chromosome. Males have only one X chromosome, so a single recessive allele is expressed. Females have two X chromosomes and are affected only if both carry the recessive allele, so they are usually unaffected carriers.
How do you identify an outlier in a box-and-whisker plot?
Calculate the interquartile range, IQR = Q3 − Q1. A data point is an outlier if it is more than 1.5 × IQR above the third quartile or more than 1.5 × IQR below the first quartile. Outliers are plotted as separate points and the whiskers end at the most extreme non-outlier values.
Written and checked against the IB Biology SL specification · Updated October 2026