Mendel did the maths before anyone knew DNA existed. The patterns he found are still doing the work.
Every sub-topic below feeds at least one of these questions.
What patterns of inheritance exist in plants and animals?
What is the molecular basis of inheritance patterns?
The required syllabus content for D3.2, in order. Each card is one lesson-sized checkpoint.
They should also understand that a diploid cell has two copies of each autosomal gene.
Use the terms “P generation”, “F1 generation”, “F2 generation” and “Punnett grid”.
Genotype as the combination of alleles inherited by an organism
Phenotype as the observable traits of an organism resulting from genotype and environmental factors
Effects of dominant and recessive alleles on phenotype
Phenotypic plasticity as the capacity to develop traits suited to the environment experienced by an organism, by varying patterns of gene expression
Phenylketonuria as an example of a human disease due to a recessive allele
Single-nucleotide polymorphisms and multiple alleles in gene pools
ABO blood groups as an example of multiple alleles
Incomplete dominance and codominance
Sex determination in humans and inheritance of genes on sex chromosomes
Haemophilia as an example of a sex-linked genetic disorder
Pedigree charts to deduce patterns of inheritance of genetic disorders
Continuous variation due to polygenic inheritance and/or environmental factors
A gene is a region of DNA coding for a particular trait. Alleles are alternative versions of a gene; each individual has two alleles per gene (one from each parent).
Diploid organisms have two copies of each chromosome — one from mum, one from dad — and therefore two alleles per gene. The two alleles can be the same (homozygous) or different (heterozygous). The combination of alleles is the genotype; what's observable is the phenotype.
The phenotype of a heterozygote depends on how the two alleles interact.
The dominant allele's phenotype is expressed in heterozygotes. The recessive allele's phenotype only appears in homozygotes. Conventionally: A (dominant) vs a (recessive).
Both alleles contribute fully — the heterozygote shows both phenotypes. ABO blood group: I^A I^B → blood type AB shows both A and B antigens.
The heterozygote shows an intermediate phenotype between the two homozygotes. (Not usually examined separately in IB but useful context.)
Mendel's classic technique. Predict the genotypes and phenotypes of offspring from any cross — and the expected ratios.
Monohybrid cross of two heterozygotes (Aa × Aa):
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
Genotype ratio: 1 AA : 2 Aa : 1 aa. Phenotype ratio: 3 dominant : 1 recessive. Both expected ratios — actual offspring may differ due to chance, especially with small sample sizes.
If an organism shows the dominant phenotype, it could be homozygous (AA) or heterozygous (Aa). A test cross with a homozygous recessive reveals which.
A pedigree shows trait inheritance across generations. Reading one lets you infer how the trait is inherited (dominant/recessive, autosomal/sex-linked) and predict probabilities for future offspring.
In humans, XX = female, XY = male. Recessive X-linked alleles affect males more often because they only have one X — no second copy to mask the recessive.
Two classic examples:
Clotting factor VIII or IX gene is on the X chromosome. Males with the disease allele have no second copy — they show the disease. Females need two copies (rare); usually females are carriers (X^H X^h).
Photoreceptor genes on X chromosome. ~8% of males vs ~0.5% of females affected — same ratio applies as for any X-linked recessive trait.
ABO is the IB's classic example of multiple alleles with codominance.
Three alleles for one gene: I^A, I^B, i.
Some traits show a continuous range (height); others fall into clear categories (blood group). The pattern reveals how many genes are involved.
Smooth variation across a range. Caused by polygenic inheritance (many genes contributing small effects) plus environmental influences. Produces bell-curve distributions in populations.
Trait falls into distinct categories. Usually controlled by one or a few genes with relatively simple inheritance. Counts produce bar charts, not smooth curves.
An extra 7 sub-topics for HL — same syllabus, deeper mechanism.
Box-and-whisker plots to represent data for a continuous variable such as student height
Segregation and independent assortment of unlinked genes in meiosis
D3.2.17—Punnett grids for predicting genotypic and phenotypic ratios in dihybrid crosses involving pairs of unlinked autosomal genes
Loci of human genes and their polypeptide products
Autosomal gene linkage
Recombinants in crosses involving two linked or unlinked genes
Use of a chi-squared test on data from dihybrid crosses
If you can't define one of these in a sentence, that's where to revise next.
“What are the principles of effective sampling in biological research?”
“What biological processes involve doubling and halving?”