Unit Overview
Inheritance
CIE IGCSE BiologyΒ· 5 min read π 12-15% of both core and extended exam papers
1. Unit at a glance
We begin with the molecular building blocks of inheritance: DNA, chromosomes, genes and alleles, and how these code for the proteins that determine observable traits in living organisms. We then explore two types of nuclear division: mitosis for growth and asexual reproduction, and meiosis for gamete production in sexual reproduction (extended only content).
Finally, we apply these foundational concepts to solve monohybrid cross problems, including special cases like codominance, ABO blood grouping, and sex-linked inheritance, as well as interpret pedigree charts to trace trait transmission across generations.
Work through the subtopics in the order listed below to build your understanding sequentially:
Chromosomes, Genes, Alleles and Protein Synthesis
Explains the structure of chromosomes, definitions of genes and alleles, and the basic process of protein synthesis that drives trait expression.
β β β± 7 min
Mitosis and Meiosis (Extended Only)
Compares the processes, locations and outcomes of mitosis and meiosis, and their distinct biological roles in living organisms.
β β β β β± 8 min
Monohybrid Inheritance; Codominance, ABO and Sex Linkage
Teaches construction and interpretation of Punnett squares for monohybrid crosses, including codominance, ABO blood groups and sex-linked traits.
β β β β± 10 min
2. Common Pitfalls
Wrong move:
Stating meiosis produces diploid daughter cells, mixing it up with mitosis outcomes
Why:
Both processes start with DNA replication, but meiosis involves two division rounds rather than one
Correct move:
Remember mitosis produces 2 identical diploid body cells, meiosis produces 4 genetically distinct haploid gametes
Wrong move:
Using lowercase letters for codominant alleles in cross problems
Why:
Codominant alleles are both fully expressed when present, so they do not follow dominant-recessive notation rules
Correct move:
Use uppercase letters for the gene, with different superscripts for each codominant allele (e.g. for the blood group A allele)
Wrong move:
Assuming sex-linked recessive traits affect males and females at equal rates
Why:
Males only carry one X chromosome, so they only need one copy of the recessive allele to express the trait
Correct move:
Use X/Y notation for sex-linked crosses, with alleles written as superscripts only on the X chromosome
3. Quick Reference Cheatsheet
Key Concept/Rule | Subtopic Reference | Quick Note |
|---|---|---|
Genotype = allele combination for a trait; Phenotype = observable physical trait | Chromosomes, Genes, Alleles | Phenotype is determined by genotype + environmental factors |
Protein synthesis (outline): a gene's base sequence β mRNA copy β ribosome joins amino acids β protein | Chromosomes, Genes, Alleles | The gene stays in the nucleus; mRNA carries the copy to a ribosome in the cytoplasm (details not required) |
Mitosis: 2n β 2n, 2 identical diploid daughter cells | Mitosis and Meiosis | Used for growth, tissue repair, and asexual reproduction |
Meiosis: 2n β n, 4 distinct haploid daughter cells | Mitosis and Meiosis | Used exclusively for gamete production for sexual reproduction |
Heterozygous monohybrid cross ratio: 3 dominant : 1 recessive phenotype | Monohybrid Inheritance | Only applies to cases of complete dominance |
Male sex chromosomes = XY, Female sex chromosomes = XX | Monohybrid Inheritance | Sex-linked alleles are only carried on the X chromosome |
What's Next
Start your learning in this unit with the foundational molecular genetics subtopic, which sets the stage for understanding all subsequent inheritance concepts. Mastering the content in this unit is critical before moving on to the next unit on variation and natural selection, which builds directly on the genetic principles covered here.