# Inheritance

> CIE IGCSE Biology · CIE IGCSE Biology (0610)
> Source: https://www.owlsprep.com/study/cie-0610-u12-overview/
> Weight: 12-15% of both core and extended exam papers

This unit covers core genetic principles from the molecular basis of inheritance to predicting trait transmission across generations, a foundational topic for advanced biology and applied genetics fields.

**Prerequisites:** [Cell structure and organisation (Unit 2)](https://www.owlsprep.com/study/cie-0610-u02-overview/)

## Learning objectives

- Describe the relationship between chromosomes, genes, alleles and DNA, and explain how protein synthesis controls trait expression
- Compare mitosis and meiosis, and evaluate their respective roles in growth, asexual reproduction and sexual reproduction (extended only)
- Solve monohybrid cross problems including special cases of codominance, ABO blood grouping, and sex-linked inheritance
- Interpret pedigree charts to predict the probability of genetic traits being passed to offspring

## 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](https://www.owlsprep.com/study/cie-0610-u12-chromosomes-genes-alleles-and-protein/) — Explains the structure of chromosomes, definitions of genes and alleles, and the basic process of protein synthesis that drives trait expression.
- [Mitosis and Meiosis (Extended Only)](https://www.owlsprep.com/study/cie-0610-u12-mitosis-and-meiosis/) — Compares the processes, locations and outcomes of mitosis and meiosis, and their distinct biological roles in living organisms.
- [Monohybrid Inheritance; Codominance, ABO and Sex Linkage](https://www.owlsprep.com/study/cie-0610-u12-monohybrid-inheritance-codominance-abo-and/) — Teaches construction and interpretation of Punnett squares for monohybrid crosses, including codominance, ABO blood groups and sex-linked traits.

## Common pitfalls

- **Wrong:** Stating meiosis produces diploid daughter cells, mixing it up with mitosis outcomes
  - Why it fails: Both processes start with DNA replication, but meiosis involves two division rounds rather than one
  - Correct: Remember mitosis produces 2 identical diploid body cells, meiosis produces 4 genetically distinct haploid gametes
- **Wrong:** Using lowercase letters for codominant alleles in cross problems
  - Why it fails: Codominant alleles are both fully expressed when present, so they do not follow dominant-recessive notation rules
  - Correct: Use uppercase letters for the gene, with different superscripts for each codominant allele (e.g. $I^A$ for the blood group A allele)
- **Wrong:** Assuming sex-linked recessive traits affect males and females at equal rates
  - Why it fails: Males only carry one X chromosome, so they only need one copy of the recessive allele to express the trait
  - Correct: Use X/Y notation for sex-linked crosses, with alleles written as superscripts only on the X chromosome

## 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 → mRNA copy → ribosome joins amino acids → 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 → 2n, 2 identical diploid daughter cells | Mitosis and Meiosis | Used for growth, tissue repair, and asexual reproduction |
| Meiosis: 2n → 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.

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/cie-0610-u12-overview/