# Inheritance

> CIE A-Level Biology · CIE A-Level 9700
> Source: https://www.owlsprep.com/study/cie-9700-u17-overview/
> Weight: n/a

This unit explores how genetic information is passed from parents to offspring, covering meiosis, Mendelian and non-Mendelian inheritance patterns that underpin all biological variation.

**Prerequisites:** Understanding of DNA structure and gene function (Unit 16); Knowledge of mitotic cell division

## Learning objectives

- Explain how meiosis generates genetic variation required for sexual inheritance
- Construct and interpret genetic crosses for monohybrid, dihybrid and multiple allele systems
- Predict inheritance patterns for sex-linked and autosomal linked traits
- Distinguish between different types of inheritance and their phenotypic outcomes

## Unit at a Glance

This unit starts with the cellular foundation of inheritance: meiosis, the process that produces haploid gametes and generates genetic variation through independent assortment and crossing over. We then build progressively from simple Mendelian inheritance patterns to more complex non-Mendelian systems.

Each sub-topic adds a new layer of complexity, starting with single-gene Mendelian crosses, moving to multiple alleles, sex-linked traits, linked genes, and finally traits controlled by multiple genes. This progression builds the core genetics knowledge needed for later topics in evolution and biotechnology.

This unit covers the following sub-topics:
- [Meiosis](https://www.owlsprep.com/study/cie-9700-u17-meiosis/) — Covers the stages of meiosis and how it generates heritable genetic variation
- [Monohybrid and dihybrid inheritance](https://www.owlsprep.com/study/cie-9700-u17-monohybrid-and-dihybrid-inheritance/) — Introduces Mendelian inheritance and how to construct and interpret genetic crosses
- [Codominance and multiple alleles](https://www.owlsprep.com/study/cie-9700-u17-codominance-and-multiple-alleles/) — Covers non-Mendelian single-gene inheritance with more than two alleles per gene
- [Sex linkage](https://www.owlsprep.com/study/cie-9700-u17-sex-linkage/) — Explores inheritance patterns of genes located on the X and Y sex chromosomes
- [Linkage and crossing over](https://www.owlsprep.com/study/cie-9700-u17-linkage-and-crossing-over/) — Covers autosomal gene linkage and how crossing over alters inheritance ratios
- [Polygenic inheritance](https://www.owlsprep.com/study/cie-9700-u17-polygenic-inheritance/) — Explains how multiple interacting genes produce continuous phenotypic variation

## Common pitfalls

- **Wrong:** Confusing crossing over with linkage
  - Why it fails: Students often mix these processes up, leading to incorrect predictions of inheritance ratios
  - Correct: Remember: linked genes are on the same chromosome; crossing over breaks linkage to create recombinant gametes
- **Wrong:** Mistaking polygenic inheritance for multiple alleles
  - Why it fails: These terms describe very different systems, but are often confused
  - Correct: Remember: multiple alleles = many variants of one gene; polygenic = many different genes control one trait
- **Wrong:** Assuming all single-gene traits follow simple dominant/recessive inheritance
  - Why it fails: This leads to incorrect phenotypic ratio predictions for non-Mendelian traits
  - Correct: Always confirm allele interaction type (dominance/codominance) before predicting cross outcomes

## Cheatsheet

| Key Concept | Summary |
| --- | --- |
| Meiosis end product | 1 diploid parent cell → 4 genetically distinct haploid daughter cells |
| Monohybrid ratio (complete dominance) | 3 dominant : 1 recessive phenotype |
| Dihybrid ratio (independent assortment) | 9 : 3 : 3 : 1 phenotype ratio |
| Codominance | Both alleles are fully expressed in the heterozygous phenotype |
| X-linked recessive traits | More common in males, who only have one X chromosome |
| Polygenic inheritance | Produces continuous phenotypic variation, often influenced by environment |

## What's next

Start with the first sub-topic of this unit, Meiosis, to build your understanding of the cellular basis of genetic variation and inheritance before moving on to solving genetic crosses. Once you complete all sub-topics in this unit, you will progress to the next unit on Selection and Evolution, which builds directly on this unit's concepts of variation and inheritance.

- [Meiosis (First sub-topic of this unit)](https://www.owlsprep.com/study/cie-9700-u17-meiosis/)
- [Monohybrid and dihybrid inheritance](https://www.owlsprep.com/study/cie-9700-u17-monohybrid-and-dihybrid-inheritance/)
- [Codominance and multiple alleles](https://www.owlsprep.com/study/cie-9700-u17-codominance-and-multiple-alleles/)

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