Unit Overview
Inheritance
CIE A-Level BiologyΒ· 5 min read π n/a
1. 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
Covers the stages of meiosis and how it generates heritable genetic variation
β β β± 10 min
Monohybrid and dihybrid inheritance
Introduces Mendelian inheritance and how to construct and interpret genetic crosses
β β β β± 12 min
Codominance and multiple alleles
Covers non-Mendelian single-gene inheritance with more than two alleles per gene
β β β β± 8 min
Sex linkage
Explores inheritance patterns of genes located on the X and Y sex chromosomes
β β β β± 9 min
Linkage and crossing over
Covers autosomal gene linkage and how crossing over alters inheritance ratios
β β β β β± 11 min
Polygenic inheritance
Explains how multiple interacting genes produce continuous phenotypic variation
β β β± 7 min
2. Common Pitfalls
Wrong move:
Confusing crossing over with linkage
Why:
Students often mix these processes up, leading to incorrect predictions of inheritance ratios
Correct move:
Remember: linked genes are on the same chromosome; crossing over breaks linkage to create recombinant gametes
Wrong move:
Mistaking polygenic inheritance for multiple alleles
Why:
These terms describe very different systems, but are often confused
Correct move:
Remember: multiple alleles = many variants of one gene; polygenic = many different genes control one trait
Wrong move:
Assuming all single-gene traits follow simple dominant/recessive inheritance
Why:
This leads to incorrect phenotypic ratio predictions for non-Mendelian traits
Correct move:
Always confirm allele interaction type (dominance/codominance) before predicting cross outcomes
3. Quick Reference Cheatsheet
Key Concept | Summary |
|---|---|
Meiosis end product | 1 diploid parent cell β 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.