Genetic Crosses: Monohybrid Inheritance

This lesson covers monohybrid inheritance — the inheritance of a single gene with two alleles — as required by the Edexcel A-Level Biology specification (9BI0, Topic 8). You need to be able to construct genetic diagrams, interpret results and use appropriate terminology.

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Key Genetic Terminology

Term	Definition
Gene	A section of DNA that codes for a polypeptide
Allele	An alternative form of a gene at a given locus
Locus	The specific position of a gene on a chromosome
Dominant	An allele that is expressed in both homozygous and heterozygous individuals
Recessive	An allele that is only expressed in homozygous individuals
Homozygous	Having two identical alleles at a locus (e.g. AA or aa)
Heterozygous	Having two different alleles at a locus (e.g. Aa)
Genotype	The combination of alleles an organism has
Phenotype	The observable characteristics of an organism (determined by genotype and environment)
Carrier	A heterozygous individual who carries one copy of a recessive allele without showing the phenotype