Inheritance and Punnett Squares

This lesson covers genetic inheritance and how to use Punnett squares to predict the outcomes of monohybrid crosses, as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand key genetic terms, construct Punnett squares and interpret the results.

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Key Genetic Terminology

Before studying inheritance, you must know these key terms:

Term	Definition
Gene	A section of DNA that codes for a specific protein
Allele	A version of a gene (e.g. the gene for eye colour has alleles for blue, brown, etc.)
Dominant	An allele that is always expressed when present (shown with a capital letter, e.g. B)
Recessive	An allele that is only expressed when two copies are present (shown with a lowercase letter, e.g. b)
Homozygous	Having two identical alleles for a gene (e.g. BB or bb)
Heterozygous	Having two different alleles for a gene (e.g. Bb)
Genotype	The combination of alleles an organism has (e.g. Bb)
Phenotype	The observable characteristic (e.g. brown eyes)