Key Terms in Genetics (Edexcel IGCSE Biology: Double Science)

• The observable characteristics of an organism are called the phenotype
• The combination of alleles that control each characteristic is called the genotype
  • Alleles are often given a lettered code such as Bb or BB or bb to easily represent the genotype. 
• Alleles can be dominant or recessive
  • A dominant allele only needs to be inherited from one parent for the characteristic to show up in the phenotype
  • A recessive allele needs to be inherited from both parents for the characteristic to show up in the phenotype.
  • If there is only one recessive allele, it will remain hidden and the dominant characteristic will show
• If the two alleles of a gene are the same, we describe the individual as being homozygous (homo = same)
  • An individual could be homozygous dominant (having two copies of the dominant allele), or homozygous recessive (having two copies of the recessive allele)
• If the two alleles of a gene are different, we describe the individual as being heterozygous (hetero = different)
• When completing genetic diagrams, alleles are abbreviated to single letters
  • The dominant allele is given a capital letter and the recessive allele is given the same letter, but lower case e.g.
  • Homozygous dominant BB
  • Heterozygous dominant Bb
  • Homozygous recessive bb

Alleles are different forms of the same gene. You can only inherit two alleles for each gene, and they can be the same (homozygous) or different (heterozygous).

Key terms & definitions for genetic inheritance table