An allele is a different version of the same gene. Alleles of a gene are found at the same locus on homologous chromosomes and differ slightly in their base sequence, so they may code for slightly different polypeptides. For example, a pea plant gene for height has an allele for tall and an allele for short. New alleles arise by mutation.A diploid organism has two alleles for each gene, one on each chromosome of the homologous pair.
• Homozygous: both alleles are the same (TT or tt).
• Heterozygous: the two alleles are different (Tt).A dominant allele is always expressed in the phenotype when present, even if only one copy is present. It is shown by a capital letter (T). A recessive allele is only expressed when there is no dominant allele present, that is in the homozygous recessive genotype. It is shown by the lower-case form of the same letter (t).So TT and Tt plants are both tall, and only tt plants are short. A heterozygous individual that carries a recessive allele for a condition without showing it is called a carrier.Some alleles are codominant: both are expressed in the heterozygote. These are covered later in the topic.Worked example: a plant with the genotype Tt is heterozygous, has a tall phenotype, and makes two types of gamete, T and t, in equal numbers.
• Homozygous: both alleles are the same (TT or tt).• Heterozygous: the two alleles are different (Tt).A dominant allele is always expressed in the phenotype when present, even if only one copy is present. It is shown by a capital letter (T). A recessive allele is only expressed when there is no dominant allele present, that is in the homozygous recessive genotype. It is shown by the lower-case form of the same letter (t).So TT and Tt plants are both tall, and only tt plants are short. A heterozygous individual that carries a recessive allele for a condition without showing it is called a carrier.Some alleles are codominant: both are expressed in the heterozygote. These are covered later in the topic.Worked example: a plant with the genotype Tt is heterozygous, has a tall phenotype, and makes two types of gamete, T and t, in equal numbers.