Define an allele as an alternative form of a gene

Definition of an Allele

An allele is defined as an alternative form of a gene that occupies the same position (locus) on a chromosome. Alleles can vary in their sequence of nucleotides, leading to different expressions of a trait. Each individual inherits two alleles for each gene—one from each parent—resulting in various combinations that contribute to an organism’s phenotype (observable characteristics).

Key Characteristics of Alleles

1. Variability:
   • Alleles arise due to mutations in the DNA sequence of a gene. These mutations can lead to changes in the protein produced or affect the regulation of gene expression, resulting in different traits.
2. Types of Alleles:
   • Dominant Alleles: An allele that expresses its trait even when only one copy is present (e.g., in a heterozygous individual). For example, in pea plants, the allele for tall stems (T) is dominant over the allele for short stems (t).
   • Recessive Alleles: An allele that only expresses its trait when two copies are present (homozygous condition). Using the previous example, the short stem trait will only be expressed if an individual has two recessive alleles (tt).
   • Co-dominant Alleles: Both alleles contribute to the phenotype when present together. An example is seen in blood types, where both A and B alleles are co-dominant, resulting in type AB blood.
   • Incomplete Dominance: The heterozygous phenotype is intermediate between the two homozygous phenotypes. For instance, crossing red and white flowers may produce pink flowers.
3. Homozygous and Heterozygous Conditions:
   • An individual can be homozygous for a gene if both alleles are the same (e.g., AA or aa) or heterozygous if the alleles are different (e.g., Aa). The combination of alleles determines how traits are expressed.
4. Genetic Diversity:
   • The presence of multiple alleles for a single gene within a population contributes to genetic diversity, which is essential for evolution and adaptation.