Alleles (Edexcel IGCSE Biology (Modular)): Revision Note

Alleles

• A gene is a section of a molecule of DNA that codes for a specific protein

• Alleles are alternative forms (versions) of the same gene

  • Alleles have small differences in their DNA base sequences, which lead to different versions of a characteristic

• Humans, like most mammals, have two copies of each chromosome (one from each parent). Therefore, humans have two copies of each gene and therefore two alleles for each gene

• One allele is inherited from the mother and the other from the father

• For example, the ability to taste the chemical phenylthiocarbamide (PTC) is controlled by a single gene with two alleles - one for being able to taste PTC (tasters) and one for not tasting it (non-tasters)

  • PTC itself isn't found in food, but it tastes similar to bitter compounds naturally present in vegetables such as cabbage, kale and Brussels sprouts, which some people find more bitter than others

• Having alleles give rise to differences in inherited characteristics and variation within species