What are Alleles?
Alleles are corresponding pairs of genes located at specific positions in the chromosomes. Together, alleles determine the genotype of their host organism. For example, the alleles for eye color are found on chromosomes 15 and 19, and depending on which alleles someone has, he or she may have blue, brown, green, gray, or hazel eyes, and sometimes a mixture of these traits is present. Alleles which determine some aspect of the phenotype, the physical appearance of an organism, are said to be “coding alleles,” while “non-coding alleles” or “junk DNA” are alleles which do not appear to have an impact on phenotype. There are numerous combinations of alleles, ranging from simple pairs to complex relationships between multiple alleles which determine someone’s appearance. When both of the alleles in a pair are the same, the alleles are is said to be “homozygous,” while if the alleles are different, the situation is described as “heterozygous.” In the case of homozygous alleles, the expressio
How many alleles can there be for a gene and how many copies does each individual have? Suppose we choose a particular chromosome from two different people and examine the DNA from the same spot on both chromosomes. We will find that the pattern of the bases (As, Cs, Ts, and Gs) is similar, but it is often not exactly the same, even if the region is a protein-coding gene. How can a gene code for a product if the pattern is not the same in every person? The answer is that there can be many different versions or variants of a given gene. These different versions of the gene are called alleles. Different alleles of a gene code for the same trait, but they may manifest themselves in different ways. The gene for eye color contains the instructions governing eye pigment, for example, but the specific color is determined by the particular alleles one has. Everyone has the same number of chromosomes and genes, but each persons genetic code has a unique combination of alleles. This potential fo
