Topic Genetic Basis for Variation: Q) What is the difference between the term gene and allele ?

Ans :  

A gene is a sequence of nucleotides coding for a particular polypeptide (FYI : while this is the ‘conventional’ definition, note that there are also rRNA and tRNA genes that code for the specific RNA)

An allele is the alternative form of a gene. It means that mutation results in slightly different DNA sequences which in turn gives rise to slightly different protein variants. This can result in different phenotypes (e.g. your blood group depends on 2 alleles coding for specific antigens on the surface or rbc).

Source: http://en.wikipedia.org/wiki/File:ABO_blood_type.svg

When we talk about a gene, it is actually about the group of all possible DNA sequences (in different individuals) that code for the particular protein, and each of the different DNA sequences will be considered an allele of this gene.

Since we are diploid organisms, it means that we have two copies (or 2 allleles) of every gene in our genome. These two alleles occupy corresponding location in a pair of homologous chromosome. If both alleles are identical, the organism is homozygous for that gene. If they are different, the organism is heterozygous.

Note: Even though the organism's genotype for the gene may be heterozygous, only the dominant allele is expressed to form the phenotype (i.e. the recessive allele is masked). That is why in the standard Mendelian dihybrid cross of two double heterozygotes (e.g. AaBb x AaBb), there are only 4 phenotypic classes (9A_B_ : 3aaB_ : 3A_bb : 1aabb) even though there are 16 genotypic combinations! More on this in our future post...