Genes and Alleles

The inheritance of any character can be studied only when thcre are two contrasting conditions, such as yellow versus green seed colour (as observed by Mendel in peas), normal pigmentation versus absence of pigmentation (albinism) in humans and other animals, and brown versus black coat colour in guinea pigs. An individual expresses one or the other, but not both contrasting conditions at the same time. Genes that govern variations of the same charactenstic and that occupy corresponding loci on
homologous chromosomes are termed alleles.

Geneticists use the tern dele to emphasise that there are two or more alternative forms of a gene that can occur at corresponding specific loci. The possible variants of a gene at any given locus are known as alleles, each of which is assigned a single Ietter (or a group of letters) as its symbol. In the example below, we shall consider the trait - height of pea plant. Tall forms are denoted by T and the short ones are denoted by t. T and t are alleles of the same gene. Since they are present in pairs they are represented as TT, tt or Tt.

It is customary to indicate a dominant allele with a capital lctter and a recessive allele with a lower case letter. The choice of the letter or'letters tllernselves is generally determined by the first allelic variant in a particular locus. For example, Mendel studied the alleles that govern seed colour-yellow versus green. The allele that is responsible for the yellow colour of the seed is designated as Y, and the allelc for the green colour y. Because discovery of tlie yellow allelc made identification of this locus possible, we commonly refer to the locus as the yellow locus, although pea seeds are most commonly green. The term locus is used to designate not only a location on a chromosome but also a kind of "generic" gene controlling a particular kind of characteristic; thus Y (yellow) and y (green) rcpresent a specific pair of alleles of a Iocus involved in determining seed colour in peas. To prevent confusion, the dominant allele is always designated first and the recessive allele secotld (Yy, never yY). Geneticists use the term gene sometimes to specify a locus and soinctimes to specify one of the allelic variants at that locus. Usually tlie meaning is clear from the context.

There are many genes within each chromosome, each generally different from the other and each controlling the inheritance of one or more charactcristic(s). The members of a homologous pair of chromosome have similar set of alleleges arranged in the same order. The regularity of the mitotic process cnsures that cach diploid daughter cell receives a pair of each chromosome and thereforc a pair of each gcne. As the chrolnosomes separate during meiosis, and become associated with new partners at the time of fertilisation the alleles also separate and associate with new partners. The entire concept of Mendelian Genetics dcpcnds on these sample facts.