GENE AND ALLELE – DESCRIPTION OF THEIR ROLES IN HEREDITY

Genes and alleles form the basis of heredity. A gene is the sequence of nucleotide on DNA which makes protein. An allele is is the variant form of a specific gene that regulates a trait. For example, for a gene of eye color, alleles can be blue or brown.

GENE

A gene is the basic unit of inheritance as it contains information required to express a particular trait in an organism. These traits can be transfered from one generation to another forming the basis of heredity.

Gene is a segment of DNA that codes for a protein which defines a particular trait. In a eukaryotic organism, genes comprise of exons and introns.

Exons are those nucleotide sequences on DNA molecule that can code for protein. Introns on the other hand, are not capable of making protein but they can form other products like tRNA, rRNA. The intervening regions of introns are removed and exons are joined after transcription by a process called splicing. Genes of a prokaryotic organism consist of only exons and no introns.

Genes are situated on the chromosome of cell nucleus at a precise position called locus. Humans have approximately 20,000 protein coding genes distributed across 23 pairs of chromosomes.

The concept of gene as a unit of heredity was first introduced by Gregor Mendel. Mendel conducted hybridization experiments on garden peas for seven years from 1856 to 1863. He proposed that some entity was passed from parent to offspring which determined the characteristics of offspring. He called this entity as ‘factor‘. Later it was found out that the Mendelian ‘factor’ is nothing but gene. The term gene was coined by Wilhelm Johannsen in 1909.

STRUCTURE OF A GENE

In eukaryotes a gene has following components:

• PROMOTER – Located upstream at 5′ end of the gene. This region serves as binding site for RNA polymerase and initiates transcription.
• CODING REGION – Comprises of exons and introns. Exons code for protein whereas intervening introns do not. Introns are removed and exons are joined by splicing.
• TERMINATOR – Situated at 3′ end . It signals the termination of transcription.
• REGULATORY SEQUENCES – Includes enhancers which increase transcription and silencers which decrease transcription.

ALLELE

Alleles are alternative forms of the same gene which code for a pair of contrasting traits. In a pair of alleles, one allele is inherited from each parent.

Alleles are located at the same locus of homologous chromosomes. They determine distinct traits, such as color of eye or blood group. Suppose a gene for eye color is present at a locus, then the pair of its alleles determine which color of eye is expressed, like blue or brown.

If we consider the character height, T represents tall trait and t for dwarf trait. (T and t are alleles for each other). Hence, the pair of alleles for height would be TT, Tt and tt.

In a pair of dissimilar alleles, one allele dominates over the other and is called the dominant allele while the other is called recessive allele.

Similar alleles are known as homozygous ( like, TT or tt ). Dissimilar alleles like, Tt are heterozygous.

GENOTYPE – Refers to the genetic composition made by the alleles. For example – Tt / TT / tt .

PHENOTYPE – Refers to observable characteristics or traits which are determined by genotype. For example – the genotype TT expresses the phenotype tall, Tt has phenotype tall ( as T is dominant allele, it suppresses t), tt has phenotype dwarf.

The ABO blood group is an example of multiple alleles. It is controlled by gene I. The gene I has three alleles – I^A , I^B and i . Since humans are diploid, each individual possesses any two of the three I gene alleles.