Basic Terms in population genetics

Population Genetics: Study of naturally occurring genetic differences among organisms.

Genetic Polymorphism: Genetic differences  that are common among organisms of the same species.
Genetic Divergence: Genetic differences that accumulate between species.

So, Population Genetics is the study of genetic polymorphism and divergence.

Gene: Roughly, gene is a genetic term meaning physical entity transmitted from parent to offspring during the reproduction process that influences heredity.

Genotype: Set of genes present in an individual.
Phenotype: Physical or biochemical expression of genotype.

Same genotype can result in different phenotype depending on environmental factors and same phenotype can result for 2 or more genotypes. Although genes do not determine complex phenotype owing to interacting genes and environmental factors, genes do determine molecular phenotypes.

Allele: Genes can exist in different forms or state. The alternative forms of gene are call alleles.


A gene corresponds to a specific sequence of constituents (called nucleotides) along DNA. Different sequences of nucleotides that may occur in a gene represent allele.

Transcription is the process in which sequence of nucleotides present in one DNA strand of a gene is faithfully copied into the nucleotides of RNA molecule. RNA has nucleotides A, U (instead of T in DNA), G & C.

After transcription, certain segments of the RNA transcript are removed by splicing. The eliminated segments are known as introns. The regions between the introns that remained in fully processed RNA are called exons.

In addition to splicing of exons, RNA processing also includes modifications to both ends of the RNA transcript. The fully processed RNA consitutes the messenger RNA (mRNA).


mRNA undergoes translation on ribosomes in the cytoplasm to produce polypeptide. In mRNA, each adjacent group of 3 nucleotides constitutes codon. Codon specifies the corresponding amino acids and subunits in the polypeptide chain.

Genome: The totality of DNA in a cell is the genome.

Within a cell, genes are arranged in linear order along the chromosomes. The position of a gene along the chromosome is called locus. In eukaryote, at any locus, every individual contains 2 alleles - one from mother, other from father. If both alleles are same, then the individual is called homozygous. On the other hand, if they are different, then that is called heterozygous.

Each human reproductive cell contains a complete set of 23 chromosomes. A human chromosome contains averagely 3500 genes. A cell has a genome size of approximately 3x10^9 base pairs.