In this article we will discuss about the features and effects of gene.
Essential Features of Gene:
Prior to 1940, recombination was not believed to occur within genes. The genes in a chromosome were considered analogous to beads on a string. Recombination was believed to occur only between the beads or genes. The gene was not considered subdivisible.
But, on the basis of modern molecular concept, it is quite clear to us that the above statement cannot satisfy the structural aspect of gene. Similarly, if we consider the single nucleotide pair as the unit of structure of gene then it does not make any sense to the cell.
According to the modern concept, essential features of the genes are:
a) Genes determine the traits and are inherited from parents to the off-springs generation after generation.
b) Genes are situated on the chromosomes in a linear mode.
c) Every gene has a specific position in a specific chromosome and the position is known as locus.
d) A single gene may occur in several forms or in several functional states. All the forms of a normal gene are known as allele.
e) Gene may undergo sudden change, both in composition and expression and this change is known as mutation.
f) Genes can duplicate themselves very accurately.
g) Genes express themselves by producing proteins.
h) One gene is a segment of DNA which contains the information for one protein (enzyme).
Origin of New Genes in an Individual:
During the time of DNA duplication new genes can arise which may or may not exist generation after generation and existence will ultimately depend upon the natural selection. According to Bridges (1918) : “the main interest in duplications lay in their off-springs a method for evolutionary increase in lengths of chromosomes, with identical genes which could subsequently mutate separately and diversify their effects”.
Variability in the Effects of Gene and Phenocopies:
Due to the following reasons the phenotypic effects of some genes are variable:
a) The complexity of the pathways by which some genes exert their effects on phenotype,
b) The effects of regulator gene on the expression of many genes and
c) The occasionally complex interactions among environmental factors, regulator genes and pathways of gene on phenotype expression.
Regarding the variability of the effects of genes, two terms—expressivity and penetrance—are frequently used. The degree of a phenotypic expression of a gene varies from individual to individual and the gene is said to have variable expressivity.
But, when variability in expressivity of a given gene is such that the presence of the gene does not always result in a detectable phenotype effect, the gene is said to have incomplete penetrance.
The most potentially confounding factor in genetic analysis, particularly in diagnosing inherited abnormalities in human, is that environmental factors sometimes induce non- hereditary phenotypic change, called phenocopies. They are indistinguishable or closely resemble conditions resulting from mutant genes. For example. Epilepsy, which may result from either genetic or environmental causes.
Positional Effect of the Gene:
When a gene occupies its normal position in the chromosome, it produces its phenotypic expression exactly, but if the position of the same gene is somehow changed from its normal position then its phenotypic expression will also vary—this is known as position effect of gene.
Regulation of Gene Expression in Eukaryotes:
The knowledge about the mechanisms by which gene expression is regulated in higher eukaryotes is not crystal clear even today. But it has been well established that regulatory mechanism is acting at the level of transcription. Similarly, it is also quite evident that various kinds of signals-like cytoplasmic molecules, hormones, environmental stimuli etc.—regulate the gene function at the appropriate times.