gene and gene concept

gene and gene Concept:

Gene is the fundamental unit of heredity. Its presence was first proposed by Mendel in 1865. He called it as factor. The word gene was coined by Johannson in 1909. A gene is defined as  a segment of DNA having a particular sequence of nucleotides coding for a specific polypeptide chain. It occupies a definite locus in the chromosome and it is responsible for the expression of a specific character, it is a unit of mutation.

Size of a Gene:

An average gene consist of 1500 nucleotide base pairs and has a molecular weight of 10.6. The haemoglobin has two types of polypeptide chains, namely alpha and beta chains. The alpha chain is synthesized by a gene containing 423  nucleotides and the beta chain is synthesized by a gene containing 438 nucleotides.

Cistron: 

It is a modern term for gene. This term was coined by Benzer Cistron is a portion of DNA specifying a single polypeptide chain.

Muton:

It is the smallest length of DNA capable of mutation. This term is coined by Benzer. A muton consists of a single nucleotide or many nucleotides.

Recon:

A recon is the smallest unit of DNA capable of recombination.

Gene expression:

Gene expression refers to the mechanism by which a gene expresses a phenotype.

A gene contains the code or plan for a polypeptide chain in the form of a specific sequence of nucleotide bases. It transfers its codes to mRNA. It is called transcription. The code present in the mRNA is translated in the ribosomes with the help of tRNA. The tRNA picks up the required amino acids and are linked in specific sequence as per the sequence of nucleotides present in the mRNA. The amino acids form a polypeptide chain. It function as a structural protein or as an enzyme. In other words, the polypeptide contributes to the morphological or functional trait of the cell. 

Gene Concept

Gene is the fundamental hereditary unit. The gene concept was proposed by Morgan. The gene concept refers to the summery of the information about characters, genes, chromosomes, linkage and crossing over. The gene concept includes the following salient features:

1. Each organism is formed of a bundle of characters. Each character is controlled by a pair of genes.
2. Of the two genes of a character, one gene comes from the father and other from the mother.
3. Law of segregation: During gamete formation, the genes of a pair seprate in meiosis and each gamete receives only one gene. This is the law of segregation.
4. Law of Independent Assortment: The genes located on different chromosomes assort out independently of each other during gamete formation. This is called the Law of independent assortment.
5. Crossing over: During meiosis genes are interchanged between homologous chromosomes. This is called crossing over.
6. Linkage: Certain gene are inherited together as one unit for a number of generations. This is called linkage.
7. Each chromosome contains numerous genes.
8. Linear arrangement: The genes are arranged in a linear manner like beads on the chromosome.
9. Locus: Each gene has a definite place in the chromosome. This place is called Locus.
10. Allele: The two genes controlling a character are called alleles.
11. Dominance: Of the two alleles, one allele expresses its character in the F1 generation. This allele is called the dominant allele. The character expressed is called the dominant character.
12. Recessiveness: The dominant allele masks the expression of the recessive allele. The recessive allele expresses itself in the absence of dominant allele.
13. Codominance: Sometimes the two alleles of a caharcter may be equally expressed. This is called codominance.
14. Incomplete dominance: In some cases, none of the two alleles may be fully dominant. But they are partially or incompletely dominant. These alleles produced an intermediate phenotype in the hybrids. This phenomenon is called incomplete dominance.
15. Polygenes: Sometimes a single character is produced by the interaction of two or more pairs of genes. This phenomenon is called factor hypothesis or interactions of genes and such genes are called polygenes.
16. Multiple genes: Certain characters are produced by the cumulative effect of two or more pairs of genes. These genes are called cumulative genes or multiple genes.
17. Pleiotropism: Sometimes a single pair of genes produces two or more characters. This is called pleiotropism.
18. Multiple alleles: Certain characters have more than two alleles. These alleles are called epistasis.
19. Lethal genes: Certain genes control the vital activities of the organisms. These genes are called lethal genes because they kill the organism when they are in the recessive condition.
20. Penetrance: Some genotypes may be expressed in some organisms and may not be expressed in others. This phenomenon is called penetrance.
21. Expressivity: Certain genotypes may be expressed to varying degrees in different individuals. This phenomenon is called expressivity.
22. Atavism: Some individuals of a species may inherit ancestral characters. This is called atavism.
23. Plasmagenes: Certain particles in the cytoplasm also control some characters. These particles are called plasmagenes. The inheritance of plasmagenes is called cytoplasmic inheritance.
24. Replication: Genes can replicate and produce its own copies.
25. Mutation: They can change from one form to another form this is called mutation.
26. Environmental factors: The expression of genes is influenced by the environmental factors.
27. One-gene one-enzyme hypothesis: Each gene controls the synthesis of specific enzyme. This is called one gene one enzyme hypothesis.