NEET

Interaction of Genes and Cytoplasmic Inheritance   Genes interaction is the influence of alleles and non-alleles on the normal phenotypic expression of genes. It is of two types. (1) Inter-allelic or intra-genic gene interaction: In this case two alleles (located on the same gene locus on two homologous chromosomes) of gene interact in such a fashion to produces phenotypic expression e.q., co-dominance, multiple alleles. (i) Incomplete dominance (ratio): After Mendel, several cases were recorded where F1 hybrids were not related to either of the parents but exhibited a blending of characters of two parents. This is called incomplete dominance or blending inheritance.   Example : In 4-O’clock plant, (Mirabilis jalapa), when plants with red flowers (RR) are crossed with plants having white flowers (rr) the hybrid F1 plants (Rr) bear pink flowers. When these  plants with pink flowers are self pollinated they develop more...

Linkage   Introduction: "When genes are closely present link together in a group and transmitted as a single unit, the is phenomenon is called linkage".   (1) Theories of linkage (i) Sutton's hypothesis of linkage (1903): The number of groups of genes are equivalent to the number of chromosomes. (ii) Morgan's hypothesis of linkage (1910): It was given by T. H. Morgan. According to him the genes of homologous parents enter in the same gamete and tend to remain together, which is opposite in heterozygous parents. Linked group are located on the same chromosome and distance between linked group of gene limits the grade of linkage. (iii) Coupling and repulsion hypothesis: Proposed by Bateson and Punnet (1906) that dominant alleles tend to remain together as well with recessive alleles, called gametic coupling. If dominant and recessive alleles are present in different parents they tend to remain separate and called repulsion. more...

Nucleic Acids (DNA/RNA)   Two types of nucleic acids are found in the cells of all living organisms. These are DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid). The nucleic acid was first isolated by Friedrich Miescher in 1868 from the nuclei of pus cells and was named nuclein. The term nuclein was given by Altman.   DNA (Deoxyribonucleic Acid)   Introduction: Term was given by Zacharis, which is found in the cells of all living organisms except plant viruses,where RNA forms the genetic material and DNA is absent. In bacteriophages and viruses there is a single molecule of DNA, which remains coiled and is enclosed in the protein coat. In bacteria, mitochondria, plastids and other prokaryotes, DNA is circular and lies naked in the cytoplasm but in eukaryotes it is found in nucleus and known as carrier of genetic information and capable of self-replication. Isolation and purification of specific DNA more...

Genes Regulation   Term 'gene' was given by Johannsen (1909) for any particle to which properties of Mendelian factor or determiner can be given. T.H Morgan (1925) defined gene as ‘any particle on the chromosome which can be separated from other particles by mutation or recombination is called a gene. In general, gene is the basic unit of inheritance.   According to the recent information a gene is a segment of DNA which contains the information for one enzyme or one polypeptide chain coded in the language of nitrogenous bases or the nucleotides. The sequence of nucleotides in a DNA molecule representing one gene determines the sequence of amino acids in the polypeptide chain (the genetic code). The sequence of three nucleotides reads for one amino acid (codon).   (1) Gene action: Gene act by producing enzymes. Each gene in an organism produces a specific enzyme, which controls a specific more...

Protein Synthesis   Formation of protein from mRNA is called translation is also known as polypeptide synthesis or protein synthesis. It is unidirectional process. The ribosomes of a polyribosome are held together by a strand of mRNA. Each eukaryotic ribosome has two parts, smaller 40S subunit (30S in prokaryotes) and larger 60S subunit (50S in prokaryotes). Larger subunit has a groove for protection and passage of polypeptide, site A (acceptor or aminoacyl site), enzyme peptidyl transferees and a binding site for tRNA. The smaller subunit has a point for attachment of mRNA. Along with larger subunit, it forms a P-site or peptidyl transfer (donor site). There are binding sites for initiation factors, elongation factors, translocase, GTPase, etc. The raw materials for protein synthesis are amino acids. mRNA, tRNAs and amino acyl tRNA synthetases.   Amino acids: Twenty types of amino acids and amides constitute the building blocks of proteins. mRNA: more...

Chromosomes and Genes The chromosomes are capable of self-reproduction and maintaining morphological and physiological properties through successive generations. They are capable of transmitting the contained hereditary material to the next generation. Hence these are known as ‘hereditary vehicles’. The eukaryotic chromosomes occurs in the nucleus and in certain other organelles, and are respectively called nuclear and extranuclear chromosomes. Nuclear chromosomes are long, double stranded DNA molecules of linear form and associated with proteins, separated from the cytoplasm by nuclear envelope and replicated during S phase of cell cycle, while extranuclear chromosomes are present in the mitochondria and plastid. They are short, double stranded DNA molecules of circular form and are not associated with proteins and also called prochromosomes.   (i) Discovery of chromosomes Hofmeister (1848): First observed chromosomes in microsporocytes (microspore mother cells) of Tradescantia. Flemming (1879): Observed splitting of chromosomes during cell division and coined the term, ‘chromatin’. Roux more...

Multiple Allelism   Multiple allelism (i) Mode of origin: Genes having only two distinct alleles. If mutation occurs in the same gene but in different directions in different individuals, the population as a whole will have many different alleles of that gene. Each allele may produce a different phenotype, and various combinations of alleles produce several genotypes and phenotypes in the population.   (ii) Characteristics (a) There are more than two alleles of the same genes. (b) All multiple alleles occupy the corresponding loci in the homologous chromosomes. (c) A chromosome or a gamete has only one allele of the group. (d) Any one individual contains only two of the different alleles of a gene, one on each chromosome of the homologous pair carrying that gene. (e) Multiple alleles express different alternative of a single trait. (f) Different alleles may show codominance, dominance-recessive behaviour or incomplete dominance among themselves. (g) more...

Genetic Variations   Variations are differences found in morphological, physiological and cytological behaviouristic traits of individuals belonging to same species race and family. They appear in offspring or siblings due to: –  

• Reshuffling of genes/chromosomes by chance separation of chromosomes
• Crossing over
• Chance combination of chromosomes during meiosis and fertilization.

  Types of variations   (1) Somatic variations: These variations influence the somatic or body cells. They appear after birth and are, also called acquired characters, modifications or acquired variations. Somatic variations are non-inheritable and usually disappear with the death of the individual. They are formed due to three reasons i.e. environmental factors, use and disuse of organs, and conscious efforts.   (i) Environmental factors: They have lesser effect on animals as compared to plants. Important environmental factors are as follows: (a) Medium : Amphibious or emergent aquatic plants possess heterophylly, i.e. different types of submerged, floating more...

Sex Determination   Fixing the sex of an individual as it begins life is called sex determination. The various genetically controlled sex-determination mechanisms have been classified into following categories (i) Chromosomal theory of sex determination: The X-chromosome was first observed by German biologist, Henking in 1891 during the spermatogenesis in male bug and was described as X-body. The chromosome theory of sex determination was worked out by E.B. Wilson and Stevens (1902-1905). They named the X and Y chromosomes as sex-chromosomes or allosomes and other chromosomes of the cell as autosomes. Sex chromosomes carry genes for sex. X-chromosomes carries female determining genes and Y-chromosomes has male determining genes. The number of X and Y chromosomes determines the female or male sex of the individual, Autosomes carry genes for the somatic characters. These do not have any relation with the sex. (a) XX-XY type or Lygaeus type : This type of more...

Sex linked inheritance Sex chromosomes of some animals and man besides having genes for sex character also possess gene for non-sexual (somatic) characters. These genes for non-sexual characters being linked with sex chromosomes are carried with them from one generation to the other. Such non-sexual (somatic) characters linked with sex chromosomes are called sex linked characters or traits, genes for such characters are called sex linked genes and the inheritance of such characters is called sex linked inheritance. The concept of sex-linked inheritance was introduced by THOMAS H. MORGAN in 1910, while working on Drosophila melanogaster. The sex chromosomes in man and Drosophila are almost same in structure. The X and Y chromosomes, although different (non-homologous) in shape, size and structure, have atleast some similar (homologous) part which is known as homologous segment and the remaining part as non-homologous or differential segment. Genes for sex linked characters occur in both more...