12th Class

The process by which exchange of chromosomal segment take place is called crossing over. Crossing over may be defined as "the recombination of linked genes" brought about as a result of interchange of corresponding parts between the chromatid of a homologous pair of chromosomes, so as to produce new combination of old genes. The term was given by Morgan and Cattle. Janssen (1909) observed chiasmata during meiosis-I (Prophase). Morgan proposed that chiasmata lead to crossing over by breakage and reunion of homologous chromosomes. Crossing over results in new combination while non-cross over result in parental type, which leads to variations. Crossing over and chiasma There are two views extended to explain the relationship between crossing over and chiasma formation. They are summarised here under : Chiasma type theory : According to Janssen, 1909 the act of crossing over is followed by chiasma formation. He suggests that the crossing over takes more...

The fact that nucleus contains the units of inheritance was proposed by Oscar Hertwig in 1870. The mechanism was clearly understood with the development of Mendel's laws of inheritance. Further researchers proposed that cytoplasm also contains the hereditary material. The evidence for cytoplasmic inheritance was first presented by Correns in Mirabilis Jalapa and by Baur in Pelargonium zonale in 1908. The cytoplasm in such cases contain self perpetuating hereditary particles formed of DNA. These may be mitochondria, plastids or foreign organism, etc. The total self duplicating hereditary material of cytoplasm is called plasmon and the cytoplasmic units of inheritance are described as plasmagenes. Criteria for cytoplasmic inheritance : The cases of cytoplasmic inheritance are found to exhibit maternal influence. The reason is very simple. Very little cytoplasm is contained in the sperm cell of an animal. Most of the cytoplasm is contributed to the zygote by the ovum or egg. more...

Gene expression in prokaryotes Gene expression refers to the molecular mechanism by which a gene expresses a phenotype by synthesizing a protein or an enzyme. Which determines the character. The gene contains the blue print or the information for the protein or an enzyme. The category includes mechanism involved in the rapid turn-on and turn-off gene expression in response to environmental changes. Regulatory mechanism of this type are very important in microorganisms, because of the frequent exposure of these organisms to sudden changes in environment. Gene concept can be studied by operon model. Operon are segment of genetic material which function as regulated unit that can be switched on and switched off, which was given by French scientists. Jacob and Monod (1961) working at Pasteur institute. They were studying lactose utilization in mutants of E.coli. An operon consists of one to several structural genes (three in lac operon and five more...

Defined as structure of nitrogen bases(nucleotides) in mRNA molecule which contain the information for the synthesis of protein molecule. It is discovered by frame shift mutation by Crick. Codon is the sequence of nitrogen bases (nucleotides) in mRNA, which codes for a single amino acid. Nirenberg and Mathaei (1961) experimentally proved that a single amino acid is determined by a sequence of three nitrogen bases which is known as triplet code. Khorana has got Nobel prize on genetic code. Salient Features Triplet : A single amino acid is specified by a sequence of three nucleotides in mRNA i.e., called codon. Due to triplet nature, it consist 64 codon. Universal : A codon specifies the same amino acid in all organisms from viruses to human beings. Commaless : There is no pause, so it reads continously. Non-overlapping : No overlapping between adjacent nucleotide. Initiation codon : The synthesis of polypeptide chain more...

Heredity is the study of transmission of genetic characters and variations from one generation to the next. Heredity involves the transfer of chromosomes from parents to offspring or one individual to another. Therefore, chromosome is the base of heredity. The physical basis of heredity are genes while chemical basis of heredity is DNA. Pre-Mendelian view points Vapour theory (Pythogoras) : Different body parts produce minute particles. Fluid theory : Empedocles, proposed that each body part produced a fluid. The fluid of different body parts of the two parents mixes up and is used in the formation of embryo. Preformation theory : Malpighi believed that homunculus or miniature individual is present in sperm or egg. Antony Von Leeuwenhoek was first to observe human sperm. Particulate theory : Maupertuis proposed that the body of each parent gives rise to minute particles. These particles unite together to form the daughter individual. Encasement theory more...

Gene : (Mendel called them factor) In modern sense an inherited factor that determines a biological character of an organism is called gene (functional unit of hereditary material). Allelomorphs or alleles : Alleles indicates alternative forms of the same gene. e.g., Tall TT and dwarf tt are alternation forms of the same gene etc. Gene locus : The specific place on a chromosome where a gene is located. Wild and mutant alleles : An original allele, dominant in expression and wide spread in the population is called wild allele. An allele formed by a mutation in the wild allele, recessive in expression and less common in the population is termed as mutant allele. Homozygous (Bateson and Saunders, 1902) : Both the genes of a character are identical is said to be homozygous or genetically pure for that character. It gives rise to offspring having the same character on self-breeding e.g., more...

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.g., co-dominance, multiple alleles. (i) Incomplete dominance or Blending inheritance (1: 2:1 ratio) : After Mendel, several cases were recorded where \[{{F}_{1}}\] 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 : First case of incomplete dominance or blending inheritance was reported in 4-O’clock plant, (Mirabilis jalapa) by Carl Correns (1903) when plants with red flowers (RR) are crossed with plants having white flowers (rr) the hybrid \[{{F}_{1}}\] plants (Rr) bear pink flowers. When these F1 plants more...

Introduction : "When genes are closely present link together in a group and transmitted as a single unit this phenomenon is called linkage". Theories of linkage Sutton's hypothesis of linkage (1903) : The number of groups of genes are equivalent to the number of chromosomes. 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. 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. When BBLL and bbll more...

Gregor Johann Mendel first "geneticist", also known as father of genetics was born on July 22, in 1822 in Silisian, a village in Heizendorf (Austria). In 1843, he joined Augustinian monastry at Brunn (then in Austria, now Brno Czechoslovakia). In 1856, Mendel got interested in breeding of Garden pea (Pisum sativum). He selected pure breeding varieties or pure lines of pea. Breeding experiments were performed between 1859 – 1864. The results were read out in two meetings of Natural History Society of Brunn in 1865 and published in 1866 in "Proceedings of Brunn Natural History Society" under the topic "Experiments in Plant Hybridisation". Mendel died in 1884 without getting any recognition during his lifetime. Rediscovery of Mendel's work : In 1900, Hugo de Vries of Holland, Carl Correns of Germany and Erich von Tshermak of Austria came to the same findings as were got by Mendel. Hugo de Vries found more...

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 (reported) by Friedrich Miescher in 1869 from the nuclei of pus cells and was named nuclein. The term nucleic acid was given by Altman (1899).     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 more...