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 red (RR), pink (Rr) and white (rr) flowered plants in the ratio of ( generation).
Example : In Snapdragon or dog flower (Antirrhinum majus) the dominant character of leaf (Broadness) and flower (Red) shows incomplete dominance over recessive characters (Narrowness and white) in dihybrid cross.
(ii) Codominance (ratio): In codominance, both the genes of an allelomorphic pair express themselves equally in hybrids. ratio both genotypically as well as phenotypically in generation.
Example : Codominance of coat colour in cattle.
In cattle gene R stands for red coat colour and gene r stands for white coat colour. When red cattle (RR) are crossed with white cattle (rr), the hybrids have roan coloured skin (not the intermediate pink). The roan colour is actually expressed by a mixture of red and white hairs, which develop side by side in the heterozygous hybrid. In generation red, roan and white appear in the ratio of . The phenotypic ratio equal to genotypic ratio RR, Rr, rr ().
Example: Codominance in andalusian fowl
In andalusian fowl a cross between pure black and pure white varieties results in blue hybrids.
Example: Codominance of blood alleles in man
(a) MN blood type in man is an example of codominance. The persons with MN genotype produce both antigen M and N and not some intermediate product indicating that both the genes are functional at the same time.
(b) In ABO blood group system gene A and B responsible for blood group A and B are codominant. The hybrid has AB blood group.
Differences between incomplete dominance and codominance
Incomplete dominance
Codominance
Effect of one of the two alleles is more conspicuous.
The effect of both the alleles is equally conspicuous.
It produces a fine mixture of the expression of two alleles.
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. When BBLL and bbll are crossed, the \[{{F}_{1}}\] is BbLl and the test cross of it will show progeny in 7 : 1 : 1 : 7 ratio i.e. BbLl : Bbll : bbLl : bbll (coupling) when BBll is crossed with bbLL the \[{{F}_{1}}\] is BbLl or the test cross progeny will show 1 : 7 : 7 : 1 ratio i.e., BbLl : Bbll : bbLl : bbll (repulsion). Coupled and repulsed genes are known as linked genes. Linkage has coupling phase and repulsion phase. In coupling phase both the linked genes have their dominant alleles in one chromosome and recessive alleles in other chromosomes. The heterozygotes with such constitution is called cis heterozygote. Cis-arrangement is a original arrangement. Which form two types of gametes as (AB) and (ab). In Human X–chromosomes carry 102 genes and Y
In repulsion phase the normal alleles as well as mutant alleles lie in opposite chromosomes of the homologous pair, such heterozygote is called as trans heterozygote. It is not original arrangement, caused due to crossing over, which form two types of gametes as (Ab) and (aB).
(iv) Chromosomal hypothesis of linkage: It was given by Morgan and Castle. According to them linked genes are bound by chromosomal material and are transmitted as a whole.
(2) Types of linkage: Depending upon the absence or presence of nonparental or new combination of linked genes, linkage has been found to be complete or incomplete.
(i) Complete linkage: Such cases in which linked genes are transmitted together to the offsprings only in their original or parental combination for two or more or several generations exhibit complete linkage. In such cases the linked genes do not separate to form the new or non-parental combinations. This phenomenon is very rare. Some characteristics in males of Drosophila are found to exhibit complete linkage.
(ii) Incomplete linkage: In majority of cases, the homologous chromosomes undergo breakage and reunion during gametogenesis. During reunion the broken pieces of the chromatids are exchanged, producing some nonparental or new combinations. Therefore, the linkage is 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 segment from a living organism achieved by Nirenberg H.Harries is associated with DNA-RNA hybridization technique.
(1) Chemical composition: The chemical analysis has shown that DNA is composed of three different types of compound.
(i) Sugar molecule: Represented by a pentose sugar the deoxyribose or 2-deoxyribose which derived from ribose due to the deletion of oxygen from the second carbon.
(ii) Phosphoric acid: \[{{H}_{3}}P{{O}_{4}}\] that makes DNA acidic in nature.
(iii) Nitrogeneous base: These are nitrogen containing ring compound. Which classified into two groups:
(a) Purines: Two ring compound namely as Adenine and Guanine.
(b) Pyramidine: One ring compound included Cytosine and Thymine in RNA uracil is present instead of Thymine.
Nucleosides: Nucleosides are formed by a purine or pyrimidine nitrogenous base and pentose sugar. DNA nucleosides are known as deoxyribosenucleosides.
Nucleotides: In a nucleotide, purine or pyrimidine nitrogenous base is joined by deoxyribose pentose sugar (D), which is further linked with phosphate (P) group to form nucleotides.
Composition of nucleoside and nucleotides of DNA and RNA
(D= Deoxyribose sugar, R = Ribose sugar, P = Phosphoric acid)
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 metabolic activity. It means each gene synthesizes a particular protein which acts as enzyme and brings about an appropriate change.
(i) One gene one enzyme: This theory was given by Beadle and Tatum (1958), while they were working on red mould or Neurospora (ascomycetes fungus). Which is also called Drosophila of plant kingdom. Wild type Neurospora grows in a minimal medium (containing sucrose, some mineral salts and biotin). The asexual spores i.e. conidia were irradiated with x-rays or UV-rays (mutagenic agent) and these were crossed with wild type. After crossing sexual fruiting body is produced having asci and ascospores. The ascospores produced are of 2 types -
(a) The ascospores, which are able to grow on minimal medium called ‘prototrophs’.
(b) Which do not grow on minimal medium but grow on supplemented medium called ‘auxotrophs’.
(2) Molecular structure of gene: Gene is chemically DNA but the length of DNA which constitutes a gene, is controversial 3 term i.e. cistron, muton and recon were given by Seymour Benzer to explain the relation between DNA length and gene.
(i) Cistron or functional gene or gene in real sense: Cistron is that particular length of DNA which is capable of producing a protein molecule or polypeptide chain or enzyme molecule.
(ii) Muton or unit of mutation: Muton is that length of DNA which is capable of undergoing mutation. Muton is having one or part of nucleotide.
(iii) Recon: Recon is that length of DNA which is capable of undergoing crossing over or capable of recombination. Recon is having one or two pairs of nucleotides.
(iv) Complon: It is the unit of complementation. It has been used to replace cistron. Certain enzymes are formed of two or more polypeptide chains. Whose active groups are complimentary to each other.
(v) Operon: Operon is the combination of operator gene and sequence of structure genes which act together as a unit. Therefore it is composed of several genes. The effect of operator gene may be additive or suppresive.
(vi) Replicon: It is the unit of replication. Several replicons constitute a chromosome.
(3) Some specific terms 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: It carries the coded information for synthesis of one (unicistronic) or more polypeptides (polycistronic). Its codons are recognised by tRNAs.
tRNAs: They picks up specific amino acid from amino acid pool and carrying over the mRNA strand.
Amino Acyl tRNA Synthetases: The enzymes are specific for particular amino acids and their tRNAs.
(1) Activation of Amino Acids: An amino acid combines with its specific aminoacyl tRNA synthetase enzyme (AA-activating enzyme) in the presence of ATP to form aminoacyl adenylate enzyme complex (AA-AMP-E). Pyrophosphate is released. Amino acid present in the complex is activated amino acid. It can attach to CCA or 3¢ end of its specific tRNA to form aminoacyl or AA-tRNA (charged tRNA / adaptor molecule)
Amino Acid (AA) + ATP + Aminoacyl tRNA Synthetase (E) \[\underset{\begin{smallmatrix}
\text{amino acid adenylate} \\
\text{enzyme complex}
\end{smallmatrix}}{\mathop{\to \,AA-AMP-E}}\,\,+PPi\]
AA-AMP-E + tRNA \[\to \]AA-tRNA + AMP + Enzyme.
(2) Initiation: It is accomplished with the help of initiation factors. Prokaryotes have three initiation factors – IF3, IF2 and IF1. Eukaryotes have nine initiation factors – eIF1, eIF2, eIF3, eIF4A, eIF4B, \[eI{{F}_{4C}},\text{ }eI{{F}_{4D}},\text{ }eI{{F}_{5}},\text{ }eI{{F}_{6,}}\] ,mRNA attaches itself to smaller subunit of ribosome with its cap coming in contact with 3¢ end of 18 S rRNA (16S RNA in prokaryotes). It requires \[eI{{F}_{2}}\] (\[I{{F}_{3}}\] in prokaryotes). The initiation codon AUG or GUG comes to lie over P-site. It produces 40S – mRNA complex. P-site now attracts met tRNA (depending upon initiation codon). The anticodon of tRNA (UAC or AUG) comes to lie opposite initiation codon. Initiation factor \[\mathbf{eI}{{\mathbf{F}}_{\mathbf{3}}}\] (\[I{{F}_{2}}\] in prokaryotes) and GTP are required. It gives rise to \[40S-mRNA\text{ }-\text{ }tRN{{A}^{Met}}\]. Methionine is nonformylated (tRNA\[_{m}^{Met}\]) in eukaryotic cytoplasm and formylated (tRNA\[_{f}^{Met}\]) in case of prokaryotes. The larger subunit of ribosome now attaches to \[40S-mRNA-tRN{{A}^{Met}}\] complex to form 80S mRNA -tRNA complex. Initiation factors \[\mathbf{eI}{{\mathbf{F}}_{\mathbf{1}}}\]and \[\mathbf{eI}{{\mathbf{F}}_{\mathbf{4}}}\](A, B and C) are required in eukaryotes and \[\mathbf{I}{{\mathbf{F}}_{\mathbf{1}}}\] in prokaryotes.\[M{{g}^{2+}}\]is essential for union of the two subunit of ribosomes. A-site becomes operational. Second codon of mRNA lies over it.
(3) Elongation/chain formation: A new AA-tRNA comes to lie 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 (1883): He believed the chromosomes take part in inheritance.
W.Waldeyer (1888): He coined the term ‘chromosome’.
Benden and Boveri (1887): They found a fixed number of chromosomes in each species.
(ii) Kinds of chromosomes
(a) Viral chromosomes: In viruses and bacteriophages a single molecule of DNA or RNA represents the viral chromosome.
(b) Bacterial chromosomes: In bacteria and cyanobacteria, the hereditary matter is organized into a single large, circular molecule of double stranded DNA, which is loosely packed in the nuclear zone. It is known as bacterial chromosome or nucleoid.
(c) Eukaryotic chromosomes: Chromosomes of eukaryotic cells are specific individualized bodies, formed of deoxyribonucleo proteins (DNA + Proteins).
(iii) Number of chromosomes: The number of chromosomes varies from two, the least number an organism can have, to a few hundred in different species. The number of chromosomes a species possesses has no basic significance, nor it necessarily shows relationship between two different species that have the same number.
Both dog and fowl have 78 chromosomes. Thus, it is not the number of chromosomes, but the genes in them which differentiate species. Their number also does not indicate the size or complexity of the organism. Amoeba proteus has 250 chromosomes and man has 46. The related species tend to have similar chromosome. Man and his nearest relatives, the apes, have chromosomes similar in size, shape and banding pattern. The least number of chromosomes are found in Ascaris megalocephala i.e. 2 while in a radiolarian protist (Aulocantha) has maximum number of chromosomes is 1600. The male of some roundworms and insects have one chromosome less than the females. For instance, the male and female roundworm Coenorhabditis have 11 and 12 chromosomes respectively and the male and female cockroach (Blatta) have 23 and 24 chromosomes respectively.
Diploid number of chromosomes in some animals
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) Multiple alleles confirm to the Mendelian pattern of inheritance.
(iii) Definition: More than two alternative forms (alleles) of a gene in a population occupying the same locus on a chromosome or its homologue are known as multiple alleles.
(iv) Examples of multiple allelism : A well known example of a trait determined by multiple alleles is the blood groups in man and skin colour.
Blood groups in man
(a) Blood proteins : According to Karl landsteiner (1900) a Nobel prize winner, blood contains two types of proteinous substances due to which agglutinations occurs.
(1) Agglutinogen or antigen : It is a protein found on the cell membrane of RBC’s.
(2) Agglutinin or antibody : This the other proteinous substance, found in the plasma of the blood.
Whenever the blood of a person receives the foreign proteins (antigen) his blood plasma starts forming the antibodies in order to neutralize the foreign antigens.
(b) Agglutinations : Two types of antigens are found on the surface of red blood corpuscles of man, antigen A and B. To react against these antigens two types of antibodies are found in the blood plasma which are accordingly known as antibody – anti-A or a and anti-B or b. Agglutination takes place only when antigen A and antibody a occur together or antigen B and antibody b are present in the blood. Under such condition antibody a reacts with antigen A and makes it highly sticky. Similarly antigen B in presence of antibody b become highly sticky with the result RBC’s containing these antigens clump to form a bunch causing blockage of the capillaries. Agglutination in blood is therefore antigen-antibody reaction.
(c) Types of blood groups
(1) ABO blood group : Landsteiner divided human population into four groups based on the presence of antigens found in their red blood corpuscles. Each group represented a blood group. Thus there are four types of blood groups viz. A, B, AB and O. He observed that there was a reciprocal relationship between antigen and antibody according to which a person has antibodies for those antigens which he 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 and emerged leaves, e.g. Ranunculus aquatilis, Limnophila heterophylla and this meristic activities are due to change in depth and medium of water.
(b) Light: Partial shade causes elongation of internodes.
(c) Temperature: Plants of hot areas have extensive roots but smaller shoots. Human skin becomes darker with increase in environmental temperature.
(d) Nutrition: Honey bee larva feeding on royal jelly develops into queen while the ones obtaining ordinary nourishment (bee bread) grow into workers.
(e) Water: Water deficiency leads to several modifications in plants like succulent, spines, reduced leaves, thick bark, hair etc.
(ii) Use and disuse of organs: In higher animals and human beings, greater use of an organ leads to its better development as compared to other organs which are less used, e.g., stronger muscular body in a wrestler.
(iii) Conscious efforts: Acquired variations due to conscious efforts include education, training of pets boring of pinna, bonsai, etc.
(2) Germinal variations: They are inheritable variations formed mostly in germinal cells which are either already present in the ancestors or develop a new due to mutations. Germinal variations are of two types, continuous and discontinuous
(i) Continuous variations: They are fluctuating variations and also called recombinations because they are formed due to recombination of alleles as found in sexual reproduction. Darwin (1859) based his theory of evolution on continuous variations.
(ii) Discontinuous variations: They are mutations, which are ultimate source of organic variations. Discontinuous variations are caused by chromosomal aberrations, change in chromosome number and gene mutations. In pea seed coat colour changes gray to white is an example of spontaneous mutation.
Importance of variations
(1) Variations continue to pile up forming new species with time.
(2) They are essential in the struggle for existence.
(3) Adaptability is due to variations.
(4) Variations allow breeders to improve races of plants and animals.
(5) Discontinuous variations introduce new traits.
(6) Inbreeding between closely related organisms reduces variation.
Heredity
Heredity is the study of transmission of characters and variations from one generation to the next.
(1) Basis of heredity: Heredity involves the transfer of chromosomes 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 sex-determining mechanism was first studied in the milk weed bug, Lygaeus turcicus by Wilson and Stevens. Therefore, it is called Lygaeus type. These are two different patterns of sex determination in Lygaeus type.
(1) Female homogametic XX and male heterogametic XY : The homogametic sex (XX) is female and produces ova all of one type, i.e. having X-chromosome. The male is heterogametic-XY and produces sperm of two types. 50% of which possess X-chromosome and other 50% Y-chromosome. This is simple XX-XY type and is found in man, Drosophila and certain insects.
Example : In Drosophila total number of chromosomes is eight, of which six are autosomes, common to both male and female. The fourth pair is of sex chromosomes. In male this is represented by XY i.e. Karyotype of male Drosophila 6+XY and in female XX i.e. 6+XX. Ova produced by female are all similar possessing 3+X chromosomes, whereas the sperm produced by male are 3+X and 3+Y in equal numbers.
(2) Female heterogametic and male homogametic : In fowl, other birds and some fishes, certain moths and butterflies, the female sex is heterogametic, with X and Y chromosome often represented by Z and W and laying two types of eggs, one half with X or Z chromosome and the other half with Y or W chromosome. The male sex is homogametic having XX or ZZ chromosomes. It produces sperm all of one type.
(b) XX-XO type or Protenor type : Mc clung in male squash bug (Anasa) observed 10 pairs of chromosomes and an unpaired chromosome. Their females have eleven pairs of chromosomes (22). Thus all the eggs carry a set of eleven chromosomes but the sperm are of the two types: fifty percent with eleven chromosomes and the other fifty percent with ten chromosomes. The accessory chromosome was X-chromosomes. Fertilization of an egg by a sperm carrying eleven chromosomes results in a female, while its fertilization by a sperm with ten chromosomes produces male. It is said to be evolved by the 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 segments of X and Y chromosomes. Many sex linked characters (About 120) are found in man. Such characters are mostly recessive.
(i) Types of sex linked inheritance
(a) Diandric sex linked or X linked traits : Genes for these characters are located on non-homologous segment of X chromosome. Alleles of these genes do not occur on Y chromosome. Genes of such characters are transferred from father to his daughter and from his daughter to her sons in F2 generation. This is known as Cris-cross inheritance. As the genes for most sex linked characters are located in X chromosome, they are called X-linked characters e.g. colour blindness and haemophilia in man and eye colour in Drosophila.
(1) Sex linked inheritance in Drosophila : Drosophila melanogaster has XX and XY sex chromosomes in the female and male respectively. Its eye colour is sex linked.
Allele of the eye colour gene is located in the X chromosome, and there is no corresponding allele in the Y chromosome. The male expresses a sex-linked recessive trait even if it has a single gene for it, whereas the female expresses such a trait only if it has two genes for it. The normal eye colour is red and is dominant over the mutant white eye colour. The following crosses illustrate the inheritance of X-linked eye colour in Drosophila.
(i) Red-eyed female ´ White-eyed male : If a homozygous red-eyed female fly is mated with a hemizygous (having a single allele for a trait) white-eyed male fly, all the F1 flies, irrespective of their sex, are red eyed. When the red-eyed male and female flies of F1 are intercrossed (equivalent to self-pollination in peas), the F2 flies are in the ratio of 2 red-eyed females to 1 red-eyed male to 1 white-eyed male. Thus, the red-eyed and white-eyed flies are in the ratio of 3 : 1 in F2 generation (Mendelian monohybrid ratio).
If XR represents a gene for red eye and Xr that for white eye colour, the above cross may be diagramed as follows. The above cross shows that a recessive X-linked more...
(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 red (RR), pink (Rr) and white (rr) flowered plants in the ratio of ( generation).
Example : In Snapdragon or dog flower (Antirrhinum majus) the dominant character of leaf (Broadness) and flower (Red) shows incomplete dominance over recessive characters (Narrowness and white) in dihybrid cross.
(ii) Codominance (ratio): In codominance, both the genes of an allelomorphic pair express themselves equally in hybrids. ratio both genotypically as well as phenotypically in generation.
Example : Codominance of coat colour in cattle.
In cattle gene R stands for red coat colour and gene r stands for white coat colour. When red cattle (RR) are crossed with white cattle (rr), the hybrids have roan coloured skin (not the intermediate pink). The roan colour is actually expressed by a mixture of red and white hairs, which develop side by side in the heterozygous hybrid. In generation red, roan and white appear in the ratio of . The phenotypic ratio equal to genotypic ratio RR, Rr, rr ().
Example: Codominance in andalusian fowl
In andalusian fowl a cross between pure black and pure white varieties results in blue hybrids.
Example: Codominance of blood alleles in man
(a) MN blood type in man is an example of codominance. The persons with MN genotype produce both antigen M and N and not some intermediate product indicating that both the genes are functional at the same time.
(b) In ABO blood group system gene A and B responsible for blood group A and B are codominant. The hybrid has AB blood group.
Differences between incomplete dominance and codominance
In repulsion phase the normal alleles as well as mutant alleles lie in opposite chromosomes of the homologous pair, such heterozygote is called as trans heterozygote. It is not original arrangement, caused due to crossing over, which form two types of gametes as (Ab) and (aB).
(iv) Chromosomal hypothesis of linkage: It was given by Morgan and Castle. According to them linked genes are bound by chromosomal material and are transmitted as a whole.
(2) Types of linkage: Depending upon the absence or presence of nonparental or new combination of linked genes, linkage has been found to be complete or incomplete.
(i) Complete linkage: Such cases in which linked genes are transmitted together to the offsprings only in their original or parental combination for two or more or several generations exhibit complete linkage. In such cases the linked genes do not separate to form the new or non-parental combinations. This phenomenon is very rare. Some characteristics in males of Drosophila are found to exhibit complete linkage.
(ii) Incomplete linkage: In majority of cases, the homologous chromosomes undergo breakage and reunion during gametogenesis. During reunion the broken pieces of the chromatids are exchanged, producing some nonparental or new combinations. Therefore, the linkage is more... 
Nucleosides: Nucleosides are formed by a purine or pyrimidine nitrogenous base and pentose sugar. DNA nucleosides are known as deoxyribosenucleosides.
Nucleotides: In a nucleotide, purine or pyrimidine nitrogenous base is joined by deoxyribose pentose sugar (D), which is further linked with phosphate (P) group to form nucleotides.
Composition of nucleoside and nucleotides of DNA and RNA
(D= Deoxyribose sugar, R = Ribose sugar, P = Phosphoric acid)
(1) Activation of Amino Acids: An amino acid combines with its specific aminoacyl tRNA synthetase enzyme (AA-activating enzyme) in the presence of ATP to form aminoacyl adenylate enzyme complex (AA-AMP-E). Pyrophosphate is released. Amino acid present in the complex is activated amino acid. It can attach to CCA or 3¢ end of its specific tRNA to form aminoacyl or AA-tRNA (charged tRNA / adaptor molecule)
Amino Acid (AA) + ATP + Aminoacyl tRNA Synthetase (E) \[\underset{\begin{smallmatrix}
\text{amino acid adenylate} \\
\text{enzyme complex}
\end{smallmatrix}}{\mathop{\to \,AA-AMP-E}}\,\,+PPi\]
AA-AMP-E + tRNA \[\to \]AA-tRNA + AMP + Enzyme.
(2) Initiation: It is accomplished with the help of initiation factors. Prokaryotes have three initiation factors – IF3, IF2 and IF1. Eukaryotes have nine initiation factors – eIF1, eIF2, eIF3, eIF4A, eIF4B, \[eI{{F}_{4C}},\text{ }eI{{F}_{4D}},\text{ }eI{{F}_{5}},\text{ }eI{{F}_{6,}}\] ,mRNA attaches itself to smaller subunit of ribosome with its cap coming in contact with 3¢ end of 18 S rRNA (16S RNA in prokaryotes). It requires \[eI{{F}_{2}}\] (\[I{{F}_{3}}\] in prokaryotes). The initiation codon AUG or GUG comes to lie over P-site. It produces 40S – mRNA complex. P-site now attracts met tRNA (depending upon initiation codon). The anticodon of tRNA (UAC or AUG) comes to lie opposite initiation codon. Initiation factor \[\mathbf{eI}{{\mathbf{F}}_{\mathbf{3}}}\] (\[I{{F}_{2}}\] in prokaryotes) and GTP are required. It gives rise to \[40S-mRNA\text{ }-\text{ }tRN{{A}^{Met}}\]. Methionine is nonformylated (tRNA\[_{m}^{Met}\]) in eukaryotic cytoplasm and formylated (tRNA\[_{f}^{Met}\]) in case of prokaryotes. The larger subunit of ribosome now attaches to \[40S-mRNA-tRN{{A}^{Met}}\] complex to form 80S mRNA -tRNA complex. Initiation factors \[\mathbf{eI}{{\mathbf{F}}_{\mathbf{1}}}\]and \[\mathbf{eI}{{\mathbf{F}}_{\mathbf{4}}}\](A, B and C) are required in eukaryotes and \[\mathbf{I}{{\mathbf{F}}_{\mathbf{1}}}\] in prokaryotes.\[M{{g}^{2+}}\]is essential for union of the two subunit of ribosomes. A-site becomes operational. Second codon of mRNA lies over it.
(3) Elongation/chain formation: A new AA-tRNA comes to lie
(b) XX-XO type or Protenor type : Mc clung in male squash bug (Anasa) observed 10 pairs of chromosomes and an unpaired chromosome. Their females have eleven pairs of chromosomes (22). Thus all the eggs carry a set of eleven chromosomes but the sperm are of the two types: fifty percent with eleven chromosomes and the other fifty percent with ten chromosomes. The accessory chromosome was X-chromosomes. Fertilization of an egg by a sperm carrying eleven chromosomes results in a female, while its fertilization by a sperm with ten chromosomes produces male. It is said to be evolved by the
