1) a) Define Linkage. Enlist types of Linkage
b) Describe different phases of Linkage and give Significance of Linkage in Crop Improvement.
Linkage: It is the tendency of two or more genes to remain together in the same chromosome during inheritance is referred as linkage.
Types of linkage:
1) Based on crossing over
a) Complete linkage
b) Incomplete linkage
2) Based on gene involved
a) Coupling linkage
b) Repulsion linkage
3) Based on chromosome involved
a) Autosomal linkage
b) X-chromosomal linkage
b) Describe different phases of Linkage and give Significance of Linkage in Crop Improvement.
Phases of linkage: a) Coupling phase b) Repulsion phase
a) Coupling phase
The linkage between two or more either dominant (AB) or recessive (ab) allels is referred to as coupling. A good example of coupling was reported by Hutchinson in maize for genes governing colour of seed (coloured or colourless) and shape of seed (full or shrunken). The colored seed is governed by dominant gene (C) and full seed is also govered by dominant gene (S) He made cross between plants having coloured full seeds (CCSS) and colourless shrunken seeds (ccss) The F, seeds were colouredfull. When the F, was test crossed with double recessive parent the following results were obtained instead of 1:1:1:1 ratio.
Parents Colouredfull x Colourless shrunken
Genotype CCSS x ccss
F1 CcSs
Test cross CcSs ccss
1) Colouredfull (CS) 4032 Parental type
2) Coloured shrunken (Cs) 149 Recombinant type
3) Colourless full (cS) 152 Recombinant type
4) Colourless shrunken (cs) 4035 Parental type
Total = 8368
This indicates that parental combinations are higher than recombinations, indicating presence of linkage. The parental recombinations occurred in 96.4% instead of 50% and recombinations were 3.6% instead of 50% in this case.
Signficance of lingake in plant breeding:
1. Effect on selection.
2. Effect on Genetic variance.
3. Effect on Genetic correlation
2) a) Define Gene Interaction & Enlist Different types of gene interaction with their rations.
b) Explain Complementary gene interaction with suitable Example.
Gene Interaction: When expression of one gene depends on the presence or absence of another gene in an individual, it is known as gene interaction.
Types of gene interaction
1. Recessive Epistasis
(9:3:4 Ratio).
2. Dominant Epistasis
(12:3:1 Ratio).
3. Dominant (Inhibitory) Epistasis
(13:3 Ratio).
4. Duplicate Recessive Epistasis
(9:7 Ratio).
5. Duplicate Dominant Epistasis
(15:1 Ratio).
6. Polymeric Gene Interaction
(9:6:1 Ratio)
7. Dihybrid gene interaction
(9:3:3:1 Ratio).
•Complementary genes
Non allelic genes that act together to produce a phenotype different from that produced by either alone.
In the case where identical phenotypes are produced by both homozygous recessive genotypes, the F₂ ratio becomes 9:7 the genotypes aaB-, A-bb and aabb produce one phenotype. Both dominant alleles, when present together, complement each other and produce
a different phenotype. In sweet pea, the development of purple flowers require the presence of two dominant genes P, and 2 or both the genes in homozygous recessive condition produce white flowers.
Since both the dominant alleles P, and P, when present together, they complement each other and produce a new phenotype and hence called complementary genes. 6. Dominant and Recessive interaction (13: 3) or Inhibitory gene interaction
In this type, a dominant allele at one locus can mask the expression of both alleles at second locus. Only two F, phenotypes result when a dominant genotype at and the recessive genotype at the other (bb) locus produce the same phenotyp B-, A bb and aabb produce one phenotype and aaB-produces another in the ratio of 13:3.
node colour in sorghum In sorghum, when crosses were made between plants with purple node and green node, the F, was with purple node. The F, segregated for 3 purple: green. In certain other crosses between plants with green node. Since purple is dominant over green, the F, is expected to be purple, but it is observed to be green. The gene for purple node is unable to express itself probably because of the presence of another gene. This gene is called inhibitory gene. It is capable of inhibiting the production of purple colour. Plants are purple, only if they possess the gene for purple colour, in the absence of the inhibitory gene In the presence of the inhibitory gene plants with the gene for purple Are unable to exhibit the purple colour and a enly green. Plants, which do not have the gene for purple colour, are also green whether they
3) a) Define Chromosomal Aberration. Enlist its different types and sub-types.
b) What is Structural Chromosomal Aberration? Enlist its different types & Explain Inversion.
Any Change which alters the normal structure of chromosome is known as chromosomal abression. It may take in somatic as well as germ cell.
Different types:
1) Alteration of gene number in chromosome
i) Deletion:
a) Terminal deletions
b) Interstitial or intercalary deletion.
ii) Duplication:
a) Tandem duplication
b) Reverse tandem duplication
c) Displaced duplication
d) Translocation duplication
2) Alteration of sequence of genes in chromosome
i) Inversions:
a) Paracentric inversion
b) Pericentric inversion
ii) Translocation:
a) Simple translocation
b) Shift
c) Reciprocal translocation.
Explanation of Inversion:
Strurtevant (1926) in Drosophila. Structural change in a chromosome in which a segment is oriented in a reverse order. Inverted segment is rotated to full 180".
Types of Inversion:
a) Paracentric inversion- Centromere dose not involved. Breaks occur in one arm of the chromosome.
Inversion heterozygote: Involve one chromosome of homologous pair.
Inversion Homozygote: Involve both chromosome of homologous pair. Meiosis is normal. Crossing over within inversion loop in a paracentric inversion heterozygote results in the formation of dicentric bridge and an acentric fragment after exchange. Dicentric chromosome leads to formation of bridge at anaphase. Acentric segment is loss due to lack of movement. Out of 4 gametes, 2 are normal and 2 deficient for same genes.
b) Pericentric inversion - When Centromere is involved. When break occurs in each of the two arms of a chromosome, the Centromere is included in the detached segment resulting in a pericentric inversion. Crossing over within the inversion loop results in the formation of chromatids with duplication and deficiency. Out of 4 chromatids, 2 are cross products and two are normal. The viable progeny is produced by non crossover chromosomes.
4) a) Define Inheritance. Enlist different laws of Inheritance & Explain any one.
b) Enlist different exceptions to Mendel’s laws of Inheritance & explain any one exception.
Inheritance: Transmission of characters from one generation to another generation is known as Inheritance.
laws of Inheritance
1. Law of Dominance
2. Law of Segregation
3. Law of Independent Assortment
4. Law of Codominance
5. Law of Incomplete Dominance
6. Law of Multiple Alleles
Law of Codominance:
In cases of codominance, two different alleles at a specific gene locus are both fully expressed in the phenotype of a heterozygous individual. This means that neither allele is dominant over the other, and they do not blend together as in incomplete dominance. Instead, both alleles are independently and visibly expressed side by side.
One classic example of codominance is seen in human blood type inheritance. The gene responsible for determining blood types has three main alleles: A, B, and O. A and B are codominant, while O is recessive.
Blood Type A: If an individual has two A alleles (genotype AA) or one A and one O allele (genotype AO), the A antigen will be expressed on the surface of their red blood cells, and they will have blood type A.
Blood Type B: If an individual has two B alleles (genotype BB) or one B and one O allele (genotype BO), the B antigen will be expressed on their red blood cells, resulting in blood type B.
Blood Type AB: If an individual has one A and one B allele (genotype AB), both the A and B antigens will be expressed on the surface of their red blood cells, leading to blood type AB. This is an example of codominance because both A and B alleles are fully expressed and present simultaneously.
Blood Type O: If an individual has two O alleles (genotype OO), they lack the A and B antigens on their red blood cells, resulting in blood type O.
One classic example of codominance is seen in human blood type inheritance. The gene responsible for determining blood types has three main alleles: A, B, and O. A and B are codominant, while O is recessive.
Blood Type A: If an individual has two A alleles (genotype AA) or one A and one O allele (genotype AO), the A antigen will be expressed on the surface of their red blood cells, and they will have blood type A.
Blood Type B: If an individual has two B alleles (genotype BB) or one B and one O allele (genotype BO), the B antigen will be expressed on their red blood cells, resulting in blood type B.
Blood Type AB: If an individual has one A and one B allele (genotype AB), both the A and B antigens will be expressed on the surface of their red blood cells, leading to blood type AB. This is an example of codominance because both A and B alleles are fully expressed and present simultaneously.
Blood Type O: If an individual has two O alleles (genotype OO), they lack the A and B antigens on their red blood cells, resulting in blood type O.
b) Enlist different exceptions to Mendel’s laws of Inheritance & explain any one exception
Exceptions to mendel's laws
1. Incomplete dominance
2. Co-dominance
3. Lethal genes
4. Over dominance
5. Penetrance
6. Expressivity
7. Pleiotropism
8. Multiple alleles
9. Gene interaction and Various modifications
5) a) Define Genetics and Enlist different branches of Genetics.
b) Explain Importance and Scope of Genetics in crop improvement
Genetics is defined as the branch of biological science which deals with the study of mechanism of heredity and causes of variations in living beings.
Branches of genetics:
1. Transmission genetics2. Molecular and Bio-chemical genetics
3. Population and Bio-metrical genetics
Importance of genetics:
Development of new wheat strain called Marquis developed in Canada which is resistant to disease which matures two weeks earlier.Changing the genetics make up harmful insects geneticist have become successful to reduce their fertility.
1. Green revolution
2.Biotechnology
3. Genetic Therapy.
Scope of genetics:
It is useful in the improvement of plants and domestic animals for economic importance. It is used in improvement of health and intelligence.It is useful in the field of medicine eg, in the study of disease transmission from one generation to other like diabetes heart trouble cancer, tubor-culosis (TB). It is useful in abnormalities and diseases through preventive medicines. It is useful legal tool in judiciary, court, law involving dispute of parents DNA tests can be used to decide real parents
6) a) What is Polyploidy? Give detailed Classification of Polyploidy.
b) How Polyploidy can be induced & State its uses in Plant Breeding.
Polyploidy: An organism or individual having more than two basic or monoploid sets of chromosomes is called ployploid ans such condition is known as polyploidy.
b. How polyploidy can be induced :
Polyploidy is manily induced by treatment with a chemical known as colchicines. This is an alkaloid which is obtained from the seeds of aplant known as colchicum auguminale; which belongs to the family liliaceae. Colchicines does not affect colchicum from which it is extracted, because this plant has an anticolchicine substance. Colchicines is applied in a very low concentration, because high concentration is highly toxic to the cells.
Significance in plant Breeding
1. Cytoplasmic inheritance has been useful in explaining the role of various cytoplasmic organelles in the transmission of character in different organisms.
2. Studies of cytoplasmic inheritance have played key role in mapping of chloroplast and mitochondrial genome in several species.
3. Development of cytoplasmic male sterility. CMS lines have been developed in several crops like maize, rice, sorghum, cotton etc.
4. Availability of CMS lines has facilitated the production of hybrid seed in these crops in cheaper cost.
5. Role of mitochondria in the manifestation of heterosis is gaining increasing importance these days.
6. Mutation of chloroplast DA and mitochondrial DNA leads to generation of new variants.
7) a) What is Multiple Allele? Give main features/ Characteristics of Multiple Allele.
b) Define Genetic Code. Describe in detail important features/ Properties of Genetic code.
Multiple allele: Existence of more than two alleles at a locus is referred to as multiple alleles,
Main features of multiple allele:
1. They belong to the same locus.
2. At a time one allele is present at a locus. 3. They control the same character.
4. There is no crossing over within multiple allele series.
5. The wild type is dominant on all other types of alleles.
6. Multiple alleles do not exhibit complementation.
Genetic code referes to the relationship between the sequence of bases in RNA and the sequence of amino acids in a polypeptide chain.
Nature of genetic code:
1. The code is triplet
2. The code is universal
3. The code is commaless
4. The code is non overlapping
5. The code is non ambigous
Important features:
1. Reciprocal Differences Character which are governed by cytoplasmic inheritance invariably exhibit marked differences in recoprocal crosses in F1, whereas in case of nuclear inheritance such differences are not observed in case of sex linked genes.
2. Maternal effect- In case of cytoplasmic inheritance, distinct maternal effects are observed. This is mainly due to more contribution of cytoplasm to the zygote by female parent tham male parent.
3. Mappability- Nuclear genes can be easily mapped on chromosomes, but it is very difficult to map cytoplasmic genes of prepare linkage map for such genes.
4. Non-mendelian segregation- The mendelian inheritance exhibit typical segregation pattern. Such typical segregation is not observed in case of cytoplasmic inheritance.
5. Somatic segregation- Character which are governed by cytoplasmic genes usually exhibit segregation in somatic tissues such as leaf variegation.
6. Infection-like cytoplasm- Cytoplasmic tritts in some organisms exhibit infections like transmission. They are associated with parasites, symbionts, or viruses present in the cytoplasm.
7. Governed by plasma genes- The true cases of cytoplasmic inheritance are governed by chloroplast or mitochondrial DNA.
8) a) Define Mutation. State different types of Mutation. What are features of Quantitative Traits?
b) Define Mutagen. Enlist Physical Mutagen & Describe application of Induced mutation in Crop Improvement.
Mutation: It refers to sudden heritable change in the phenotype of an individual. In molecular terms
mutation is defined as the permanent and relatively rare change in the number or sequence of nucleotide.
Main features of polygenic character or quantitative traits are as follows:
1. They are governed by several genes.
2. Effect of each gene is not detectable.
3. The variation is continuous.
4. Separation in to clearcut classes is not possible.
5. Such characters are highly influenced by environmental factors.
6. Statistical analysis is based on means, variances and covariances.
7. Such characters exhibit transgressive segregation,
8. The heritability of such characters is low as compare to oligogenic characters.
Mutagens refer to physical or chemical agents which greatly enhance the frequency of mutations.
Physical mutagens includes:
1. X-rayser
2. Gamma rays
3. Alpha particies
4. Beta rays particles
5. Fast and therial neutrons
6. Ultra violet rays.
Induced mutations are useful in crop improvement in five principal ways, viz.
1. Development of improved varieties.
2. Induction of male sterility.
3. Production of haploids.
4. Creation of genetic variability
5. Overcoming self incompatibility.
9) a) What is Mitosis? Describe various phases of Mitosis & give its Significance in Crop Improvement.
b) What is Meiosis? Describe various phases of Meiosis & give its Significance in Crop Improvement.
Mitosis is type of cell division in which each chromosome of the parent cell splits up longitudinally into two identical halves each half going to each of the daughter cells.
Phases of Mitosis
The spindle using nuclear division which produces two identical daughter cells from a mother cell is called mitosis.
The period in which one cycle of cell division is completed is called cell cycle. A cell cycle consists
of two phases, viz. 1.Interphase and 2.Mitotic phase.
1. Inetrphase
Interphase is generally known as DNA synthesis phase and mitotic phase refers to the period of nuclear division. Inetrphase consists of G., S and G₂ sub phases.
a) G1 phase
It is a pre-DNA replication phase. It lies between telophase and S phase.
This is the longest phase. Protein and RNA synthesis takes place during this phase.
b) S phase
This comes after G. The chromosome and DNA replication takes place during this phase.
c) G2 phase
This is a post-DNA replication phase. This is the last Stage of interphase.
Protein and RNA synthesis occur during this stage.
2 Mitotic phase
A phase of separation of replicated DNA into two identical daughter nuclei without recombination is called mitotic phase. Thus, the daughter nuclei have the same chromosome, combination as that of parent nucleus. The mitotic phase consists of four stages, viz., 1.prophase, 2.metaphase, 3. anaphase and telophase.
a) Prophase
Prophase starts immediately after G2 stage of interphase. Chromosomes look like thin thread and uncoiled in the early prophase, but become shortened, coiled and more distinct during mid prophase. In the late prophase, chromosomes appear more conspicuous, short and thick and longitudinally double. The two chromatids of each chromosome held at centromere are visible under light microscope. The nucleolus becomes smaller in size. The nuclear membrane and nucleolus disappear at the end of prophase Among all the four phases of mitosis, prophase takes longest duration
b) Metaphase
This phase hegins alier prophase. The spindle tubes are formed an chromosomes are oriented in the centre at equatorial plate. Chromosomes ar attached to the spindle tubes at the centromere. Chromosomes are clearl visible at metaphase Sister chromatids of each chromosome are joines together at the point of centromere, but their arms are free.
c Anaphase
This is the phase when chromatids separate at the centromere and move towards opposite sides or poles. Chromatids of each chromosome become free at the centromere, but each chromatid is attached to spindle tube. These chromatids suddenly move apart, one goes to one pole and the other toward the other pole. After separation, each chromatid becomes a chromosome
d Telophase
When chromosomes reach the pole, the last stage, telophase begins. The spindle tubes disintegrate, A new nuclear membrane is formed at each pol covering the chromosomes. The nucleoli also reappear at each pole Chromosome again become thinner and longer by uncoiling and unfoldind, and look like a single thread under light microscope. Then the nucleus enter interphase.
e) Cytokinesis
The division of nucleus is known as Karyokinesis. It is followed by divisio of cytoplasm, which is known as cytokinesis. The division of cytoplasm into two daughter cells may take place in two ways. In plants, the division ocytoplasm takes place due to formation of cell plate. The formation of such plate begins in the centre of cell, which moves towards periphery in both side dividing the cytoplasm in to two daughter cells. In animals, the separation ocytoplasm starts by furrowing of plasma takes place.
Phases of Meiosis
Definition: Two spindle divisions which reduce the chromosome number from diploid to haploid consitutue meiosis.The main fuction of meiosis is to produce gametes in an organism.
Stages of Meiosis-I
First Meiotic division- The first meiotic division consists of four different phases
1. Prophase-I
- This phase starts after inter phase and is of maximum duration.
- This consists of five sub stages.
Leptotene-
- Chromosomes look like thin thread under light microscope. They are inter-woven liken loose ball of wool.
- Chromosomes are scattered throughout the nucleus in a random manner.
- In some cases, chromosomes are visible on the chromosomes in the form of condensed regions
- RNA and protein synthesis also take place
Zygotene-
- Homologous chromosomes being to pair is called as synapsis.
- Chromosomes become shorter and thicker.
- The synthesis of remaining 0.3% DNA which has not taken place during S phase also occurs during this stage.
- Synaptonemal complex also develops during this stage.
Synaptonemal complex- A protein frame work which is found between paired chromosomes.
Pachytene-
- Chromosomes look like bivalents. Each bivalent has two chromatids. Thus each pair has four chromatids generally known as tetrad.
- The structure produced by pairing between homologous chromosomes is known as bivalent.
- The chromosome number looks like haploid number.
- Nucleolus is present and attached to a chromosome.
- Formation of chiasma and crossing over takes place during pachytene stage.
- Chiasma- Point of contact between non-sister chromatids.
Diplotene-
- Separation of homologous chromosomes begins. It starts at centromere and moves towards the end.
- The separating chromosomes are attached at some points. These points are called as chiasmata.
- These chiasmata are terminalized towards the end of diplotene,
- Chiasma terminalization- Chiasma slowly moves towards the ends of homologous chromosome.
- Chromosomes are further condensed and become still shorter and thicker.
- Nucleolus decreases in size.
Diakinesis
- This stage begins after complete terminilization of chiasmata.
- Chromosomes are further condensed.
- Bivalents are distributed throughout the cell
- Nucleolus and nuclear membrane disappear towards the end of diakinesis
10) a) Define Cell. Enlist different cell organelles & describe their functions.
b) Define chromosome & give classification of Chromosome.
Nucleus
1. Nucleous of nucleus forms the ribosomas and Synthesis the RNA.
2. Chromatin of nucleus plays important role in the inheritance of the characters from the parents to their offspring
Plastids
1. Leucoplasts are associated with storage of starch, protein and fat.
2. Chloroplasts are associated with photosynthesis.
Mitochondria
1. They the sites of cell respiration. The oxidation of carbohydrates, lipids and proteins occur in the mitochondria.
They supply energy to various processes of cell in the form of ATP.
2. Associated with cytoplasmic inheritance as it contain DNA
Endoplasmic reticulum
1.Associated with the synthesis of proteins (rough ER), lipids and glycogen (rough and smooth ER).
2. Act as an inter-cellular
3. Contain lot of enzymes. transport system for various substances.
4. Provide passage for mRNA from nucleus to the cytoplasm.
Ribosomes
Protein synthesis with the help of mRNA..
Lysosomes
1. Digestion of intracellular substances and particles.
2.Autophagy:- during adverse conditions they digest their own inclusions.
3. Autolysis: - perform the function of removing dead cells.
Golgi bodies
1.Packaging of food materials such as proteins, lipids and phospholipids for transport to other cells.
2. Lysosomes are believed to be originated from golgi bodies.
Centrioles
Formation of spindle apparatus during cell division.
Cell wall
1.To protect inner part of cell.
2. To give a definite shape to the cell.
3. To provide mechanical support to the tissues.
Plasma membrane.
1. It regulates the passage in and out of the cell.
2. It acts as a selectively permeable membrane.
3. checks the entry of toxic elements from outside in to the cytoplasm.
11) a) Why Mendel got Success in his experiment? Enlist different characteristics studied by Mendel
b) Give diagrammatic explanation of Watson & Crick Model of DNA.
Why Mendel got Success in his experiment
1. He had selected garden pea as his experimental material.
2. He concentrated his attention on only one character at a time.
3. He kept accurate record of progenies in successive generations
4 He divided the off springs in each generation into definite class according to visible contrasting characters
5. He studied large population to avoid sampling error
6 He analyzed the data by statistical method
Characters studied by Mendel-
1. Plant Stature- Tall and Dwarf
2. Position of flower- Axial and Terminal
3. Shape of Pod- Inflated and Constricted
4. Colour of Pod- Green and Yellow
5. Seed Shape- Round and Wrinkled
6. Seed colour-Yellow and Green
7. Seed-coat Colour-Grey and White
