Botany- Genetics Model Ans

Que.1- Define cell division. Enllst different stages of Meiosis. Write down the significance of Meiosis and Mitosis.

Ans: Definition

Cell division: The process in which a cell divides into two cells, each containing identical genetic material, as that of the original cell.

Stages of Meiosis:

• Prophase-1

• Metaphase-1

• Anaphase-1

• Telophase-1

Significance of mitosis

1. Genetic stability: Mitosis produces two daughter cells which have the same number of chromosomes as that of parent cell.

2 Growth: The number of cells within the organism is increased by mitosis and this is the basis for growth of organisms.

3. Cell replacement: Replacement of oid cells and dead cells in an organism is achieved by mitosis.

4. Asexual reproduction: Production of new Individuals through asexual Reproduction is achieved by mitosis.

Significance of meiosis

1. Melosis enables the chromosome number of a sexually reproducing species to be kept constant from generation to generation

2. Meiosis introduces the genetic variation in the offspring's of sexually reproducing individuals by means of independent assortment and crossing over (recombination).

Que.2- Define linkage. State the phases of linkage. Explain the types of Linkage.

Ans. Definition

Linkage: When two or more genes present on the same chromosome and they do not exhibit Independent assortment, they are said to be linked and the phenomenon of transmission of linked genes is called linkage.

Phases of linkage

• Coupling phase 

In this phase, both the linked genes have their dominant alleles in one chromosome and recessive alleles in the other.

• Repulsion phase

In this phase there is tendency of one dominant one recessive gene to ente the gamete and remain on one chromosome and the other dominant with recessive gene in other chromosome.

Types of linkage

Linkage is classified on the basis of following three criteria.

I. Based on crossing over

(i) Complete linkage

Linkage in which crossing over does not occur is known as complete linkage or absolute linkage. In complete linkage test cross progenies posses only parental types.

(ii) Incomplete linkage

In some cases, frequency of crossing over occurs between linked genes, it is know as incomplete linkage. In incomplete linkage, the test cross yields some as recombinants besides parental combinations.

II. Based on status of genes Involved

(i) Coupling linkage

It refers to linkage either between dominant genes or between recessive genes.

(ii) Repulsion linkage

It refers to linkage of some dominant genes with some recessive genes.

III. Based on chromosomes involved

(i) Autosomal linkage

It refers to linkage of such genes, which are located in other than sex chromosomes.

(ii) X-chromosomal linkage

It refers to the linkage of genes, which are located in sex chromosomes.

Que.3- Define crossing over. Enlist the theories of crossing over. Write factors affecting crossing over. Significance of crossing over.

Ans. Definition

Crossing Over: Crossing over may be defined as "interchange of chromosomal segments between non-sister chromatids of a homologous chromosome pair".

Theories of crossing over 

• Breakage And Reunion Theory 
• Copy Choice Theory 
• Partial Chiasma Type Theory 

1.  Single Cross Over 
2.  Double Cross Over

Factors affecting crossing over

i. In drosophila male and female silk moth, it is completely suppressed however in drosophila female, the frequency very high.

ii. Mutation reduces crossing over in all chromosomes of drosophila.

iii. Due to accidental change in sequence of genes Le Inversion the crossing over Is reduced.

iv) High and low temperature variations in drosophila Increase the rate of crossing over.

v) X-ray increases crossing over in the vicinity of centromere.

Significance of Crossing Over

i. Crossing over provides a direct evidence for the linear arrangement of genes in the chromosome.

ii. Since crossing over results in recombination of genes variations are produced.

iii. Crossing over helps in the construction of chromosome maps.

iv. Crossing over is universal in occurrence, occurs in plants, animals, bacteria, viruses and moulds.

Que.4- Define polyploidy. Give its classification and use of Polyploidy In crop improvement.

Ans. Definition

Polyploidy refer to any organism in which the number of chromosome sets exceeds two i.e.

an organism with more than two set of chromosomes or genome.

Classification of polyploidy:

1. Autopolyploid

In autopolyploids the multiple sets of chromosomes are identical (eg). Genome an identical with each other. (eg). Banana 2n=3x=33

A. Autotriploid

The triploid organisms have three sets of chromosomes.

B. Autotetraploid

The organisms with four genomes (4n) in the nuclei of their somatic cells are called tetraploids.

2. Allopolyploid

A species or types of plant derived from doubling the chromosomes In the F1 hybrid of two species, is called an amphidiploids. In amphidiploids the two species are known.

Eg.

Gossypium hirsutum - 2n-4x=52 

Que.5- Describe different phases of mitosis. Explain the significance of mitosis.

Ans. Phases of mitosis:

2.1. Prophase

1. Coling and condensation of chromosome takes place which make them visible as thread like Structures.

2. Each chromosome has two identical longitudinal splits called identical or sister chromatids, which are attached by common centromere.

3. Migration of centrioles to opposite ends of the cell.

4. Disappearance of nucleolus and beginning of the breakdown of the nuclear membrane

5. Formation of spindle fibre.

2.2. Metaphase

1. Formation of spindle fibres is completed and chromosomes are attached to the spindle fibres at the point of centromere.

2. Movement and arrangement of all chromosomes in metaphase plate or equatorial plate.

3. Sister chromatids of each chromosome are joined together at the point of centromere, but their arms are free.

4. Chromosomes are clearly visible.

2.3. Anaphase

1. This is the shortest phase of the mitotic division

2. This stage begins with splitting of centromere into two, which allow the sister chromatids to separate and move to opposite poles.

3. The separated sister chromatids are called as new chromosomes.

4. The arms of each chromosome drag behind their centromeres giving them characteristic shapes depending up on the location of centromere.

2.4. Telophase

1. Chromosomes reach the opposite poles and spindle fibres begin to disintegrate.

2. Nuclear membrane is reestablished.

3. Nucleoli is reformed.

4. Chromosomes again become thinner and longer by uncoiling and unfolding.

2. Nuclear membrane is reestablished.

3. Nucleoli is reformed.

4. Chromosomes again become thinner and longer by uncoiling and unfolding.

Que.6- Define chromosomes. Explain the structure of chromosomes.

Ans. Definition

Chromosomes: Chromosomes are the darkly stained bodies seen during the metaphase stage of mitosis.

Structure of chromosomes:

1. Centromere (Primary Constriction)

- It is a localized region of the chromosome with which spindle fibres are attached during metaphase is known as centromeres of primary constriction or kinetochore.

Centromere has four important functions, viz., (i) orientation of chromosomes movement of chromosomes during anaphase. (ii) at metaphase (iii) formation of chromatids, and (iv) chromosomes shape.

Centromere may occupy various positions on the chromosome, viz., terminal, sub-terminal, median etc.

2. Chromatid

• One of the two distinct longitudinal subunits of a chromosome is called chromatid.

• These subunits of a chromosome get separated during anaphase.

• Chromatids are of two types viz., sister chromatids and non-sister chromatids.

• Sister chromatids originate from homologous chromosomes.

3. Secondary Constriction

• Some chromosome exhibits secondary constriction in addition to primary to constriction.
• It may be present either in short or long arm of the chromosome.
• The chromosomal region between secondary constriction and nearest telomere (end of the chromosome) is called as satellite or trapant.
• The chromosome having satellite is known as satellite chromosome.

4. Telomere

• The two ends of the chromosomes are known at telomeres.
• Telomeres are highly stable and they do not fuse or unite with telomeres of other chromosomes.
• The structural integrity and individuality of the chromosome is maintained by telomeres.

5. Nucleolar Organizer Region (NOR)

• During interphase, nucleolus of the cell is always associated with secondary construction of satellite chromosome.
• So the secondary constriction is also called as NOR.
• The NOR contain several copies of gene coding for ribosomal RNA.

6. Chromomeres

• The chromosome of some of the species show small bead like structures called as chromomeres.
• The distribution of chromosomes in the chromosome is constant.
• Available evidence indicates that chromomere represents a unit of DNA replication, chromosome coiling, RNA synthesis and RNA processing.

7. Chromonema

• Under light microscope, thread like coiled structures are found in the chromosomes and chromatids which are called chromonema (plural chromonemata).
• Chromonema is considered to be associated with three main functions.
• It controls size of chromosomes, results in duplication of chromosomes and is the gene bearing portion of chromosomes.

8. Matrix

• A mass of aromatic material in which chromonemata are embedded is called matrix.
• Matrix is enclosed in a sheath which is known as pellicle.
• Both matrix and pellicle are non genetic materials.

Que10. Define mutation. Explain different types of mutation. Also explain the merit and demerits of mutation breeding.

Ans. Definition

Mutation may be defined as sudden heritable change in a gene which is not due to segregation or recombination.

Types of Mutations:

1) Germinal mutation:- Change in reproductive cells (sperms & eggs) of the individuals take place. The change my take place In gamete or zygote.

2) Somatic mutation:- Change in somatic or vegetative cells. However they are not hereditary they perish with individual which they occur.

3) Reverse mutation:- The mutated gene undergoes change i.e. back to normal hence they are called reverse or back mutation. They are rare and less frequent than normal mutation.

4) Spurious mutation:- When recessive gene appear phenotypically by the crossing over it is known spurious mutation. They are generally hidden mutation e.g. pink eyes colour in drosophila.

5) Homozygous mutation:- Change occur due to structural variations in chromosome İ.e. chromosomal aberrations or change in chromosome number.

6) Biochemical mutation:- They affect the biochemical process i.e. loss of ability to synthesize vitamins and amino acids. They are studied In neurospora by beadle and taum.

7) Lethal mutation:- they causes death of organisms similarly loss or alternation in a essential function of an organism.

Que-11. Define sex determination. Enlist various theories. Explain any one of them.

Ans. Definition

Sex determination:

A sex determination system is a biological system that a diterminds The development of sexual Carecterastics in an organism is Called as Sex determination.

Theory of Sex Determination:

• Chromosomal Theory
• Quantitative Theory
• Gene Balance Theory
• Metabolic Differentiation Theory
• Hormonal Theory 
• Environmental Factors Theory

Explanation:

Environmental Factors Theory

I. In bonellia when larvae get attached to the proboscis of female develop into male. But become female, if settle down in mud and live in sea water.

II. In mollusca If young animals are reared alone, become female, but if reared in close contact of adult, they become males.

III. Whitney and shull showed that in rotifers crowding of daphnids together males were produced.

IV. Many fowering plant earthworm and snails hermaphrodites havine genes for both sexes but sexes exposed by separate part to exclusion of others.

V. In dinophilus sex is determine by such if egg i.e. large egg produce female and small one males.

VI. In equisetum females are produced by growth condition and male under poor condition.

Que-12. Define gene interaction. Give its type with there ratio and explain any one.

Ans. Definition

Gene Interaction: The phenomenon in which two or more pairs of genes affect the same Character.

Types of Gene Interaction:

Interaction of allelic genes

i. Lethal genes(2:1)

Non-allelic gene interaction

i. Interaction without modification of normal F1 ratio(3:1)

ii. Interaction with modification of normal F1 ratio 

- Complementary factory (9:7) 

- Supplementary factory (9:3:4) 

- Epistasis (12:3:1)

Supplementary factor interaction

Involves two pairs of non-allelic genes Affect the same character One of the dominant gene has visible effect itself • Second dominant gene expresses itself when supplemented by the other dominant gene of a pair Coat color (black, albino and agouti) of mice follows supplementary gene interaction.

F2 generation shows segregation in the ratio 9:3:4. This behavior is based on ratio of dihybrid cross, so the trait must be governed by two pairs of genes.