law of heridity

8/6/2019 Law of Heridity

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Mala TrivediPh.D

DIVISION OF GENETICS AND PLANT BREEDING

CENTRAL INSTITUTE OF MEDICINAL AND AROMATIC PLANTS (CIMAP) (CSIR), LUCKNOW,

INDIA


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AN INTRODUCTION TO GENETICS

Genetics is the science of heredity. Hereidity istransmission of traits from one generation to other.

Genetic information directs cellular functions, determine an

organisms external appearance and serve as link betweengeneration in every species.


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Gregor Johann Mendel (1822-1884)

-First published his work in 1865, on garden pea, Mendels success wasdue to, elegant model of experimental design.

-Good methodology necessary for experimental biology.

-Selection of good plant material, garden pea (Pisum sativum) (Self

fertilizing and easy to cross pollinate), Reproduces well , mature in oneseason

-Selection of seven contrasting visible traits. Used true breeding seeds.

-Restricted his experiments to one or few contrasting traits, kept

accurate quantitative records,a necessity in genetic experiments.


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Some common Terms (Terminology) Inherited character Those characters which

transfer from one generation to other are known as Inheritedcharacters/ traits. In other words - The characters in a population

are under the control of separate particulate factor called

GENES. That are transmitted from one generation to other by

the chromosome.

GENESUnit of hereidity, controlling character of population

CHROMOSOME- These are thread like structures present in

the nucleus and serve as vehicle for transmitting genes from one generation to

other. Homologous chromosome Chromosome that synapse or pair during

meiosis. Chromosome that are identical to their genetic loci and centromere

placement


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MONOHYBRID CROSS- It involves

only one pair of contrasting

traits

-P1 Parental generation

-F1 First filial generation

-F2 Second filial generation


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Mendels First Three Postulates ()

These are also called principles ofinheritance

1. Unit Factors in pair Genetic characters are controlled by unit

factors that exist in pairs in individual organisms, i.e., 3

combinations DD, dd, Dd, in monohybrid cross for plant height.

2. Dominance/ Recessive ()-When two unlike unitfactors responsible for a single character are present in a single

individual, one unit factor is dominant to the other, which is said

to be recessive.

3. Segregation During the formation of gametes, the

paired unit factors separate or segregate randomly so that each

gamete receives one or the other with equal likelihood.


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Modern Genetic terminology

Phenotype - The physical appearance of a trait. Alleles () - Alternative form of a single gene

Genotype - When the alleles are written in pairs to

represent two unit factors (genes) present in an individual for

particular character e.g DD, Dd and dd are the symbols for

tallness called genotype.

Homozygous - When both alleles are same e.g.

DD, dd Heterozygous- When alleles are different e.g.Dd.


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TEST CROSS

It is used to find out

the genotype of and

individual expressing

dominant phenotype.It will always

produce 1:1 ratio.


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THE DIHYBRID

CROSS(

)Two factors are

involved.

Here ratio will be

9:3:3:1. In spite of change in P1

phenotype, both the F1

and F2 results remain

unchanged.


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Mendels fourth postulate- Independent Assortment-

(

)

During gamete() formation, segregating pairs of

unit factors assort independently of each other. Therefore

all possible gametes are formed in equal frequency.

The frequencies of all possible F2 phenotype can be

predicted by applying Product Law


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PPRODUCT LAW

WWhen two independent events occur

simultaneously, the combined probability of two out

comes is equal to the product of their individual

probabilities of occurrence.


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TTHE TRIHYBRID

CROSS

Three factors/ characters

are involve


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Forked line method

This method is much easier, here each trait is first

considered separately and later on combined usingforked-line method also called branched method.

It relies on simple application of the law of probability

or product law, established for dihybrid cross.


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Forked line method


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Unit factors, Genes and Homologous Chromosomes


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The current accepted concept is that a chromosome is

composed of a large number of linearly ordered, information

containing units called genes.

Mendels unit factors actually constitute a pair of gene located

on a pair of homologous chromosomes.

Location of a particular gene on chromosome is called locus.

The different form taken by a given gene, called alleles.


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Independent Assortmentand

Genetic Variation:

One of the major consequences of independent assortment is the

production of an individual of genetically dissimilar gametes. The

number of possible gamete each with different chromosome

composition, is 2n where n is equal to haploid number.

Thus if a species has a haploid chromosome number of 4, then 24

or 16 different gamete combinations can be formed as a result of

independent assortment.

For human being n=23 therefore over 8 million different types of

gametes are formed. Each offspring represent 64 x 1012 potential

genetic combinations.


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Evaluating Genetic Data :

Chi-Square Analysis

NullHypothesis()

No real difference between

the measured values

( or ratio) and the

predicted values ((ratio) exists. The apparent

difference can be attributed

purely to chance.

(o - e)2

2=

e


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Mendels 3:1 and 9:3:3:1 ratios are based of following

assumptions-

1.Each allele is dominant or recessive, 2.Segregation is operative,

3.Independent assortment occurs and 4.Fertilization is random.

The last three are influenced by chance events and show random

fluctuation. This concept, called chance deviation.

1.The outcomes of segregation, independent assortment, and

fertilization, are subjected to random fluctuations from their

predicted occurrences as a result of chance deviation.

2.As the sample size increases, the average deviation from the

expected results decreases. Therefore, a large sample size

diminishes the impact of chance deviation on the final outcome.


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Human Pedigrees ():The family tree showing phenotype of trait of interest, for each

member called pedigree. Proband- An individual in whom agenetically determined trait of interest is first detected.


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GENETIC STUDIES FORDISEASE RESISTANCE() IN OPIUM POPPY (Papaver somniferum L.)

AGAINST COLLAR ROT CAUSED BY

Rhizoctonia solaniAG-4

Mala Tiwari nee Trived

Ph.D


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A.Plant Material

A total 106 accessions() of opium poppy (Papaversomniferum L.) germplasm collected and maintained in the

National Gene Bank of Medicinal and Aromatic Plants at CIMAP,

Lucknow, India were used in the experiment. Of these, 82

accessions(inbred lines,)

9 selections(),8 commercial varieties,

2 mutants

4 introductionsfrom Rajasthan and

1 from Thailand.


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Frequency distribution for collar rot and

assessment of genetic ratios

G2 (chi-square)-test was used to test the goodness of fit in different

generations of the crosses, where

(O E) 2

Ewhere, 7 =Summation taken over all

O O= Observed frequency

E = Expected frequencyG2 = 7


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Field Experiments

Selection of parents and development of hybrids

Development of different generations of the crosses Selection of genotypes and assessment of variability

Recording of agronomic data

1. Morphometric traits

Plant height (cm). Days to 50% flowering

Days to maturity.

Number of leaves per plant:

Estimation of Leaf area per plant (cm2)

Peduncle Length per plant (cm) Number of capsules per plant.

Number of stigmatic rays per capsule.

Straw yield per plant (g).

Seed yield per plant (g).


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2. Physiological traits

CC

hlorophyll content in leaves : [Arnon (1949)]

3.3. Biochemical traits

Peroxidase activity in the leaf tissues. [Shannon et. al. (1966)]

Estimation of Nitrogen and seed protein content (%) [Kjeldhal method

Jackson, (1973)]

Oil content in seeds

4.Chemical traits Estimation of morphine () in straw : [Gupta and Verma, (1996)]


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Collar rot X Collar rot P1

RESISTANT SUSCEPTIBLE

All susceptible F1

RESISTANT :SUSCEPTIBLE F2

1 :3


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RAKSHITA recently released variety of

Opium poppy (Papaver somniferum L. )


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Central Institute of Medicinal and Aromatic Plants ,

Lucknow, India


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Central Institute of Medicinal and Aromatic Plants ,

Lucknow, India


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law of heridity

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