INTRODUCTION TO GENETICS

Toipcs to be covered:❖MENDEL’S WORK ON TRANSMISSION OF

TRAITS

❖GENETIC VARIATION

❖Molecular basis of genetic information

Dr. Shweta Thakur

St. Bede’s College,

Shimla

❖MENDEL’S WORK ON TRANSMISSION OF

TRAITS

Mendel

Gregor Mendel, well-known as the

was born in Austria in 1822. As

a monk, Mendel revealed the basic principles of

heredity through experiments in his monastery's

garden. His experiments illustrated that the

inheritance of rigid traits in pea plants which

follows a particular pattern, is consequently

becoming the organization of modern genetics

and leading to the study of heredity.

Early Life

Gregor Johann Mendel was born on July 22,

1822.

He spent his early life in rural background, until

the age of 11, when a local schoolmaster who was

impressed with his aptitude for learning

recommended that he should be sent to secondary

school in Troppau to continue his education.

As he was progressing in his life, his studies

were a major economic strain on his family, but he

still excelled in his studies, and in 1840, he

graduated from the school with honors.

Mendel graduated from two-year program at the

Philosophical Institute of the University of Olmutz in

1843.

Experiments and Theories

• Around 1854, Mendel discovered thetransmission of hereditary traits in planthybrids. It was also commonly accepted that,over generations, a hybrid would revert to itsoriginal form, the implication of whichsuggested that a hybrid could not createnew forms.

• Mendel chose to use peas for hisexperiments due to their many distinctvarieties, and because offspring could bequickly and easily produced.

• He that had clearlyopposite characteristics—

Tall with short, smooth with wrinkled, those containinggreen seeds with those containing yellow seeds, etc.and, after analyzing his results, reached two of hismost important conclusions:

The Law of Segregation, which established that thereare dominant and recessive traits passed on randomlyfrom parents to offspring (and provided an alternativeto blending inheritance, the dominant theory of thetime),

The Law of Independent Assortment, traits werepassed on independently of other traits from parentto offspring.

Drawbacks of Mendel’s work

1. Mendel did little to promote his work.

2. It was generally thought that Mendel had

shown only what was already commonly known at

the time that hybrids eventually revert to their

original form.

3. The importance of variability and its

evolutionary implications were largely overlooked.

4. Furthermore, Mendel's findings were not

viewed as being generally applicable, even by

Mendel himself, and concluded that they applied

to certain species or types of traits.

Mendel was elected as abbot of the school in 1868 where he taught

for the previous 14 years, Gregor Mendel died on January 6, 1884,

at the age of 61.

Several decades later, Mendel’s research informed the work of

several noted geneticists, botanists and biologists conducting

research on heredity, that its significance was more fully appreciated,

and his studies began to be referred to as

and came out with some results in 1900, they found that that both the

data and the general theory had been published in 1866 by Mendel.

They gave credit to Mendel with priority.

His work was not considered important by Darwin, as it could

not explain theory of evolution. As genetic theory continued to

develop, the relevance of Mendel’s work fell in and out of favor, but

of the field, and this is the reason that he is

considered as "father of modern genetics."

Mendel Experiment Plant

Mendel worked on Pisum sativum. He selected this plant because of

special advantages. These are given below:

1. Some plants were tall and climbing, whereas others were short and

bushy. Some had coloured flowers, some white. Mendel noted 7 pairs of

such contrasting traits. The traits which always appear in two

opposing conditions, one dominant and other recessive are called

contrasting traits.

2. The traits of each kind of

and normally

restored to self pollination.

3. Cross pollination can be easily achieved by removing the stamens

before the pollen grains mature and dusting the pistil of this flower with the

pollen from a desired plant.

4. Pea plant has a short life cycle so that results can be seen within a

year.

5. It can be raised, maintained and handled conveniently.

6. It produces many seeds in one generation. This helps in drawing

correct conclusions.

7. Pea plants pure for each of the seven characters he selected was

available.

Law of Purity: Mendel confirmed that the seven

characters noted by him were true breeding that is

they passed on from parents to offspring. For this

he raised seeds by self-pollination from each of

the seven types of plants and sowed them in the

garden. The plants that grew from the seeds showed

that the seven characters were transmitted from

parent to offspring. Seeds from tall plants produced

tall plants and yellow seeds produced plants with

yellow seeds. Mendel called each of these a pure

character.

Mendel investigated the pairs of pea plants with

one contrasting trait. Mendel studied on the

following seven characters with contrasting traits (Fig.

1.1):

1. Flower color: Violet or

purple/white

2. Flower position: Axial/terminal

3. Seed color: Yellow/green

4. Seed shape: Round/wrinkled

5. Pod shape: Inflated/constricted

6. Pod color: Green/yellow

7. Stem height or length:

Tall/dwarf

Monohybrid Cross

For monohybrid cross, Mendel began with a pair

of pea plants with two contrasting traits i.e., one

tall and another dwarf. The cross-pollination of

tall and dwarf plants resulted in tall plants. All the

hybrid plants were tall. He called this as a first

hybrid generation (F1) and offspring were called

Filial1 or F1 progeny. He conducted an experiment

with all the seven contrasting pairs. He observed

that the entire F1progeny showed one pattern in

their behavior i.e., they resembled either one of

the parents. Another parent character was

completely absent.

He continued his experiment with self-pollination of F1 progeny

plants. Surprisingly, he observed that one out of four plants

were dwarf while other three were tall. The tall and the short

plants were in the ratio of 3:1. He also noted that no progeny was

in intermediate height i.e., no blending. The result was same for

other traits of plants too. And he called them second hybrid

generation and offspring were called Filial 2 or F2 progeny.

Mendel observed that traits were absent in F1 generation

had reappeared in F2 generation. He called such suppressed

traits as recessive traits and expressed traits as dominant

traits. He also concluded that some ‘factors’ are inherited by

offspring from their parent over successive generations. Later,

these ‘factors’ were called genes. Genes are responsible for the

inheritance of traits from one generation to another. Genes

consist of a pair of alleles which code for different traits. If a pair of

alleles is same i.e., TT or tt, such alleles are called homozygous

pair while those that are different or non-identical (e.g. Tt) are

called heterozygous pair.

Alleles

The cross between two

monohybrid traits (TT and tt)

is called monohybrid cross.

Monohybrid cross is responsible

for the inheritance of one gene.

It can be easily shown through a

Punnett Square.

Dihybrid Cross

Mendel studied the inheritance of two characters

simultaneously. A breeding experiment producing two

characters at the same time is called dihybrid cross. In one

such experiment, Mendel considered form of the seed

and colour of the cotyledons. He crossed a pea plant

having round seeds and yellow cotyledons with a pea plant

having wrinkled seeds and green cotyledons. All the plants

of F1 generation had round seeds and yellow

cotyledons. F1 plants on selfing produced four different

kinds of plants in F2 generation. These were in the ratio

of 9 with two dominant traits (round seeds and yellow

cotyledons): 3 with one dominant and one recessive trait

(round seeds and green cotyledons): 3 with other dominant

and recessive trait (wrinkled seeds and yellow cotyledons):

1 with two recessive traits (wrinkled seeds and green

cotyledons).

Among the four kinds of F2 plants, two types had parental

combinations of seed traits i.e. round seeds and yellow cotyledons,

and wrinkled seeds and green cotyledons; whereas the other two

types had new combinations of seed traits i.e. round seeds and

green cotyledons, and wrinkled seeds and yellow cotyledons.

The phenotype ratio of a dihybrid cross is purely the mixture of

the two monohybrid ratios. Since both the round and yellow traits

are dominant their F2 monohybrid ratios would be 3 round: 1

wrinkled: 3 yellow: 1 green. By multiplying these ratios together

we get:

(3 round: 1 wrinkled) x (3 yellow: 1 green) = 9 (round yellow):

3(round green): 3(wrinkled yellow): 1(wrinkled green)

From the F2 ratios of dihybrid cross, Mendel concluded that the

inheritance of cotyledon colour is independent of the inheritance

of seed shape. This led to law of independent assortment.

❖ GENETIC VARIATION

It refers to diversity in gene frequencies. Genetic variation can

refer to differences between individuals or to difference between

populations. They are inheritable variations which can occur due to

following reasons.

1. Mutations: They are which develop due

to permanent change in genotype. Mutations are of three types:

(a) Change in chromosome number- There are changes in the

numerical strength of chromosome (Polyploidy and Aneuploidy).

(b) Chromosome Aberration- There is a structural modification of

chromosome due to deletion, duplication, inversion and

translocation.

(c) Gene mutation- They are discontinuous variation caused by

changes in nucleotide type and sequence of DNA segment

representing a gene due to inversion, substitution and frame shift

mutations.

2. Gene recombination: They are new combination of genes which

are caused by crossing over, chance separation and combinations of

chromosome. Gene recombinations are helpful in dispersing

mutations in the gene pool.

3. Hybridization: It is the of

organisms which are

. It helps in intermingling

of genes belonging to different group of

same variety, species and sometimes

different species.

4. Genetic drift: It is a change in gene

number and gene frequency in a small

isolated population due to intensive

inbreeding and loss of genes with

decrease in population.

❖MOLECULAR BASIS OF GENETIC INFORMATION

Direct evidences for DNA as the genetic material

Transformation Experiments (Fig.1.4)

1. Frederick Griffith, 1928 encountered a

phenomenon known as genetic transformation.

2. Transformation is a

produced in one strain of bacteria by a

substance isolated from another strain of same

kind of bacteria. A substance which brings about

transformation is called transforming agent.

3. Griffith experimented with Streptococcus

pneumoniae, a bacterium that causes pneumonia.

There are two different strains of bacterium.

a. Smooth (S): It is a virulent strain, has a glistening appearance

owing to presence of polysaccharide (poymer of glucose and

glucoronic acid). It is pathogenic.

b. Rough (R): It lacks capsule forming rough colonies. It is non

virulent and does not cause pneumonia.

Both S and R forms occur in several types: SI, SII, SIII, SIV and RI, RII,

RIII, RIV

c. All these subtypes of S and R bacteria differ with each other in type

of antigens.

4.(a)Griffith injected laboratory and it was

found that mice suffered no illness because R-II pneumococci is

(b) Mice injected with suffered from

pneumonia so as a result of .

(c) Griffith injected , mice

(d) Mice were injected with a mixture of living avirulent RII and heat

killed SIII virulent mice was found to possess symptoms of

pneumonia with high mortality. After postmortem of dead mice it was

found that their heart blood had both RII and SIII pnuemococci.

Conclusion: Presence of

This process is

called Griffith effect/ Bacterial

transformation.

Identification of transforming principle/substance.1. Griffith could not understand, the cause of bacterial

transformation and it was firstly identified by Avery, MacLeod

and Mc Carty. They partially purified transforming principle from

cell extract (cell free extract of SIII bacteria) and demonstrated that

it was DNA.

2. Avery, MacLeod and Mc Carty modified known schemes for

isolating DNA and prepared samples of DNA from SIII bacteria.

3. They after a

period of time they placed a sample of SIII containing RII

bacterial culture on an agar surface

4. Some of the that grew were . To show that

this was a permanent genetic change, they dispersed many of

newly formed SIII colonies and placed them on second agar

surface resulting in colonies that were SIII type.

5. If RII colony from original mixture was

dispersed, in subsequent

generations. Hence RII colonies retained RII

character whereas transformed SIII colonies

bred true as SIII.(a) SIII strain kills mouse

SIII encapsulated + live mouse →dead mouse

b) RII strain does not kill mouse

RII strain + live mouse → live mouse

(c) Heat killed SIII strain does not kill mouse

SIII (heat)→ SIII + live mouse → live mouse

(d) RII strain + heat killed SIII strain both of which are

separately non lethal but together kills mouse

RII strain + SIII heat killed + live mouse → Dead mouse

(a) Chemical analysis showed that major component is deoxyribose

containing nucleic acid.

(b) Physical measurement showed sample contained highly viscous

substance having properties of DNA.

(c) Transforming activity is not lost by reaction i.e with enzymes

trypsin, chymotrypsin, RNAase and lipase.

(d) Treatment of material with DNA ase enzyme inactivated

transforming principle.

Results of Avery Experiment

(a) R type bacteria + Carbohydrates of S type bacteria - R

type bacteria

(b) R type bacteria + Protein of S type bacteria - R type

bacteria

(c) R type bacteria + DNA of S type bacteria - S type

bacteria

(d) R type bacteria + DNA of S type bacteria + DNAase - R

type bacteria

Blender Experiment

Fig. 1.6: Diagrammatic representation showing sulphur labeled protein capsule

and phosphorus labeled DNA bacteriophages undergoing 3 stages: Infection,

Blending and Centrifugation

1.It was performed by Hershey and Chase, 1952.

2.DNA was injected by phage into bacterium which

required all the information required to synthesize

progeny phage particles.

3.Single particle of phage T2 consists of DNA encased in

protein shell. Protein shell have sulphur containing

amino acids (methionine and cysteine)

4.DNA is the only phosphorus substance in phage.

5.Phage DNA was made radioactive by growing infected

bacteria on a medium containing radioactive

phosphate 32P. Since phage proteins do not contain

phosphorus so only DNA would be labeled. Similarly

phage proteins were labeled with 35S. This labeling

would enable one to distinguish between DNA and

protein of the phage.

6.(a) Hershey and Chase allowed both kind of labeled

phage particle to infect E.coli bacteria.

(b) Infected bacteria were agitated in warming blender

(c) After shaking, only 32P found associated with

bacterial cell and

and not in bacterial cell.

(d) When phage progeny was studied for radioactivity it

was found that phage progeny was labeled only 32P

and not 35S.

(e) This clearly indicates that only

Hershey and Chase experiment proved that DNA

entering host cell carries all genetic information for

synthesis of new phage particles.

Bacterial Conjugation

1. Lederberg and Tatum (1946) found that F+male

E.coli conjugated F- female E.coli.

2. Unidirectional transfer of F+ factor of male cell

to F- (female cell)

3. F+ female was found to be a fragment of DNA

which occurred in cytoplasm of bacterial cell.

4. The bacterium with F plasmid is the donor F+

male. Fertility factor genes confer bacteria with the

ability to transfer genetic material to the recipient cell.

The bacterium without F factor is called F- female.

5. In bacterial conjugation F plasmids are transferred

and not the entire bacterial genome.

6. Various steps are followed in bacterial conjugation

(a) F+ cells produces hair like appendages cell sex

pili which facilitates cell to cell contact with F- strain by

forming a conjugation tube. The formation of sex pili is

governed by genes of F factor.

(b) Replication of F factor making a copy.

(c) Transfer of copy F plasmid to the recipient cell

via conjugation tube.

(d) Conjugation tube dissolves. Now F- strain is

also F+ as discussed above in point number 3.

Evidences for RNA as genetic material of some

viruses

1. Gierer and Schramm inoculated tobacco plants with

purified RNA from TMV. TMV like lesions later were

identified on tobacco plant.

2. Fraenkel-Conrat and Singer first developed the

techniques for separating TMV particles into RNA and

protein. RNA and protein were used separately for

infectivity, it was shown that RNA alone caused infection.

They developed techniques for forming reconstituted or

reconstructed viruses containing protein from one mutant

strain of TMV and RNA from another. Such hybrid

viruses were allowed to infect tobacco leaves and the

progeny were examined.

(Fig.1.11).

Fig.1.11: Diagrammatic representation showing the nature of infectivity when

TMV particles separates into RNA and protein, it was observed RNA alone

causes infection

3. Chimeric virus particles were synthesized by

combining RNA of one strain and protein from

another. These particles had serological properties

of those strains from which protein was derived,

while in some it resembled the strains from which

RNA was used. Reciprocal chimaeras were

synthesized and allowed to infect the host plant.

It was observed that the progeny viruses had

proteins of that viral strain from which RNA of

the chimeric virus particles was derived which

proved that specificity of viral proteins was

determined by RNA alone and not by the viral

proteins (Fig 1.12).

Fig.1.12: Diagrammatic representation showing chimeric virus particles

which were synthesized by combining RNA of one strain and protein from

another. It was observed that virus progeny of Type A caused infection as it

contains the RNA of Type A strain

RNA

Another nucleic acid is called ribonucleic acid, or

RNA. Like DNA, RNA is a polymer of nucleotides. Each of the

nucleotides in RNA is made up of a

In the case of RNA, the

five-carbon sugar is ribose, not deoxyribose. Ribose has a

hydroxyl group at the 2′ carbon, unlike deoxyribose, which

has only a hydrogen atom. RNA nucleotides contain the

nitrogenous bases adenine, cytosine, and guanine. However, they

do not contain thymine, which is instead replaced by uracil,

symbolized by a U. RNA exists as a single-stranded molecule

rather than a double-stranded helix. Molecular biologists have

named several kinds of RNA on the basis of their function.

These include messenger RNA (mRNA), transfer RNA

(tRNA), and ribosomal RNA (rRNA) molecules that are

involved in the production of proteins from the DNA.

Fig.1.13: Ribose which is found in RNA has a hydroxyl group at the 2′

carbon whereas deoxyribose which is found in DNA has only hydrogen

atom at 2′ carbon