principles of inheritance and variations presentation part- i

PRINCIPLES OF

INHERITANCE

AND

VARIATION

GENETICS

Branch of science that deals with the study of heredity

and variations

Greek word Genesis –To grow

The term genetics was coined by Bateson(1906)

HEREDITY

The transmission of characters from one generation to the other or from the parents to the offsprings

VARIATIONS

The differences between the parents and the offsprings

Offsprings of the same parents

or

Individuals of the same species

FATHER OF GENETICS

Gregor Johann Mendel an Austrian Monk

Born on July 22, 1822

Conducted several hybridization experiments on Garden Pea (Pisum sativum)

Cultivated and tested approx. 28,000 pea plants

for 7 years (1856-1863) and

gave his laws governing inheritance of traits

MENDELIAN GENETICS

Mendelian genetics (or Mendelian inheritance or Mendelism)

is a theory of genetic inheritance which was developed by

Gregor Mendel.

QUESTIONS

Q1. What is genetics?

Q2. What is heredity?

Q3. What are variations?

Q5. Who coined the term genetics?

Q6. Who is the called the Father of Genetics?

Q7. Which material Mendel selected for his experiments?

Q8. What is the botanical name for garden pea?

Q9. For how many years Mendel conducted his experiments?

MENDEL’S WORK NOT RECOGNIZED

Mendel’s work was not widely published

Crude microscopes were there

Mendel’s approach for using Mathematics to

explain biological phenomenon was totally new and unacceptable to many biologists of his time

Although Mendel’s work suggested that factors were

discrete units

He couldn't present any physical evidence for the existence of factors or what they were made of

Also biologists of his time did not accept the factors

( now called genes) as discrete units

In 1865 Mendel delivered two lectures on his findings

to the Natural Science Society in Brno (Czech Republic)

They published the results of his studies in

their journal the following year under the title

“Experiments on plant Hybrids”

Mendel did little to promote his work

A few references from that time period

indicated that much of it had been misunderstood, furthermore Mendel’s findings were not viewed as being applicable even by Mendel himself

Who guessed that they only applied to certain species or certain types of traits

QUESTIONS

Q1. Why Mendel’s work remained unrecognized?

Q2. Who first published Mendel’s work?

Q3. Name the title under which Mendels results were published?’

Q4.When and where Mendel delivered lectures about his work?

Q5.What are factors?

RECOGNITION OF MENDELS WORK

Mendel’s work was not recognised until the turn of

20th century , Mendel died in1884

16 years after Mendel’s death in 1900 with independent rediscovery of Mendel’s laws

Three scientists Hugo de Vries , Carl Correns and

Erich Von Tshermak rediscovered Mendel’s work

They independently duplicated his experiments

and Mendel’s work got recognition

By this time due to advancement in microscopy scientists were able to study cell division

This led to the discovery of nuclear structures that

appeared to double and divide just before cell division

These were called chromosomes

In 1902 Walter Sutton and Theodore Boveri noted that the

behavior of chromosomes was parallel to that of genes and used the chromosome movement to explain Mendel’s Laws

Sutton and Boveri argued that pairing and separation of chromosomes would lead to the segregation.

Sutton united the concept of segregation with Mendelian

Principles and called it the Chromosomal Theory of Inheritance

As a result a fundamental unifying theory of genetics proposed by Thomas Hunt Morgan came forward in 1915

unearthing the cause for Variations produced as a result of sexual reproduction

Those principles became the core of classical genetics

REASON FOR SUCCESS

Mathematical logic and statistical analysis

were methodically applied to problems in

biology for the first time in Mendel’s investigations

His experiments had a large sampling size

so the data collected by him was

reliable and had a greater credibility

Confirmation of his supposition from experiments with successive generations of his test plants

Proved that his results pointed to relatively common rules of inheritance than unconfirmed ideas

None of the characters demonstrate Linkage

QUESTIONS

Q1.How Mendel’s work got recognition?

Q2.Name the scientists who rediscovered Mendel’s work?

Q3.How many years after Mendel’s death did his work got recognition?

Q4.What were the reasons for recognition of Mendel’s work?

WHY MENDEL SELECTED GARDEN PEA

1. Small annual herbaceous plant2. Completes its life cycle in 3-4 months3. Produces many seeds4. Self pollinating(True breeds could be obtained)5. Can be cross pollinated 6. Has many contrasting characters7. Reproduces sexually producing two distinct male

and female gametes8. Traits are easy to isolate9. Easy growing plant10. Seeds do not show dormancy

REPRODUCTION IN FLOWERING PLANTS

Pollens contain sperms or male sex cells that are produced in stamen

Ovary contains the egg or ovule (female reproductive cell) produced in the pistil

SELF FERTILIZATION- Cross or fusion of male and female gametes (sex cells) produced by the same individual.

It occurs in bisexual organisms, including most flowering plants

CROSS FERTILIZATION- also called Allogamy, the fusion of male and female gametes (sex cells) from different individuals of the same species.

It occur in dioecious plants (those having male and female

organs on separate individuals)

QUESTIONS

Q1. What experiments did Mendel conduct?

Q2. What is garden pea?

Q3.Why Mendel selected garden pea for his experiments?

Q4. What is Self pollination?

Q5. What is cross pollination?

TERMINOLOGY

TRAITS- Characteristics that pass from parents to offsprings

FERTILIZATION- Uniting of male and female gametes to produce zygote

CROSS- Combining male and female or

parents with different traits

ALLELES- Different forms of the same gene

Each organism has two alleles for each trait as TT tallness and tt for dwarfness

GENES- The hereditary units situated on the chromosomes control the development of organism

DOMINANT TRAIT- The trait that shows itself very often in the offspring

Represented by a capital letter as T for tallnessRECESSIVE TRAIT- Traits that remain hidden or disappear

in the offspring Represented by a small letter as t for dwarfnessGENOTYPE- It is the genetic makeup of an organism

example-TT , Tt or ttPHENOTYPE- It is the visible expression or the way an

organism looks example-color of the eye, color of hair

HETEROZYGOUS- A gene combination with one dominant and one recessive trait

Example- Tt ( the two alleles for a trait are different)

HOMOZYGOUS- A gene combination with two dominant and two recessive traits

Example-TT or tt ( the two alleles for a trait are same)

MONOHYBRID CROSS- Crossing parents with one pair of contrasting characters (true breeding plants)

Example- TT and tt where T and t are the contrasting traits for the height of the plant

DIHYBRID CROSS- Crossing parents with two pairs of contrasting characters (cross breeding plants)

Example-TTPP and ttpp where T and t are contrasting features for the height of the plant P and p are the contrasting traits for the color of the flower

QUESTIONS

Q1. Define the following terms:

Fertilization ,traits, alleles, genes

Q2. What is dominant and recessive trait ?

Give examples.

Q4 Explain the terms Genotype and Phenotype giving examples.

Q5. Differentiate between homozygous and heterozygous with examples.

PUNNETT SQUARE

Developed by a British geneticist Reginald Punnett in 1875

It is a graphical representation of calculating the probability of all possible genotypes of offsprings in a genetic cross

It is used to help solve genetic problems

MENDEL’S EXPERIMENTAL METHODS

Mendel hand pollinated flowers

Snip stamens to prevent self pollination

Covered each flower with a bag

Traced the traits through several generations

HOW MENDEL BEGAN

Mendel produced pure strains which were similar but had contrasting characters, by allowing the plants to self pollinate for several generations

Mendel identified seven traits

TRAIT DOMINANT RECESSIVE

Seed shape Round Wrinkled

Seed color Yellow Green

Pod shape Full Constricted

Pod color Green Yellow

Flower color Violet White

Flower position Axial Terminal

Stem height Tall Dwarf

TRAITS IDENTIFIED BY MENDEL

DIAGRAM SHOWING CONTRASTING TRAITS IN PEA PLANT

MENDEL’S LAWS OF INHERITANCE

LAW OF DOMINANCE

Characters are controlled by discrete units called factors

Factors occur in pairs

In a dissimilar pair of factors one member

dominates (dominant) the other (recessive)

unless both factors in the pair are recessive

F1 GENERATION- If we cross homozygous tall & homozygous dwarf

Traits- Tall (dominant) Dwarf (recessive)

Cross - T T X t t

/ \ / \

Gametes- t

F1 Generation Ratio - Genotype- Tt 4 : 0 (heterozygous tall)

Phenotype- tall 4 : 0 (tall)

T T t t

T T

t Tt Tt

t Tt Tt

F2 GENERATION - If we cross two heterozygous tall plants

Traits Tall (heterozygous) Tall (heterozygous)

Cross T t X T t

Gametes / \ / \

F2 Generation ratio - Genotype : 1 : 2 : 1tall tall dwarf

(homozygous) (heterozygous) (homozygous)

Phenotype: 3 : 1

(Tall ) ( Dwarf)

T t

T TT Tt

t Tt tt

T t T t

EXPLANATION TO THE LAW OF DOMINANCE

The law of dominance is used to explain

the expression of only parental characters in the F1 generation

And the expression of both (parents and offsprings of F1) in F2 generation

It also explains the proportion 3:1 obtained at F2 generation

LAW OF SEGREGATION

1. This law states that during gamete (Egg or Sperm) formation the two alleles of a gene get separated

2. Alleles for a trait then recombine at fertilization producing genotype for the traits of the offspring

example- A Monohybrid cross

Application of Law of Segregation

For a homozygous parent - The two gametes will be similar (pure alleles) T T and t t

/ \ / \

(pure alleles) T T t t

For heterozygous parent - Tt will have one dominant allele T and one recessive allele t

T and t

/ \

T t

(dominant) (recessive)

These gametes are pure for that particular allele

Therefore law of segregation is also called law of purity of gametes

APPLICATION OF THE LAW OF SEGREGATION

For heterozygous parent

Tall -T & Dwarf - t Female gametes( 1 : 1 )

Male gametes

( 1 : 1 )

A monohybrid cross

Phenotype ratio- 3: 1

T t

T TT Tt

t Tt tt

L AW OF INDEPENDENT ASSORTMENT

This law states that when two pairs of traits are combined

in a hybrid, segregation of one pair of characters is independent of the other pair

Example- A Dihybrid cross – Tracks the inheritance of two traits or genes

Pure tall with purple flowers (TTPP)

and dwarf with white flowers (ttpp)

Parental generation P1(dominant) (recessive)

Traits tall & purple Dwarf & whiteCross T T P P X t t p p

/ \Gametes

X 4 X 4

TP TP TP TP tp tp tp tpFormula - 2n (where n is no of heterozygote's )

Q How many gametes will be produced for following allele arrangement?

a.) RrYy b) Aa Bb Cc Dd

TP tp

QUESTIONS

Q1.What are the three laws of Mendel ?

Q2. State Mendel’s Law of Dominance.

Q3. What is the phenotype and genotype ratio of F1 generation according to Mendel’s first Law?

Q4. Explain Mendel’s Law of Segregation?

Q5. Which law is called the Law of purity of gametes?

Q6. Which Law explains the segregation of characters? Explain with an example.

GENERATIONS IN A CROSS

Parental or P1 generation- represents the parents

F1 Generation- Ist filial generation of offsprings or progeny

F2 Generation- IInd Filial generation of the offsprings

TYPES OF CROSSESMONOHYBRID CROSS

A cross between one pair of contrasting characters

MONO HYBRID CROSS (parents or pure lines)

P1 generation Trait: Height of the stem

Alleles: T Tall & t Dwarf

Cross: Tall plants X Dwarf plants (pure)

Traits TALL DWARF

Cross T T X t t

/ \ / \

Gametes T T t t

Pure Female gametes

P1 Cross

Pure

Male gametes

F1 generationGenotype - Tt Ratio - 4 : 0Phenotype - Tall Ratio - 4 : 0

(All alike)

T T

t Tt Tt

t Tt Tt

REVIEW OF P1 CROSS

Parents

homozygous dominant X homozygous recessive

\ /

F1 Gen offsprings

Genotype heterozygous

Phenotype (all alike )F1 generation

Genotype - Tt Ratio - 4 : 0

Phenotype - Tall Ratio - 4 : 0

(All alike)

F1 MONOHYBRID CROSSTrait- Seed shapeAlleles- R (Round) r (wrinkled) HeterozygousCross- Round & wrinkled seeds X Round & wrinkled seeds

R r R r/ \ / \

Gametes

F2 GENERATION

Genotype Ratio- 1 : 2 : 1 RR Rr rr

Phenotype Ratio- 3 : 1 ( Round seeds) (Wrinkled seeds)

R r R r

R r

R Rr Rr

r Rr rr

REVIEW OF F1 CROSSHeterozygous X Heterozygous

\ /

F2 Gen Offsprings

Homozygous Heterozygous Homozygous

Genotype (dominant) (recessive)

RR Rr rr

25% 50 % 25%

Genotype Ratio - 1 : 2 : 1

Phenotype Ratio - 3 : 1

RESULTS OF MONOHYBRID CROSS

Inheritable factors or genes are responsible for the transfer of all heritable characters

Phenotype is based on genotype

Each trait is based on two genes (one from mother & one from father)

True breeding individuals are homozygous

QUESTIONS

Q1. What is a true breeding pea line?

Q2. State one difference between a pure breed and hybrid?

Q3. What is a monohybrid cross?

Q4. What is P1 generation?

Q5. What is the genotype and phenotype ratio in an F1 generation of monohybrid cross?

Q6. Explain the results of a monohybrid cross?

DIHYBRID CROSS

Inheritance of two genes taking two characters into account

Traits : Pure tall & purple flowers TTPP (dominant)

Pure dwarf & white flowers ttpp (recessive)

P1Generation TTPP ttpp

/ \

Gametes : TP TP TP TP tp tp tp tp

TP tp

F1 DIHYBRID CROSS

F1 Generation

Genotype ratio 16 : 0

( Heterozygous) tall & purple

Phenotype ratio 16 : 0

All tall & Purple flowers

TP TP TP TP

tp TtPp TtPp TtPp TtPp




F2 DIHYBRID CROSS

P1 generation

Traits : (Heterozygous tall and purple flowers)

Cross : TtPp x4 X TtPp x 4

Gametes : TP Tp tP tp TP Tp tP tp

Where n = 2

2n = 2³

=8

F2 DIHYBRID CROSS

F2 GenerationGenotype ratio-

1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1

TTPP TTPp TTpp TtPP TtPp ttPp ttPP TtPp ttppPhenotype ratio 9 : 3 : 3 : 1

Tall purple Tall white Dwarf purple Dwarf white

TP Tp tP tp

TP TTPP TTPp TtPP TtPp

Tp TTPp TTpp TtPp Ttpp

tP TtPP TtPp ttPP ttPp

tp TtPp Ttpp ttPp ttpp

TRIHYBRID CROSSA cross between two individuals that are heterozygous

for three different traits

TRAITS : tall purple axial flowers (TTPPAA)dwarf white terminal flowers (ttppaa)

CROSS : TTPPAA X ttppaaGAMETES : TPA X 8 tpa X 8

where n= 3 2n= 2³

= 8

PUNNET SQUARE FOR F1 TRIHYBRID CROSSTPA TPA TPA TPA TPA TP A TPA

tpa TtPpAa TtPpAa TtPpAa TtPpAa TtPpAa TtPpAa TtPpAa




tpa TtPpa TtPpAa TtPpAa TtPpAa TtPpAa TtPpAa TtPpAa

tpa TtPpAa TtPpAa

TtPpAa

TtPpAa TtPpAa TtPpAa TtPpAa



TPA

TtPpAa

TtPpAa

TtPpAa

TtPpAa

TtPpAa

TtPpAa

TtPpAa

TtPpAa

F1 TRIHYBRID CROSS RATIOS

GENOTYPE RATIO - 64 : 0

PHENOTYPE RATIO - 64 : 0

F1 generation –All 64 offsprings are going to be of the same type (Tall Purple and Axial)

(heterozygous)

F2 TRIHYBRID CROSS

Cross members of F1generation to get F2 generation

TRAITS : Tall Purple Axial (heterozygous)

CROSS : TtPpAa X TtPpAa

GAMETES : TPA , TpA , Tpa , tpa, Tpa, tPA, tpA , tPa

where n=3

2n = (2)³

= 8

F2 TRIHYBRID CROSS

TPA TpA Tpa TPa tpa tPA tpA tPa

TPA TTPPA

A1

TTPpA

A1

TTPpA

a1

TTPPA

a1

TtPpAa

1

TtPPAA

1

TtPpAA

1

TtPPAa

1

TpA TTPpA

A1

TTppA

A2

TTppA

a2

TTPpA

a1

TtppAa

2

TtPpAA

1

TtppAA

2

TtPpAa

1

Tpa TTPpA

a1

TTppA

a2

TTppaa

3

TTPpa

a4

Ttppaa

3

TtPpAa

1

TtppAa

2

TtPpaa

4

TPa TTPPA

a1

TTPpA

a1

TTPpa

a4

TTPPa

a4

TtPpaa

4

TtPPAa

1

TtPpAa

1

TtPPaa

4

tpa TtPpAa

1

TtppAa

2

Ttppaa

3

TtPpaa

4

ttppaa

5

ttPpAa

6

ttppAa7 ttPpaa

8

tPA TtPPAA

1

TtPpAA

1

TtPpAa

1

TtPPAa

1

ttPpAA

6

ttPPAA

6

ttPpAA

6

ttPPAa

6

tpA TtPpAA

1

TtppAA

2

TtppAa

2

TtPpAa

1

ttppAa

7

ttPpAA

6

ttppAA

7

ttPpAa

6

tPa TtPPAa

1

TtPpAa

1

TtPpaa

4

TtPPaa

4

ttPpaa

8

ttPPAa

6

ttPpAa

6

ttPPaa

8

TRIHYBRID CROSS

Phenotype ratio :

Tall purple axial- 27 (1)

dwarf purple axial- 9 ( 6)

tall purple terminal- 9 ( 4)

tall white axial- 9 ( 2)

tall white terminal- 3 ( 3)

dwarf white axial- 3 ( 7)

dwarf purple terminal - 3 ( 8)

dwarf white terminal- 1 ( 5)

* ( Numbers in the brackets represent the numbers in Punnet square for F2 trihybrid cross)

QUESTIONS

Q1. What is a dihybrid cross?

Q2. What is the genotype and phenotype ratio in F1 dihybrid cross?

Q3. What is a genotype and phenotype ratio in an F2 dihybrid cross?

Q4. What is a trihybrid cross?

Q5. What is the genotype and phenotype ratio in a trihybrid cross?

THANK YOU

PREPARED BY

RITU SHARMA