Dr. Madhumita BhattacharjeeAssiatant ProfessorBotany Deptt.P.G.G.C.G. -11,Chandigarh

Gregor Johann Mendel

• 1822- 1884• Austrian monk• Experimented with pea

plants (Pisum sativum)• He thought that

‘heritable factors’ (genes) retained their individuality generation after generation

Terms to Know and Use

• Gene – A DNA blueprint controlling synthesis of a protein

• Trait - variant for a gene: i.e. a purple flower, determined by alleles

• Dominant trait - expressed over recessive trait when both are present

• Recessive trait - not expressed when the dominant trait is present

• Co-Dominant – expressed as blended traits

• Allele - a variation of a gene responsible for different traits, often represented as A or a

• Locus - location of a gene,or allele, on a chromosome

• Chromosome - strand of DNA containing the genes

• Haploid - one copy of a chromosome

• Diploid - two copies of a chromosome

• Gamete - a spermatozoa or oocyte (egg) cell, they are haploid

• Zygote - cell resulting from the fusion of two gametes, they are diploid

• Genotype - the type of alleles on a chromosome: genetic makeup

• Phenotype - The way a genotype is expressed: i.e. the color of a flower

• True breeding line - organisms that always pass the same genotype to their offspring

• Hybrid - offspring resulting from crossbreeding two true breeding lines: F1

Garden Pea Experiments 1856-64

• Mendel disagreed with the “Blending Theory” of inheritance.

• Started with 34 kinds peas Pisium sativum

• After 2 years he had 22 purebreds

Pea Characteristics

Trait on the left is dominant. Trait on the right is recessive.

Mendel’s Hypotheses• There are alternate forms

of ‘genes’=alleles

• For each trait, organisms have 2 genes, one from mother & other from father

• Pollen and egg each carry 1 allele/trait because alleles segregate

• Dominant allele is expressed & recessive allele has no noticeable effect in presence of dominant allele

Mendel’s Experiments

1. Plants must possess constant differentiating characteristics.

2. The hybrids of such plants must, during the flowering period, be protected from the influence of all foreign pollen, or be easily capable of such protection.

3. The hybrids and their offspring should suffer no marked disturbance in their fertility in the successive generations.

Mendel's Laws of Inheritance• Law of Dominance In F1 generation only

dominant allele is expressed• Law of Segregation during gamete

formation allele pairs separate or segregate, into different gametes

• Law of Independent Assortment suggested that each allele pair segregates independently of other gene pairs during gamete formation (Demonstrated with a dihybrid cross).

A A

A

A

AA

AA

AA

AA

Fat

her

cont

r ibu

tes:

Mother contributes:

oror

True Breeding

a a

A

A

Aa

Aa

Aa

Aa

Fat

her

cont

r ibu

tes:

Mother contributes:

oror

Cross Breeding

Law of Dominance & law of segregation

Test Cross

Dihybrid Cross

Cytological interpretation of Mendelesim

Two chromosomes of one parent are represented on the left.Possible alleles passed on to the offspring are on the right.(Consider smooth or wrinkled peas AND tall or short plants)

Smooth Tall Smooth Smooth wrinkled wrinkledTall short Tall short

Parallel behavior between Mendelian genes and chromosomes:

Illustrates:Mendel’s Principle of Independent Assortment

What is Linkage?

• Linkage is defined genetically: the failure of two genes to assort independently.

• Linkage occurs when two genes are close to each other on the same chromosome. .

• Genes far apart on the same chromosome assort independently: they are not linked.

• Linkage is based on the frequency of crossing over between the two genes. Crossing over occurs in prophase of meiosis 1, where homologous chromosomes break at identical locations and rejoin with each other.

Discovery of Linkage

• In 1900, Mendel’s work was re-discovered, and scientists were testing his theories with as many different genes and organisms as possible.

• William Bateson and R.C. Punnett were working with several traits in sweet peas, notably a gene for purple (P) vs. red (p) flowers, and a gene for long pollen grains (L) vs. round pollen grains (l).

B+P Genes in a Test Cross

• Purpose of a test cross: the offspring phenotypes appear in the same ratio as the gametes in the parent being tested.

• Here, we want to see how many gametes are in the original parental configuration (PL or pl) and how many are in the recombinant configuration (Pl or pL). The parental types have the same combination of alleles that were in the original parents, and the recombinant types have a combination of the mother’s and father’s alleles.

• Original parents: PP LL x pp ll• F1 test cross: Pp Ll x pp ll

Pheno-type

obs

purple long

392

purple round

116

red long

127

red round

365

total 1000

Coupling vs. Repulsion

• The original test cross we did was PL/pl x p l. Among the offspring, PL and pl were parental types, and pL and Pl were the recombinant types. There was 24.3% recombination between the genes.

• The condition of having the dominant alleles for both genes on the same parental chromosome, with both recessives on the other parental chromosome, is called “coupling”: the P and L genes are “in coupling phase”.

• The opposite condition, having one dominant and one recessive on each parental chromosome, is called “repulsion”. Thus, if the original parents were P l x p L, their offspring would have the genes in repulsion phase: Pl / pL.

Chromosome Mapping• Each gene is found at a fixed position on a particular chromosome.

Making a map of their locations allows us to identify and study them better. The basis of linkage mapping is that since crossing over occurs at random locations, the closer two genes are to each other, the less likely it is that a crossover will occur between them. Thus, the percentage of gametes that had a crossover between two genes is a measure of how far apart those two genes are.

• As pointed out by T. H. Morgan and Alfred Sturtevant, who produced the first Drosophila gene map in 1913. Morgan was the founder of Drosophila genetics, and in his honor a recombination map unit is called a centiMorgan (cM).

• A map unit, or centiMorgan, is equal to crossing over between 2 genes in 1% of the gametes.

Chromosomal MappingThe distance between genes is proportional to the

frequency of recombination events.

recombination recombinant progeny

frequency total progeny

1% recombination = 1 map unit (m.u.)

1 map unit = 1 centimorgan (cM)

=