Mendelian Genetics

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Transcript Mendelian Genetics

Dr. Madhumita Bhattacharjee
Assiatant Professor
Botany 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 - 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
• 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
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).
Mother contributes:
True Breeding
Mother contributes:
Cross Breeding
Law of Dominance & law of
Test Cross
Dihybrid Cross
Cytological interpretation of
Smooth Tall
Smooth wrinkled
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)
Parallel behavior between Mendelian
genes and chromosomes:
Mendel’s Principle of Independent
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
• 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
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
• A map unit, or centiMorgan, is equal to crossing over between 2 genes in
1% of the gametes.
Chromosomal Mapping
The distance between genes is proportional to the
frequency of recombination events.
recombinant progeny
total progeny
1% recombination = 1 map unit (m.u.)
1 map unit = 1 centimorgan (cM)