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1. Mendelism and its applications

2. Gregor Mendel and Pisum sativum (garden pea)
Image Resource:
http://cdn.yourarticlelibrary.com/wp-content/uploads/2013/12/b428.jpg

3. Father of Genetics: Gregor Mendel (1822-1884)
Rural upbringing: knowledge of Plants and Animal husbandry
1847: became a priest; adopted clerical name ‘Gregor’
Taught at local high school
Study: University of Vienna (1851-1853)
Teaching monk: started genetic experiments with Pisum sativum
Completed experiments (1864); presented results → Local Natural History Society
Published detailed work report in 1865 under the title “Experiments in Plant
Hybridization,”
Hugo de Vries (Holland) Carl Correns (Germany) and Eric von Tschermak-Seysenegg (
Austria) rediscovered Mendel’s work in 1900.
Image Resource:
http://2.bp.blogspot.com/_To_nsDE_7TA/TVLz6fCYHWI/AAAAAAAABCM/jN8A5eLQgl
Q/s1600/GregorMendel.jpg

4. The Principle of Segregation or Law of Segregation
In a heterozygote, two different alleles segregate from each other
during formation of gametes
Or
In a heterozygote, two factors controlling a trait get separated and
included in different gametes at the time of gamete formation.
Each individual carries two copies (alleles) of every gene and only one of
these is transmitted to each child by each parent.

5. The Principle of independent assortment
Also known as “Law of independent assortment ”
The alleles of different genes segregate, or as we sometime say,
assort independently of each other
May be called as Alleles from unlinked loci are assorted independently

6. The garden pea, Pisum sativum, is easily grown in experimental
gardens or in pots in a greenhouse.
Pea flowers contain both male and female organs.
male organs: anthers→ sperm-containing pollen
female organ: ovary → eggs
Pea reproduction: petals of the flower close down tightly,
preventing pollen grains from entering or leaving
• Enforces a system of self-fertilization
So Pea plants: true-breeding
Mendel’s Experimental Plant: Pisum sativum
Image Resource:
http://2.bp.blogspot.com/-
gxIPHeuStcI/UdnKjOL6tpI/AAAAAAAAB2Q/bowPvrVphZk/s1600/025.JPG

7. Image Resource:
https://scottishforestgarden.files.wordpress.com/2012/08/dscf0032.jpg
https://www.jic.ac.uk/media/parallax/_DSC9047.jpg

8. http://www.backyardseedsavers.com/store/wp-content/uploads/2013/10/Pea-arista-
green.jpg

11.  monohybrid cross because a single trait was being studied
 Mendel observed that only one of the two contrasting characteristics
 appeared in the hybrids and that when these hybrids were self-
fertilized, they produced two types of progeny, each resembling one
of the plants in the original crosses.
 Furthermore, he found that these progeny consistently appeared in a
ratio of 3:1.
 Thus, each trait that Mendel studied seemed to be controlled by a
heritable factor that existed in two forms, one dominant, the other
recessive.
 These factors are now called genes, a word coined by the Danish
plant breeder Wilhelm Johannsen in 1909; their dominant and
recessive forms are called alleles—from the Greek word meaning “of
one another.”
 Alleles are alternate forms of a gene

12. Monohybrid Cross

14. Source:
Principles of Genetics, 6th Edition by Snustad and Simmons

15. The Principle of Dominance:
In a heterozygote, one allele may conceal the presence of another.
This principle is a statement about genetic function. Some alleles evidently control
the phenotype even when they are present in a single copy.
The Principle of Segregation:
In a heterozygote, two different alleles segregate from each other during the
formation of gametes.
This principle is a statement about genetic transmission.
An allele is transmitted faithfully to the next generation, even if it was present with a
different allele in a heterozygote.
The biological basis for this phenomenon is the pairing and subsequent separation of
homologous chromosomes during meiosis.

16. DIHYBRID CROSSES: THE PRINCIPLE OF INDEPENDENT ASSORTMENT
Mendel also performed experiments with plants that differed in two traits
He crossed plants that produced yellow, round seeds with plants that produced
green, wrinkled seeds.
The purpose of the experiments was to see if the two seed traits, color and texture,
were inherited independently.
Because the F1 seeds were all yellow and round, the alleles for these two
characteristics were dominant.
Mendel grew plants from these seeds and allowed them to self-fertilize.
He then classified the F2 seeds and counted them by phenotype.

19. Reasons for Mendel’s Success

20. Applications of Mendel’s Principles
THE PUNNETT SQUARE METHOD
For one or two genes, it is possible to write down all the gametes
and combine them systematically to generate the array of zygotic
genotypes
Then Principle of Dominance can be used to determine the
associated phenotypes.
This procedure, called the Punnett square method after the
British geneticist R. C. Punnett
However, in more complicated situations, like those involving more
than two genes, the Punnett square method is unwieldy

22. Used for predicting the outcome of a cross involving two or more than two
genes.
instead of enumerating the progeny in a square by combining the gametes
systematically, we tally them in a diagram of branching lines
all the genes assort independently. For each gene, we expect the
phenotypes to appear in a 3:1 ratio
Dd X Dd will produce a ratio of 3 tall plants:1 dwarf plant. Similarly others
will do
THE FORKED-LINE METHOD
The forked-line method for predicting the outcome of an intercross
involving three independently assorting genes in peas.

23. THE PROBABILITY METHOD
alternative method to the Punnett square and forked-line methods—and a
quicker one—is based on the principle of probability
Mendelian segregation is like a coin toss;
when a heterozygote produces gametes, half
contain one allele and half contain the other
In a Heterozygote, probability that a
particular gamete Dominant allele is
therefore 1/2 , similarly 1/2 for recessive
allele (Frequencies)
Can we use these frequencies to predict
the outcome of crossing two
heterozygotes?
The chance that a zygote will be AA is simply the probability that each of the uniting
gametes contains A, or (1/2) x (1/2) = (1/4), similarly probability of aa = 1/4 and
for Aa it will be (1/4) x (1/4) = (1/2).
By applying the Principle of Dominance, we conclude that (1/4) +(1/2) =(3/4) of the progeny
will have the dominant phenotype and 1/4 will have the recessive

24. Que: A cross between plants heterozygous for four different genes, each
assorting independently. Aa Bb Cc Dd
What fraction of the progeny will be homozygous for all four
recessive alleles?
Ans: consider one genes at a time.
For the first gene, the fraction of offspring that will be recessive
omozygotes is 1/4, as it will be for the second, third, and fourth
genes.
Therefore, by the Principle of Independent Assortment, the fraction
of offspring that will be quadruple recessive homozygotes.
This will be (1/4)x(1/4) x (1/4) x (1/4) = (1/256).

25. Testing Genetic Hypotheses
The chi-square test is a simple way of evaluating whether the
predictions of a genetic hypothesis agree with data from an
experiment
Data collected from observations or from experimentation enable
scientists to test hypotheses—that is, to determine if a particular
hypothesis should be accepted or rejected?
let’s consider the data that Mendel obtained from his dihybrid cross involving the
color and texture of peas. In the F2, 556 peas were examined and sorted into four
phenotypic classes

26. DeVries’s experiment with flower color and
foliage type in varieties of campion. The
inset shows the variety with red flowers
and hairy foliage

27. DeVries proposed that flower color and foliage type were controlled by two
different genes, that each gene segregated two alleles—one dominant, the
other recessive—and that the two genes assorted independently; that is, he
simply applied Mendel’s hypothesis to the campion.
However, when we compare DeVries’s data with the predictions of the
Mendelian hypothesis, we find some disturbing discrepancies.
Are these discrepancies large enough to raise questions about the
experiment or the hypothesis?

29. index to the set of 2
distributions is
determined by
subtracting one from the
number of phenotypic
classes.
In each of our examples,
there are 4 -1=3 degrees
of freedom.
Chi Square or This critical value is the point where the discrepancies between
observed and expected numbers are not likely to be due to chance.