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THE LOGIC OF MENDEL

All of us have some characters similar to our parents or grand parents
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Our grand parents, most of them are farmers, selected the
best seeds for their next season
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Characters are passed to next generation through seeds
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Bees visit
flowers for honey
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Here we are trying to understand logically, how a normal man like Mendel who had lowest
grades in Biology could do such a meticulously planned experiment? Above all he had failed for a
teaching certificate in natural sciences! On the other hand Mendel studied physics, mathematics
and chemistry along with important aspects of biology
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We are logically going to understand what brought Mendel to understand the concept
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He knew
nothing about meiosis, never considered chromosomes or whether they had anything to do with
what he was investigating
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All he did was breed
peas, and do some high quality thinking
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He also studied practical and theoretical
philosophy and physics at the University of Olomouc, Czech republic
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He also worked as a substitute high school teacher
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He returned and
again worked as physics teacher, but failed on the oral part of examination
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Mendels work can be divided into the following steps: (A) Preparation for experiments (B)
Choice of experimental material (C) Planning and execution of experiment (D) Interpretation of
experimental results and (E) Further testing of his observations
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Preparation for Experiments
How do to an experiment? The laboratory should be accessible! That is Mendel approached the
head of the department of Natural history and agriculture where he was working as a priest for
permission to use the 2 hectar experimental garden intended to study differences in plants
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Why he wanted the entire full 2 hecters
rather a few cents or pots ? If we need to have a reliable results, the sample size should be very
high ! Therefore how much land you need to cultivate around 20,000 pea plants? Ask your
grandfather!
B
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Now the question is which plant? Shall we take a mango tree? Here comes the
advantage of critical observation
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It is Pea plants, because very easy to cultivate,
flowers are big (think why?), number of plantlets produced from one flower will be more and
above all its generation time is less
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Examples of variation?
Flower color, seed color, seed size and shape, flower position etc
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From the Mendel’s observations it is the Pea
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Planning and execution of the experiment
Selection of variations: He selected the following characters: 1
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Seed color [Yellow Vs Green], 3
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Pod shape [inflated
vs constricted], 5
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Flower position [Axial vs Terminal] and 7
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That was the second logic of Mendel
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Does Mendel randomly selected
seeds? We should know which characters seeds are inherited? We are studying variation, should
we select seeds which show consistent inheritance or variable inheritance? The choice should be
of consistent inheritance
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That is Mendel used well defined seeds as the starting material in all his
experiments involving thousands of plants
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This
is the spectacular logic he applied and the basis of Mendel’s success
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How to do controlled breeding? We want to cross a tall
plant with a dwarf plant
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How to do this? Simply bag the flower bud, cut
the stamens of the experimental flower, take the pollen grain from the other flower using a brush,
“paint” on the stigma of the initial flower, bag it again to avoid unwanted entry of other pollen
grain
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Get the seeds from the flower and observe for the variation intended
to study!
Here is the fifth logic
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All other experiments involved self
pollination
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Why not twice or thrice?
As simple as this, first let us start from simple things! So do the cross only once, avoid complexity
and evaluate the results
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Looking behind a character how many generations we
should follow after crossing? Again comes the simplicity and reproducibility
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So in short he started with the same experimental plan is as follows:
Tall plants

Dwarf plants

True breeding Tall plant

First filial generation (F1)

True breeding dwarf plant

Cross product (Self pollinate)

Second filial generation (F2) Raise the plants look for variation how it was passed

Do the counting for the variations, tabulate

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Figure 1
The seven variations observed in Pea plant that Mendel followed: Are they easily recognizable?

Figure 2 Flower structure of Pea: Observe the reproductive parts of the flower
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From
the stigma the male nucleus of the pollen grain reaches the ovary and fertilizes an egg to produce a zygote
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The problem faced by
Mendel is controlled pollination
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In this case the
pollen grains of dwarf plant only should fall on the stigma of the dwarf plant or vice versa
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After his experiments
the results were carefully tabulated by him
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What you will conclude from these results? The character
which is appeared in the F1 is also dominating in F2 also
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The other or
alternative character which is disappearing in the F1 generation is also less in number compared to
dominant character
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(Law of Dominance)
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But it didn’t happen
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This further means that some “units” which functions as
discrete particles are responsible for characters
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It means that two discrete units
are responsible for the “Tall” character
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We can express
the alternate forms of characters (dominant or recessive) as alleles
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So alleles are alternate forms (in fact variables) of a gene
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For example a tall plant can have TT or Tt
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This binomial expression is the genotype
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Why two units of inheritance? Not three or four? We have father and mother
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A character is represented by two units
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So the gamete contains one unit
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At present the “units” proposed by Mendel is known as gene
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Now study the following cross which can be represented in the form of a Punnett square (A simple grid
representing all possible gametes and combinations
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This is the second core idea of Mendel’s theory
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This illustration represents Mendel’s core ideas
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Similarly the following figure illustrates a dihyrbid test cross

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Mendel has modelled one experiment, he executed and finally he tested it
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It can predict the genotype to be
tested based on the phenotypic ratio of the cross output
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Based on his results, he has tested his hypothesis of dominance and segregation
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If the alleles maintain the association, he is expecting
only the parental types in the F2 generations (Why not F1 generation?)
If the alleles maintain the association, the F1 gametes will be SY and sy
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If they segregate independently he was expecting four different
phenotypes
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New types were obtained in F2
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Mendel and his Mathematics predictions
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You are going to toss it together
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Probability of getting a 5 rupee tail = ½
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Hence the probability of getting both tail = ½ X ½ = ¼ ie 25%
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The probable gametes here are S and s
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Hence added probability is
¼ + ¼ = ½ ie 50% will be heterozygotes
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Mendel did all these statistical problems
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You should understand both; ie doing a genetic problem by using probability and by using a
Punnett square
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Similar to monohybrid test cross, it
is possible to predict the genotype of a phenotype by crossing with a true recessive parent
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Mendel experiments: Did he predicted
chromosomes?
(Meiosis accounts for segregation)
The segregation of Mendelian factors is because of meiosis
Mendel proposed mechanisms of heredity
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But he speculated that cells contained some type of factor that carried traits from one
generation to the next
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Most probably Mendel believed that for each character there is a factor
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They show
characteristic segregation and independent assortment are due to meiosis (Illustrated below)
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Mendel said the two alleles will segregate
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This is exactly matches what Mendel
speculated from his results
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What happenes in mitosis? A cell with genotype Ss just produces two daughter cells with
the same genotype as illustrated above and there is no segregation
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Whether all the inheritance follows Mendelian pattern? Human have several traits like hair
pattern, skin color, tasting ability, shape of ear and so on
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So if Mendel’s rules we apply, we should have only 23 chromosomes
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Now we have to think that who is the
luckiest Man? It is Mendel
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Each character was regulated by a gene and
they were located in seven different chromosomes
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Suppose if the traits selected by Mendel resides on the same chromosome, he will not get
a 9:3:3:1 ratio as expected
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If we are not getting this ratio, then genes my be on same chromosome
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The seven chromosomes of Pisum sativum
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If it was not, Mendel might have failed in his efforts
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Morgan and his Drosophila: Mendel’s hypothesis is
rejected in Morgan’s experiments
[Chromosomal theory, Connecting Mendel to Morgan, Linkage and crossing over]

It was Thomas Hunt Morgan and his students of Columbia University who did pioneering works
to explain many of this from the beginning of 1909
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He selected fruit fly because of its small size, easy to grow
and breed and its short generation time
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17 assuming that maximum recombination is 1)

Now we have to see why the new phenotypes (non-parental phenotypes) occurs?
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The new phenotypes appear because of exchange of genes between homologous chromosomes that
occurs during meiosis (swapping)
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Look at the following illustration
to understand the process
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Morgan’s found
recombination frequencies of many genes through experiments and used this frequencies to construct a
genetic map or mapping the genes
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The following
illustration helps us to find how to do a genetic map
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The recombination frequencies can be used for making genetic maps
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After finding out the frequency he was able to apply for construction of genetic
map because the more the distance between two loci, the more will be the recombination
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The unit is cm (Centimorgans) or map units (1cM
= 1 map unit)
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Walther Flemming, the grand father of mitosis discovered chromosomes
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He found cells contains the colored genetic material, the
chromosomes (Chrome = color; some = body)
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Even he
discovered that chromosomes splits longitudinally during cell division (His illustration is given below)
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We we know, in meiosis the longitudinal splitting happens after crossing
over
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Many people
experimented over decades on it
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The probable mechanism of
transmission from one generation to next was speculated by the discovery of chromosomes by Walther
Flemming, a german biologist
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This was done by
Boveri, Sutton and Morgan during the dawn of 20th century
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In fact Mendel was a Physicist (and philosopher), Darwin was a naturalist, Morgan was a zoologist

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