MENDELIAN GENETICS: Unveiling the Principles of Inheritance

This presentation delves into the fundamentals of Mendelian genetics, highlighting Gregor Mendel's groundbreaking experiments with pea plants, his formulation of the principles of inheritance that explain trait transmission in organisms & the significance of Mendel's contributions to modern genetics.

1. MENDELIAN
GENETICS
PRESENTER NAME: MOULI GHOSH
B S C . H O N S ( 3
RD
S E M ) , D E P T. O F M O L E C U L A R
B I O L O G Y & B I O T E C H N O L O G Y, S R I PAT S I N G H
C O L L E G E

2. 1.INTRODUCTION
❑Gregor Johann Mendel (1822-1884),known as the father of Genetics,
was an Augustinian friar. He conducted a series of breeding experiments
with garden pea Pisum sativum to learn something about the mechanism
of Heredity. As a result of his creativity, Mendel discovered some
fundamental principles of genetics.
❑Mendel developed a simple theory to explain the transmission of
Hereditary traits from generation to generation. Although Mendel had no
knowledge about DNA, DNA is the genetic material and Cell division
( Mitosis & Meiosis ) that is the segregation or disjunction of
Chromosomes.
❑In 1865,he published the results of hybridization experiments titled
‘Experiments on plant hybridization’ in a journal ‘The proceeding of the
Brunn society of natural history’ and postulated the Principles of
Inheritance which are popularly known as Mendel’s Laws.
Gregor Johann Mendel

3. Gregor Johann Mendel
❑But for several reasons, his work largely went unnoticed by the
scientific community at that time and it remained unrecognized
till 1900, because, Firstly, his work didn’t align with the prevailing
thoughts that were dominating the biological world , which was
heavily influenced by Charles Darwin’s Theory of Evolution ‘Origin
of new species by means of Natural Selection’.
❑Darwin’s theory of Evolution describes evolution as a
slow,gradual,continuous process of change,he gave the
concept of ‘Continuous variation’ or accumulation of
continuous changes.

4. ❑In the context of human evolution or the evolution of
other species, Darwin argued that the accumulation
of small changes over long periods could result in the
development of new species.
Horseshoe bats (Rhinolophus) in China
SARS-CoV-2
✓Another concept is discontinuous variation,
whichwas given by Mendel (occurswhen there
is an abrupt change from one phenotype to
another, as inMendel'sround and wrinkled
peas). According to Mendel,
Spontaneous Evolution is possible. ( later, Hugo de
Vries established in his mutation theory that
Discontinuous variation actually leads to Speciation).
✓In the context of the COVID-19 pandemic, it
is noteworthy that Mendel’s Concept is
much more correct than Darwin, that
‘Discontinuous variation does take place’,
❑But this concept was not accepted at that
time.

5. ❑Secondly, Mendel’s use of statistical analysis & mathematical logic in
biology ,a somewhat unconventional approach for his time, was
indeed one reason his work faced initial neglect. The use of
mathematics in biology, particularly during Mendel's time, was
indeed met with skepticism.
❑Mendel's pioneering work on genetics encountered numerous
challenges and skepticism from various quarters; further complicating
matters, Carl Wilhelm von Nägeli, who was a Swiss botanist and
contemporary of Gregor Mendel. Although Carl Wilhelm von Nägeli
attempted to replicate Mendel's experiments using Dandelion weeds and
failed to obtain similar results, so he claimed that these experiments
or results are rather fabricated, but it's crucial to recognize that the
discrepancy stemmed from the unique nature of Dandelion
flowers, which are apomictic (Apomixis isa type of asexual
reproduction in flowering plants that leads to seed production without
fertilization,where the female gametophyte or ovule in the flower
directly develops into an embryo, skipping meiosis and syngamy.This
process mimics sexual reproduction).
✓‘Mendel’s Experiment can only be succeed with the gametes
which are produced by Meiosis’,making his experimental
framework incompatible with the reproductive mechanisms of apomictic
plants like Dandelions.
Carl Wilhelm von Nägeli
Dandelion weed

6. ❑Mendel then performed his experiments on Dandelion weeds but also
failed to obtain the desired results.
so, this is in the light,that there is something wrong and eventually
Mendel got that what was wrong. But nonetheless,
His inability to reproduce its results in dandelion was a big setback to
validate his experimental Claims.
✓Therefore we can actually attribute ‘the discovery of Linkage’ to
Mendel.
✓Also we can attribute ‘the Discovery of Ploidy’ atleast in Pea plants to
Mendel.
( Mendel’s Monohybrid ratio and dihybrid ratio are only applicable to
diploid organisms where the gametes are produced by meiosis, For
example,In banana which are tetraploid, they never produce these true
meiotic kind of gametes, so those are aborted seeds of Banana.)

7. 2. MENDEL’S EXPERIMENTAL DESIGN
Mendel chose the garden pea, Pisum sativum,for his
hybridization experiments because it was easy to
cultivate and had a short life cycle, bears flowers
and fruits in the same year a seed is planted and
produces a large number of seeds, naturally it’s self
fertilizing in nature and it’s easy to cross breed
experimentally.
Mendel conducted such artificial pollination/cross
pollination experiments using several true-breeding
pea lines. A truebreeding line is one that, having
undergone continuous self-pollination, shows the
stable trait inheritance and expression for several
generations.
Mendel selected 7 pairs of traits or 14
contrasting traits out of 50 types of seed.This
selection of contrasting traits was a major
breakthrough in the field of genetics.
One is Dominant over the other.
Each pair affected one characteristic of plant, with
each member of a pair being clearly distinguishable.
✓Contrasting Traits studied by Mendel in Pea

8. ➢The characteristics of an individual are called Traits, Traits are
under the control of Genes (Mendel called them Factors) , each
gene may exist in alternative forms known as Alleles , which
code for different versions of a particular inherited character
( Mendel didn’t know about allele ).
➢The Genotype is the genetic constitution of an individual for any
particular trait,it is the set of alleles that an individual organism
possesses.The Phenotype is the observable properties of an
individual for any particular trait.
➢Relationship between genotype and phenotype:
Genotype
(genetic constitution)
Environmental
influences & random
developmental events
Phenotype (expression
of physical trait)
❑Mendel’s experiment was totally based on
Phenotypic Observation.
❑Mendel's experiments were primarily based on
phenotypic observation. Through the careful
examination of observable traits, he was able to
infer and predict the corresponding genotypes.

oIf the Trait is : Height of
the Plant
oGenotype : TT, Tt,tt
oPhenotype : Tall , Dwarf
oHomozygous : TT,tt
oHeterozygous : Tt

9. 3.MENDEL’S LAWS OF INHERITANCE
❑MONOHYBRID CROSS
❑Mendel first performed monohybrid crosses (crosses between true breeding strains
of peas that had alternative forms of a single trait.
❑Let us take the example of one such hybridisation experiment carried out by Mendel
where he crossed tall and dwarf pea plants to study the inheritance of one gene. He
collected the seeds produced as a result of this cross and grew them to generate
plants of the first hybrid generation. This generation is also called the Filial1
progeny or the F1 . Mendel observed that all the F1 progeny plants were tall, like
one of its parents; none were dwarf. He made similar observations for the other pairs
of traits – he found that the F1 always resembled either one of the parents, and that
the trait of the other parent was not seen in them.
❑Mendel then self-pollinated the tall F1 plants and to his surprise found that in the
Filial2 generation some of the offspring were ‘dwarf’; the character that was not
seen in the F1 generation was now expressed. The proportion of plants that were
dwarf were 1/4th of the F2 plants while 3/4th of the F2 plants were tall. The tall
and dwarf traits were identical to their parental type and did not show any
blending, that is all the offspring were either tall or dwarf, none were of in
between height. Similar results were obtained with the other traits that he studied:
only one of the parental traits was expressed in the F1 generation while at the F2
stage both the traits were expressed in the proportion 3:1. The contrasting traits did
not show any blending at either F1 or F2 stage.

10. ❑Based on these observations, Mendel proposed that something
was being stably passed down,unchanged, from parent to offspring
through the gametes, over successive generations. He called these things
as ‘factors’. Now we call them as genes. Genes, therefore, are the
units of inheritance. They
contain the information that is required to express a particular trait in an
organism. Genes which code for a pair of contrasting traits are known as
alleles, i.e., they are slightly different forms of the same gene.
As Mendel found the phenotype of the F1 heterozygote Tt to be exactly
like the TT parent in appearance, he proposed that in a pair of
dissimilar factors, one dominates the other (as in the F1 ) and hence is
called the dominant factor while the other factor is recessive . In this
case T (for tallness) is dominant over t (for dwarfness), that is
recessive. He observed identical behaviour for all the other
characters/trait-pairs that he studied.
❑Brachydactyly is a
genetic condition that
causes the fingers and
toes to appear shorter
in proportion to other
parts of your body, is
actually a Dominant
Genetic Trait.This is
not abundant but still
it’s designated as
Dominant Character .

11. ❑Based on his observations on monohybrid crosses Mendel proposed two general rules to consolidate his
understanding of inheritance in monohybrid crosses. Today these rules are called the Principles or Laws of
Inheritance: the First Law or Law of Dominance and the Second Law or Law of Segregation.
❑LAW OF DOMINANCE
•Characters or Traits are controlled by discrete units called ‘Factors’.
•Factors occur in pairs. They are in alternating forms and one particular factor is
Dominant over the other, one that is dominant will actually get expressed.
A recessive allele is an allele that exerts its effect only in the homozygous condition, and in
heterozygous condition its expression is masked by the dominant allele.
❑During the time when Mendel conducted his experiments, there were alternative ideas about
inheritance, and one such notion was the theory of "blending inheritance." According to this
theory ( given by Aristotle), it was believed that the blood or body fluids of parents mixed
together to produce an intermediate offspring, blending the traits of both parents. Mendel
opposed this theory or challenged this prevailing idea of blending inheritance.

12. LAW OF SEGREGATION
❑This law is based on the fact that the Discrete Factors or alleles do not show any blending and
that both the characters are recovered as such in the F2 generation though one of these is not seen
at the F1 stage. Though the parents contain two alleles during gamete formation, the factors or
alleles of a pair segregate from each other such that a gamete receives only one of the two
factors. Of course, a homozygous parent produces all gametes that are similar while a heterozygous
one produces two kinds of gametes each having one allele with equal proportion.
T T t t
➢One chromosome from mother & one chromosome from father
They form a homologous pair and these homologous pair can have
two different allele and these alleles are the alternating form of
a particular gene. Sister chromatids must have the same allele.
Non-sister chromatid may have the same allele (homozygous
Condition) or different alleles (heterozygous).
(Tt)

13. ❑The exceptions of The law of Dominance are 1) Incomplete Dominance 2) Co-Dominance. These exceptions actually prove the law.
❑CO-DOMINANCE
When both the alleles get expressed together or simultaneously, the condition is called Co-
dominance.In the case of co-dominance the F1 generation resembles both parents. A good example is
different types of red blood cells that determine ABO blood grouping in human beings. ABO blood
groups are controlled by the gene I. The plasma membrane of the red blood cells has sugar polymers
that protrude from its surface and the kind of sugar is controlled by the gene. The gene (I) has three
alleles I A , I B and i. The alleles I A and I B produce a slightly different form of the sugar while allele i
does not produce any sugar. Because humans are diploid organisms, each person possesses any two
of the three I gene alleles. I A and I B are completely dominant over i, in other words when I A and i
are present only I A expresses (because i does not produce any sugar), and when I B and i are
present I B expresses. But when I A and I B are present together they both express their own
types of sugars( Neither IA is dominant over IB nor IB is dominant over IA, both are getting
expressed simultaneously) this is because of co-dominance. Hence red blood cells have both A and
B types of sugars. Since there are three different alleles, there are six different combinations of these
three alleles that are possible, and therefore, a total of six different genotypes of the human ABO
blood types & four different Phenotypes are possible.

14. ▪The example of ABO blood grouping also provides a good example of
multiple alleles. So,there are more than two, i.e., three alleles,
governing the same character. Since in an individual only two alleles
can be present, multiple alleles can be found only when population
studies are made.
INCOMPLETE DOMINANCE
❑When neither of the alleles get expressed , this condition
is called Incomplete Dominance. The inheritance of flower
colour in the dog flower (snapdragon or Antirrhinum sp.) is
a good example to understand incomplete dominance.
❑ Both are incomplete in their expression.
while in co-dominance both alleles are
complete in their expression.
❑But incomplete dominance doesn’t promote a Blending
hypothesis. if blending really occurred, then we would not
get a recessive white plant in the F2 generation.
▪Snapdragon
plant

15. ❑Exceptions of Law of Dominance are incomplete dominance and co-dominance , these happens in
normal condition but as the Law of segregation is an Universal Law, there is no exception
under normal condition, the exception only happens in the diseased conditions.
❑Failure of segregation of chromatids during cell division cycle (Non-disjunction of chromosome)
due to some error, results in the gain or loss of a chromosome(s), called aneuploidy. So One of
the gametes gets both the Homologous chromosome and the other one gets neither of the
Homologous chromosome.
❑Down’s Syndrome : The cause of this genetic disorder is the presence of an additional copy of
the chromosome number 21 (trisomy of 21).
❑Edwards syndrome : Trisomy 18
❑Turner’s Syndrome : Such a disorder is caused due to the absence of one of the X chromosomes,
i.e., 45 with X0.
❑Klinefelter’s Syndrome : This genetic disorder is also caused due to the presence of an
additional copy of X-chromosome resulting into a karyotype of 47, XXY.

16. DIHYBRID CROSS
A cross that involves the analysis of two traits.Let us
use the genotypic symbols Y for dominant yellow
seed colour and y for recessive green seed colour, R
for round shaped seeds and r for wrinkled seed
shape. The genotype of the parents can then be
written as RRYY and rryy. The cross between the
two plants can be written down as in Figure showing
the genotypes of the parent plants. The gametes RY
and ry unite on fertilisation to produce the F1
hybrid RrYy. When Mendel self hybridised the F1
plants he found that 3/4th of F2 plants had yellow
seeds and 1/4th had green. The yellow and green
colour segregated in a 3:1 ratio. Round and
wrinkled seed shape also segregated in a 3:1
ratio; just like in a monohybrid cross.
Genotypic Ratio : 1:2:1:2:4:2:1:2:1
Phenotypic Ratio: 9:3:3:1
Total combination : 16

17. RY , ry : Parental Gametes (50%)
Ry , rY : Recombinant Gametes (50%) ; This is the prerequisite.
But in reality , in most of the cases, parental gametes are more than recombinant gametes.
This depends on the distance between two locus which determine two traits.
In the dihybrid cross, the phenotypes round yellow ; wrinkled yellow, round green and wrinkled
green appeared in the ratio 9:3:3:1. Such a ratio was observed for several pairs of characters that
Mendel studied. The ratio of 9:3:3:1 can be derived as a combination series of 3 yellow: 1
green, with 3 round : 1 wrinkled. This derivation can be written as follows: (3 Round : 1
Wrinkled) (3 Yellow : 1 Green) = 9 Round, Yellow : 3 Wrinkled, Yellow: 3 Round, Green : 1
Wrinkled, Green.
❑Since these events are independent, the multiplication of Mendelian monohybrid ratio gives
The Mendelian dihybrid ratio.
❑Based upon such observations on dihybrid crosses (crosses between plants differing in two traits)
Mendel proposed a second set of generalisations that we call,
❑ The law states that ‘The Factors for different pairs of traits assort independently
of one another, (that means segregation of one pair of characters is independent of
the other pair of characters)’.
Mendel’s Law of Independent Assortment

18. But it only happens if the parental and recombinant gametes are distributed equally. It
Doesn’t happen if the traits were on the same chromosome.
If the traits are in different chromosomes or they are largely apart in the same chromosome
then it can happen.
▪If both the locus for both traits are very close to each other , then the segregation pattern of
one trait influences the segregation pattern of other trait. This is called Linkage.
▪Also, by this time due to advancements in microscopy that were taking place, scientists were
able to carefully observe cell division. This led to the discovery of structures in the nucleus ,
these were called chromosomes (colored bodies, as they were visualised by staining). By
1902, the chromosome movement during meiosis had been worked out. Walter Sutton and
Theodore Boveri noted that the behaviour of chromosomes was parallel to the behaviour
of genes and used chromosome movement to explain Mendel’s laws.

19. BACK CROSS & TEST CROSS
❑Back cross : If the offspring is crossed with one of the parent or another plant whose
genotype and genetic constitution is similar to the parent , called Back Cross.
❑It has got Hereditary significance, used to produce superior lineage of organisms. The
significance of backcrossing lies in its ability to achieve specific breeding goals by
transferring or reinforcing desired traits while minimizing the introduction of unwanted
genetic material.
❑Backcrossing is a powerful tool in genetics and breeding, enabling the controlled transfer of
specific traits while maintaining or recovering the genetic background of a desired parent.
This technique plays a crucial role in agriculture, conservation, and other fields where the
manipulation of genetic traits is important for achieving specific goals.

20. Test Cross : A cross of an F1 individual with a homozygous recessive individual is called Test Cross.
A test cross is generally performed to know about the genotype of an organism.
Conclusion : only tall plants are
obtained. Thus it’s a homozygous
dominant (TT).
Conclusion : It’s a heterozygous
dominant plant (Tt).

21. PENETRANCE & EXPRESSIVITY
❑In some cases, not all individuals with a particular genotype show the expected phenotype due to
influences of internal and or external environment. The percentage of individuals with a given
genotype who exhibit the phenotype associated with that genotype is called Penetrance of the
genotype. For example , say everyone in a population of 100 individuals carries the same
genotype for a certain trait , yet only 80 individuals actually shows the expected phenotype, so
the penetrance is 80%. For example , Brachydactyly, an autosomal dominant trait that causes
shortened and malformed fingers, shows 50-80% penetrance. In general when we know that the
genotype is present but the phenotype is not observable, the trait shows Incomplete Penetrance.
In incomplete penetrance , a genotype is not expressed by every organism in which it is present.
Basically anything that shows less than 100% penetrance is an example of incomplete Penetrance.
❑The degree of expression of a trait is controlled by gene. A particular gene may produce different
degrees of expression in different individuals . This is known as Expressivity. For example,
Polydactyly, the condition of having more than the usual number of fingers or toes, is an
example where expressivity can vary. In some cases of polydactyly, the extra digits may be
fully functional. The additional fingers or toes have joints, muscles, and nerves, allowing
them to move and function similar to the regular digits. In other instances, the extra digit
may be present but not fully functional. It may lack joints, muscles, or nerves necessary for
normal movement. While visibly present, this extra digit is non-functional, In some cases,
individuals with polydactyly may have only a flesh extension or stub at the site where an extra
digit would normally form. It's important to note that expressivity is distinct from penetrance.
Penetrance refers to whether or not a particular genotype is expressed as a phenotype, while
expressivity deals with the degree or range of phenotypic expression when the genotype is
expressed. In polydactyly, the presence of extra digits represents the penetrance of the trait, and
the degree of functionality or development of those extra digits represents the expressivity.

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