This Project Aims at discussing Mendel's Laws of Inheritance with a brief introduction to his work, followed up by the developments that occured post mendelism

1. Mendelian
Inheritance
Bhanu Pratap Singh Yadav
MSFS NFSU Delhi

2. Who was Mendel?
He was the head of St. Thomas monastery in Brno,
Czech Republic while also teaching physics and Natural
sciences.
• In 1856, Mendel observed 2 types of seeds in pea
plants growing in his monastery and became
interested in them. He then carried out hybridisation
experiments on pure lines of Garden Pea, Pisum
sativum for 7 years from 1856-63.
• His experiments had a large sampling size, some 10K
pea plants.
• He used statistics and mathematical logics and
published his paper “Experiments in plant
hybridisation” in the annual proceedings of Brunn
Natural Science Society in 1866.
• He passed away in 1884 without getting any
recognition for his work.

3. 7 characters of Pea Plant:

4. Reasons for Choosing Pea Plant?
Can be grown in small area
Produce lots of offsprings
Produce pure plants when allowed to self pollinate several
generation.
Can be artificially cross pollinated.

5. Reasons for Mendel’s success
• Strategic Organism Choice: Mendel's selection of the garden
pea with its short life cycle enabled quick generation turnover
• Controlled Breeding: Mendel's meticulous control in cross-
breeding allowed precise understanding of parent genetics.
• Clear Distinctions in Traits: The garden pea displayed distinct
traits, facilitating easy identification and analysis.
• Quantitative Analysis: Mendel's use of numerical data and
ratios introduced a quantitative dimension to genetic studies.
• Trait Isolation: Focusing on one trait at a time with constant
conditions enabled clear conclusions about inheritance

6. Rediscovery of
Mendel’s Work:
• Mendel's contributions went unacknowledged for 34 years. Three scientists
independently rediscovered the concepts of heredity that Mendel had
previously figured out in 1900. They were Hugo de Vries, Carl Correns, and Eric
von Tschermak.
• Additionally, De Vries discovered Mendel's article and had it published in
"Flora" in 1901.
• After their rediscovery, Carl Correns was the one who transformed Mendel's
work into the "laws" that we are familiar with today.
• Scientists like Bateson, Morgan, and others popularized Mendel’s work with
further research application on animals.

7. Important Terms:
• Character- Phenotypic property of the individual like height, flower color,etc
• Trait- an inherited character such as tall/dwarf, white/purple flower color, etc
• Gene- unit factor responsible for inheritance of characters, occurs in chromosome and control the expression of a
character or multiple characters in cooperation with other genes and environment.
• Allele- alternative forms of a gene, control variations of the character controlled by that gene.
• Phenotype- observable/measureable distintive structural/functional characteristic of an individual
• Genotype- gene constitution of an individual with regard to 1 or more characters, irrespective of the expression of the
genes.
• Test cross: between an unknown genotype and recessive parental type; 3 types- mono/di/tri-hybrid.
• Monohybrid cross- cross made between 2 organisms of a species while studying single pair of alleles.
• Dihybrid cross- cross made between 2 organisms of a species while studying two pairs of alleles.
• Trihybrid cross- cross made between 2 organisms of a species while studying three pairs of alleles
• Back cross: an 𝐹1 hybrid with one of the 2 parental types.
• Locus- a fixed location on a chromosome, occupied by a gener or one of its alleles.
• Homozygote- an individual which contains identical alleles of a gene on its homologous chromosome.
• Heterozygote- an individual which contains contrasting factors for a character or 2 different alleles of a gene on its
homologous chromosome.

8. Monogenic
Inheritance
Studying the inheritance of a single
gene (one pair of alleles) at a time.
• Each organism has a large number
of characters, like height, skin
colour, etc. Each character is
represented in an individual by two
units or alleles.
• When both alleles represent the
same trait of the gene, that
condition is called as Homozygous,
e.g., TT/tt. When two alleles
represent different traits of same
character, the condition is called as
Heterozygous or Hybrid, e.g., Tta

9. Law of
Dominance:
States that-
“Characters are controlled by discrete
units called Factors (now, Alleles),
which occur in pairs. Out of the 2
alleles, only one expresses itself in the
hybrid and prevents the expression of
the other allele. The allele which
expresses itself in the hybrid is called
the Dominant allele while the subdued
allele is called Recessive.”

10. Explanation of law of Dominance:
Take 2 pea plants, one homozygous tall (TT)
and the other homozygous dwarf (tt) and
Cross the two to raise their progeny, called the
First filial or 𝐹1 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑜𝑛. All plants of
𝐹1 are Tall.
Significance: the expression of only one
phenotype, i.e., All plants of 𝐹1 are Tall shows
that T is the dominant allele.
Parents: TT X tt
Gametes: T T t t
Crossing
𝐹1 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑜𝑛:
Genotype = all Tt = 1 or 100% Tt
Phenotype = All Tall = 100%

11. Law of
Segregation:
States that-
“Two alleles of a gene controlling each
character stay together in the individual, but
segregate or separate from each other during
gamete/spore formation by meiosis such that
each gamete/spore receives only one allele of
each character. Therefore, each gamete/spore
is always pure for a character”
Hence, this is also called the Law of purity of
gametes.

12. Explanation of law of Segregation:
Take 2 pea plants, one homozygous tall(TT) and the other homozygous dwarf(tt) and Crossed,
producing all tall progeny in 𝐹1 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑜𝑛. Now, 𝐹1 plants are allowed to self-breed. The
plants of the 𝐹2 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑜𝑛 appear to be both tall & dwarf, in the phenotypic ratio of 3:1.
Further selfing of these plants shows that dwarf (tt) plants breed true = produce only dwarf
plants while amongst the tall plants 1
3 breed true = produce only tall plants, the remaining
2
3 of tall plants produce both tall and dwarf plants in the ratio 3:1.
Parents: Gametes: 𝐹1 𝑠𝑒𝑙fing:
TT
tt
T
T
t
t
Tt
Tt
Tt
Tt
𝐹1 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑜𝑛:
Tt
Tt
T
t
T
t
Gametes:
TT
Tt
Tt
tt
𝐹2 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑜𝑛:
All
hybrid
Tall
Pure
Tall
Hybrid
Tall
Dwarf
Pure
Tall
Pure
Dwarf
1 TT
2 Tt
1 tt
Further
Selfing:

13. Significance: 𝐹2 generation is produced by selfing 𝐹1 plants. Though 𝐹1 only show the
dominant trait, it carries both types of alleles as the recessive trait appears in 𝐹2 plants.
The 𝐹2 generation contains 3 types of plants – Pure Tall, hybrid tall & pure dwarf, this is
only possible when;
a) 2 alleles present in 𝐹1 plants, segregate during gamete formation
b) Gametes carry a single allele for a character.
Due to these observations by scientists, this is also known as the Law of non-mixing of
alleles.
Segregation might not occur under abnormal condition such nondisjunction.

14. Digenic
Inheritance
Studying the inheritance of two
genes (two pairs of alleles) of
two characters at a time is called
as digenic inheritance.

15. Law of
Independent
Assortment:
States that-
“The alleles of two pairs of traits separate
independently of each other during gamete/spore
formation and get randomly rearranged in the
offspring at the time of fertilization, producing both
parental and new combinations of traits.”
Here, two important events occur:
1. Assortment of two pairs of alleles
independently of each other during
gamete/spore formation.
2. Rearrangement of alleles (random reunion) in
the offspring at the time of fertilization.

16. Explanation:
1. A cross is made between a pure round yellow seeded plant and a pure wrinkled
green seeded plant; Yellow colour and round shape being the dominant traits,
producing all Round yellow seeded plants in 𝐹1 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑜𝑛.
RRYY
rryy
RY
RY
ry
ry
RrYy
RrYy
RrYy
RrYy
Parents: Gametes: 𝐹1 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑜𝑛:
Segregation of alleles during
gamete formation
RrYy
100%
Round
Yellow
Seeds
(Hybrid)

17. 𝐹1 plants are allowed to self breed and produce 𝐹2 generation. 𝐹2
generation has 4 types of plants- round yellow, round green, wrinkled
yellow and wrinkled green seeded plants in the ratio of 9:3:3:1.
RrYy
RrYy
RY
RY
Ry
rY
Ry
ry
rY ry
Gametes:
Round yellow Round yellow Round yellow Round yellow
Round yellow Round green Round yellow Round green
Round yellow Round green wrinkled yellow wrinkled yellow
Round yellow Round green wrinkled yellow wrinkled green
Round yellow Round green Wrinkled yellow Wrinkled green
9 3 3 1
: : :
𝐹1 𝑆𝑒𝑙𝑓𝐵𝑟𝑒𝑒𝑑𝑖𝑛𝑔
Phenotypic Ratio = 9:3:3:1
Seed Colour and Seed Shape
Yellow = 9 +3 = 12
Green = 3 + 1 = 4
Round = 9 + 3 = 12
Wrinkled = 3 + 1 = 4
Ratio of Yellow : Green = 12:4 or 3:1
Ratio of Round : Wrinkled = 12:4 or 3:1
Genotypic Ratio =
1:2:2:4:1:2:1:2:1
Recombinants –
Round green (3): RRyy, Rryy, Rryy
wrinkled yellow (3): rrYY, rrYy, rrYy
Parental types – 9

18. Significance-
• 𝐹2 generation has phenotypic ratio
9:3:3:1. Each trait if considered
separately, shows a ratio of 3:1
same as obtained from a
monohybrid cross.
• 2 types of recombinants, Round
Green and Wrinkled Yellow seeded
plants are produced. They can only
be produced if 2 alleles of 2
different traits are free to
recombine, i.e., separate and
combine independent of each
other.
Round yellow Round yellow Round yellow Round yellow
Round yellow Round green Round yellow Round green
Round yellow Round green wrinkled yellow wrinkled yellow
Round yellow Round green wrinkled yellow wrinkled green
Round yellow Round green Wrinkled yellow Wrinkled green
: : :
9 3 3 1
RRYY : RRYy : RrYY : RrYy : RRyy : Rryy : rrYY : rrYy : rryy
RRYY : RRYy : RrYY : RrYy : Rryy : Rryy : rrYY : rrYy : rryy
1 2 2 4 1 2 1 2 1

19. Mendelism, while influential, has
limitations. The notion of paired
alleles controlling traits isn't
universally applicable; some traits
are governed by pleiotropic genes
or multiple alleles. The law of
dominance has exceptions like co-
dominance and incomplete
dominance. Linkage challenges the
law of independent assortment.
Amidst these variations, the law of
segregation stands as a universal
principle.

20. Incomplete
Dominance:
The phenomenon where none of the
two alleles of a gene is dominant
over the other so that when both
are present together, a new
phenotype is formed which is
somewhat an intermediate between
the independent expression of the
two alleles.
Also called as Mosaic/Blending inheritance.

21. Incomplete dominance in Antirrhinum majus (Snapdragon/Dog Flower)

22. Examples of Incomplete Dominance
• Shown by Mirabilis jalapa (Four O’clock flower):
Pure Red and White flowered parents produce intermediate “pink”
flowers in 𝐹1. When 𝐹1 is crossed, it gives back pure red and white
parental types along with the intermediate type in the ratio
1(Red):2(Pink):1(White).
• Shown by Andalusian Fowls.
They have two pure forms, Black and White. If the two are crossed, 𝐹1
progeny has blue colored individuals. 𝐹2 generation produces 3 types of
fowl- 1(Black):2(blue):1(White)

23. Co-Dominance
The phenomenon in which both the
alleles of a gene express themselves
simultaneously in a heterozygote.
Both alleles neither show dominant-
recessive relationship nor
incomplete dominance but express
their traits independently and are
known as Co-dominant alleles.

24. In cattle, gene R stands for red coat color and gene W stands for
White coat color.
• When red cattle (RR) are crossed with white cattle (WW),
the 𝐹1 generation has a reddish coat with interspersed white
spots or “Roan” coat with RW genotype.
• In the 𝐹2 generation: Red, Roan, and White Cattle are
produced in the ratio 1(RR): 2(RW):1(WW).
• The Roan Coat color heterozygote – RW shows co-dominance
of both alleles.
Co-dominance in ABO blood
group:
In humans, ABO grouping
contains 4 blood group types-A,
B, AB, & O.
These are governed by Gene I
and its 3 alleles: 𝐼𝐴
, 𝐼𝐵
& 𝐼𝑂
.
𝐼𝐴
& 𝐼𝐵
are dominant of 𝐼𝑂
but
co-dominant with each other
and so produce AB group
phenotype

25. Multiple
Allelism
The existence of more than two
alternative forms (alleles) of a gene
in a population occupying the same
locus on a chromosome or its
homologue is known as multiple
allelism.
Multiple alleles are produced due to
repeated mutations of the same
gene in different directions.

26. Human ABO blood groups: Multiple Allelism
• ABO blood groups exhibit both codominance and
multiple allelism.
• People with Group A form Antigen A, People with
Group B form Antigen B, People with Group AB form
both antigens while those with Group O don’t form
any. ABO grouping is governed by Gene I and its 3
alleles: 𝐼𝐴
, 𝐼𝐵
& 𝐼𝑂
.
• 𝐼𝐴
& 𝐼𝐵
are dominant of 𝐼𝑂
but co-dominant with
each other.
• A human being carries 2 of the 3 alleles, one from
each parent therefore, the maximum possible number
of genotypes is 6 for 4 phenotypes.
Other examples-
• Coat colour in Rabbits is controlled by 4
alleles ( C, c, 𝐶ℎ, 𝐶𝑐ℎ; C is dominant over all
others for Full Color)

27. Pleiotropy
A condition in which one gene
influences more than one trait and
such a gene is called pleitropic gene.

28. Examples:
• In Gaden Pea, the gene which controls flower color also controls the
color of seed coat and the presence of red spots on leaf axils.
• In humans, Sickle cell anaemia and Phenylketonuria (PKU) are
examples of Pleiotropy.
• Phenylketonuria (PKU) serves as a prime example of pleiotropy. It
arises from mutations in the phenylalanine hydroxylase (PAH) gene,
impacting the breakdown of phenylalanine to tyrosine in the liver. The
pleiotropic effects include the accumulation of phenylalanine, leading
to intellectual disability, changes in skin and hair pigmentation, and
neurological issues. This highlights how a single gene mutation can
influence various seemingly unrelated traits.

29. Polygenic
Inheritance
A type of inheritance controlled by 3
or more genes in which the
dominant alleles have cumulative
effect with each dominant allele
expressing a part of the trait, the full
being shown only when all dominant
alleles are present.
The genes involved in such an
inheritance are called as polygenes.

30. Skin color and pigmentation
• The color of the skin is controlled by around 60 loci and
caused by a pigment called Melanin. The quantity of
melanin is due to 3 pairs of polygenes (A, B & C).
• If Very dark (AABBCC) and Very Light (aabbcc) individuals
marry, the 𝐹1 generations show intermediate or Mulatto
color (AaBbCc). When 2 Mulattos marry, the skin color of
the children (the 𝐹2 generation) will vary from very
dark/black to very light/white.
• A total of 8 allelic combinations are possible in gametes
forming 27 distinct genotypes distributed into 7
phenotypes.
• 1 (Very Dark): 6(dark): 15(fairly dark): 20(Mulatto): 15(fairly
light): 6(Light): 1(Very light) or a phenotypic ratio of
1:6:15:20:15:6:1
ABC abc
AaBbCc AaBbCc
Gametes
𝐹1 self-crossed
𝐹2 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑜𝑛

31. Epistasis
The phenomenon of masking/suppressing the
expression of a gene by another non-allelic gene.
The suppressor gene is known as epistatic gene and
the suppressed gene is known as hypostatic gene.
Types of Epistasis:
• Dominant(12:3:1 ratio): In this, the epistatic gene
is dominant over its own allele. It is therefore
effective even in heterozygous condition. Example-
Fruit color in Cucurbita pepo.
• Recessive(9:3:4 ratio): In this, the epistatic gene is
recessive to its own allele. Thus, epistatic gene can
only exert influenc in homozygous condition.
Example- Coat color in mice.

32. Dominant Epistasis: Fruit color in
Summer Squash
In Cucurbita pepo there are 3 common fruit colours- white,
yellow and green. White colour is produced due to the
presence of dominant gene W. In the absence of W, the
dominant gene Y produces yellow fruit colour and the double
recessive is green.
The effect of dominant gene `Y' is masked by dominant gene
`W' which is the epistatic gene.
When pure breeding white fruited variety is crossed with the
double recessive green variety, the F1 hybrids are all white.
When the hybrids are selfed, white, yellow and green fruited
plants arise respectively in the ratio of 12:3:1

33. • The wild type coat colour, agouti (AA) is dominant to coloured
fur (aa). Anyhow, a separate gene (C) is necessary for
pigmentation production.
• A mouse with recessive c allele at this locus is unable to
produce pigment and is albino regardless of the allele
present in locus A.
• Therefore, the genotypes AAcc, Aacc, and aacc all produce
an albino phenotype. In this case, the C gene is epistatic to
the A gene.
• The classical F2 segregation ratio of 9:3:3:1 becomes
modified into 9:3:4 in recessive epistasis.
• Here, the expression of color genes is masked by recessive
cc.
Recessive Epistasis:
Coat Color in Mice

34. Thank You