This document provides an overview of Mendelian genetics. It discusses: - Gregor Mendel, the father of genetics, who discovered the basic principles of heredity through pea plant experiments in the mid-1800s. - Key genetic terminology like traits, heredity, genetics, alleles, genotype and phenotype. - Mendel's experiments on monohybrid and dihybrid crosses, which showed that traits are inherited in predictable ratios and follow the laws of dominance and segregation. - Mendel's laws of inheritance: dominance, segregation, and independent assortment. His work established the foundation of modern genetics.

1. MENDELELIAN
GENETICS
Prepared By:
Mr. Dinabandhu Barad
MSc Tutor,
Department of Paediatric.
SUM Nursing College,
Siksha ‘O’ Anusandhan Deemed to be University

2. Gregor Johann Mendel
• Gregor Mendel, known as the "father of modern genetics," was
born in Austria in 1822.
• A monk, Mendel discovered the basic principles of heredity
through experiments in his monastery's garden.
• His experiments showed that the inheritance of certain traits in
pea plants follows particular patterns, subsequently becoming
the foundation of modern genetics and leading to the study of
heredity

3. GENETIC TERMINOLOGY
• Trait: Trait is a specific characteristic of an individual that can be passed from
parent to offspring. For example, their hair color or their blood type.
• Heredity: The process of passing of traits from one generation to another.
• Genetics: According to WHO “ the study of heredity is called genetics.

4. GENETIC TERMINOLOGY
• Hybrid: an offspring resulting from the mating between individuals of two
different genetic constitutions
• Mono hybrid cross: A cross between two individuals differing in one
characteristic
• Di hybrid cross: A cross between two individuals differing in two characteristics.
• Alleles: the alternative form of gene is called alleles
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5. GENETIC TERMINOLOGY
Genotype : the genetic make-up of an individual
Phenotype : the outward expression of genotype is called phenotype
Example: In case of a tall plant where genetic constituent can be (TT
or Tt)
Phenotype= tallness of the plant which is expressed or visible
Genotype = TT /Tt
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6. GENETIC TERMINOLOGY
• Locus: The location of allelic genes on the chromosome, e.g., A, B,
and O genes occur at the ABO locus. (Plural = loci)
• Homozygous: Possessing a pair of identical alleles for a particular
locus (TT / tt)
• Heterozygous: possessing a pair of different alleles for a particular
locus (Tt)
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7. GENETIC TERMINOLOGY
Dominant:an allele that is always expressed when present, regardless of whether
the organism is homozygous or heterozygous for that gene
Recessive: an allele that is only expressed when the organism is homozygous for
that allele and not expressed when heterozygous (when paired with a dominant
allele)
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8. MENDEL’S
EXPERIMENTS

9. MONOHYBRID CROSS
Mendel investigated the pairs of pea plants with one contrasting trait. Mendel
studied on the following seven characters with contrasting traits:
1. Flower colour: Violet/white
2. Flower position: Axial/terminal
3. Pod colour: Green/yellow
4. Pod shape: Inflated/constricted
5. Seed colour: Yellow/green
6. Seed shape: Round/wrinkled
7. Stem height: Tall/dwarf

10. MONOHYBRID CROSS
• He crossed two homozygous individuals which resulted in heterozygous
offspring. This was known as the monohybrid cross.
• “A monohybrid cross is the hybrid of two individuals with homozygous
genotypes which result in the opposite phenotype for a certain genetic trait.”
• “The cross between two monohybrid traits (TT and tt) is called a Monohybrid
Cross.”

11. MONO HYBRID CROSS

12. MONOHYBRID CROSS
• For monohybrid cross, Mendel began with a pair of pea plants with two
contrasting traits, i.e., one tall and another dwarf.
• The cross-pollination of tall and dwarf plants resulted in tall plants. All the
hybrid plants were tall. He called this as a first hybrid generation (F1) and
offspring were called Filial1 or F1 progeny.
• He conducted an experiment with all seven contrasting pairs and observed that
the entire F1 progeny showed one pattern in their behaviour, i.e., they
resembled one of the parents. Another parent character was completely
absent.

13. MONOHYBRID CROSS
• He continued his experiment with self-pollination of F1 progeny plants.
Surprisingly, he observed that one out of four plants were dwarf while the other
three were tall.
• The tall and the short plants were in the ratio of 3:1.
• He also noted that no progeny was in intermediate height, i.e., no blending was
observed.
• The result was the same for other traits of plants too, and he called them
second hybrid generation and the offspring were called Filial2 or F2 progeny.

14. MONOHYBRID CROSS
• Mendel observed that traits which were absent in F1 generation had
reappeared in the F2 generation.
• He called such suppressed traits as recessive traits and expressed traits
as dominant traits.
• He also concluded that some ‘factors’ are inherited by offspring from
their parent over successive generations.
• Later, these ‘factors’ were called genes.

15. MONOHYBRID CROSS
• Genes are responsible for the inheritance of traits from one generation to
another.
• Genes consist of a pair of alleles which code for different traits.
• If a pair of alleles is the same, i.e., TT or tt, such alleles are called
homozygous pair while those that are different or non-identical (e.g. Tt)
are called heterozygous pair.

16. DIHYBRID CROSS
• A dihybrid cross is a breeding experiment between two organisms which are
different in two contrasting traits.
• He picked the wrinkled-green seed and round-yellow seed and crossed them.
• He obtained only round-yellow seeds in the F1 generation.
• This indicated that round shape and yellow colour of seeds are dominant in
nature.

17. DIHYBRID CROSS
• Meanwhile, the wrinkled shape and green colour of seeds are recessive traits.
• Then, F1 progeny was self-pollinated.
• This resulted in four different combinations of seeds in the F2 generation.
• They were wrinkled-yellow, round-yellow, wrinkled-green seeds and round-green
in the phenotypic ratio of 9:3:3:1.

18. DIHYBRID CROSS
• Consider “Y” for yellow seed colour and “y” for green seed colour, “R” for roun
shaped seeds and “r” for wrinkled seed shape. Thus, the parental genotype wil
be “YYRR” (yellow-round seeds) and “yyrr” (green-wrinkled seeds).

19. Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R round
r wrinkled
Y yellow
y green
RrYy x RrYy
RY Ry rY ry RY Ry rY ry
All possible gamete combinations

20. Dihybrid Cross
RRYY RRYy RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy rryy
Round/Yellow: 9
Round/green: 3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1 phenotypic
ratio
RY Ry rY ry
RY
Ry
rY
ry

21. Dihybrid Cross
Round/Yellow: 9
Round/green: 3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1

22. Mendel’s Laws

23. LAW OF DOMINANCE
• In case of a heterozygote state of alleles (Tt) the trait associated with one of
these will be expressed (Called dominant) and the other one will remain
supressed.(Called recessive)
• Example:
• A plant with genotype for height if is in heterozygote state i.e. Tt , then the
plant appears to be tall as the characteristics of ‘T’ only expressed.

24. LAW OF SEGREGATION
During the formation of gametes (eggs or
sperm), the two alleles responsible for a
trait separate from each other.
Alleles for a trait are then "recombined"
at fertilization, producing the genotype
for the traits of the offspring.

25. Applying the Law of Segregation
copyright cmassengale

26. LAW OF INDEPENDENT
ASSORTMENT
• Mendel's law of independent assortment states that
the alleles of two (or more) different genes get sorted
into gametes independently of one another

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