This document provides information about the history of genetics and key concepts discovered by Gregor Mendel through his experiments with pea plants. It discusses: - Mendel's experiments on inheritance of traits like plant height and seed color through monohybrid and dihybrid crosses. - His discovery of the laws of dominance (one allele masks the other) and segregation (alleles separate into gametes independently during reproduction). - Other genetic concepts like genotypes/phenotypes, homozygosity/heterozygosity, incomplete dominance and codominance that emerged from later experiments building on Mendel's work. - How Mendel's discoveries established genetics as a science and laid the foundation for understanding inheritance of

2. History of Genetics
Mendel and Mendelian Inheritance
Important Terms used in Genetics
LEARNING OBJECTIVES
Selection of traits and Mendel’s Methodology
Monohybrid cross and its observations
Phenomenon/ Law of Dominance
Law of Segregation
Dihybrid cross and Law of Independent assortment

3. History Of Genetics
• Genetics is a branch of biology concerned with the study of genes, genetic
variation, and heredity in organisms.
• The most influential, early theories of heredity were proposed by Hippocrates
and Aristotle.
• Hippocrates' theory was similar to Darwin's later ideas on pangenesis, involving
heredity material that collects from throughout the body.
• Aristotle suggested instead that the (nonphysical) form-giving principle of an
organism was transmitted through semen (which he considered to be a purified
form of blood) and the mother's menstrual blood, which interacted in the womb
to direct an organism's early development.
• In the 18th century, with increased knowledge of plant and animal diversity and
the accompanying increased focus on taxonomy, new ideas about heredity began
to appear.
• Plant and animal breeders described a wide variety of inheritance phenomena,
include hybrid sterility and the high variability of back-crosses.
• These observations led them to ask- Why do children resemble their parents?,
and how can various diseases run in families?

4. Important Terms used in Genetics
• Factors or Genes: Mendel defined the units of inheritance as factors, which was
later in 1909 re-coined by William Johansen as genes.
• Alleles: Alleles are alternate forms of the same gene and may result in formation
of contrasting traits.
• Traits: A particular inheritable feature is referred to as trait.
• Dominant and recessive traits: An allele that expresses itself even in the
presence of its alternate form and simultaneously masks the expression of its
alternate form is termed as dominant allele, while the one being masked is
termed as recessive allele. A recessive allele is capable of expression only in
homozygous condition.
• Homozygosity (Homo= similar): When similar pair of alleles are present for a
particular trait, the phenomenon is referred to as Homozygosity and the
individual is termed homozygous.

5. Important Terms used in Genetics
• Heterozygosity (Hetero= dissimilar): When dissimilar pair of alleles are
present for a particular trait, then the phenomenon is referred to as
Heterozygosity, and the individual is termed as heterozygous.
• Genotype: The allelic constitution of an organism is its genotype, which is
the hereditary underpinning of the organism.
• Phenotype: The expressed and observable traits constitutes the phenotype
of an allele.

6. Mendel And Mendelian Inheritance
• Genetics as a set of principles and analytical procedures
did not begin until the 1860s, when an Augustinian monk
named Gregor Mendel performed a set of experiments
that pointed to the existence of biological elements that
we now call alleles which we now know as variants of
genes.
• Mendel traced inheritance patterns of certain traits in pea
plants and showed that they obeyed simple statistical
rules. Now known as laws of Inheritance.
• His work acted as a proof that application of statistics to
inheritance could be highly useful.
• The significance of Mendel's work was not understood
until early in the twentieth century, after his death, when
his research was re-discovered by scientists working on
similar problems. Hugo de Vries, Carl Correns and Erich
von Tschermak.

7. Selection of traits and Mendel’s Methodology
• Mendel conducted artificial pollination/cross
pollination experiments using several true-breeding
pea lines.
• A true-breeding line is one that, has undergone
continuous self-pollination, and shows stable trait
inheritance and expression for several generations.
• Mendel selected 14 true-breeding pea plant varieties,
as pairs which were similar except for one contrasting
trait.
• Mendel used emasculation and artificial pollen
transfer techniques to ensure cross-pollination and
prevent self-pollination.

8. Normal pea flower
Emasculation of the female parent
Pollen from selected Male
parent
Emasculation and Cross pollination technique used by Mendel

9. Inheritance Of One Gene
• To understand how traits are inherited from parents to progeny, Mendel
performed several hybridization experiments with his pea plants.
• In one of such hybridization experiment, Mendel crossed tall and dwarf pea
plants to study the inheritance of the trait of tallness.
• He collected the seeds produced from the result of this cross and grew
them to generate plants of the first hybrid generation. Mendel called it
Filial progeny-1 or F1 progeny.
• Mendel observed that all the members of F1 progeny were tall, and none
were dwarf. On performing similar hybridization for other traits, Mendel
observed appearance of only one of the traits .
• To investigate the fate of the other traits he self-fertilized the F1 progeny
and to his surprise found that in the Filial2 generation some of the
offspring were ‘dwarf ’; the trait of dwarfness, which was not seen in the F1
generation was now expressed.
• Of the total F2 progenies 25% were dwarf while the rest 75% were tall,
and the contrasting traits did not show any blending in either F1 or F2
generations, even for crosses of other traits.

10. Monohybrid cross
Parental Generation
Genotype TT tt
F1 Progeny
Gametes formed T t
Tt Tall (Heterozygous)
F1 X F1 self pollination
Gametes T t
T TT( Tall) Tt (Tall)
t Tt (Tall) tt (Dwarf)
Phenotypic Ratio: 3 (tall) : 1 (dwarf)
1( homozygous tall ) : 2 (heterozygous tall) : 1 (Homozygous dwarf)
Genotypic Ratio:

11. Mendel’s observation from Monohybrid cross
• Mendel proposed that in a true breeding, tall or dwarf pea variety the
allelic pair of genes for height are identical or homozygous, TT and tt,
respectively.
• TT and tt are called the genotype of the plant while the descriptive terms
tall and dwarf are the phenotypes.
• Phenotype of the F1 heterozygote ‘Tt’ is like the TT parent in appearance,
and 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.
• Mendel also observed that the alleles T and t did not undergo any blending
of traits and segregated during gametogenesis.
• From the above observations Mendel recognized the phenomenon of
dominance, which led to the formulation of two laws of inheritance.

12. Phenomenon of Dominance
• Mendel described the phenomenon of dominance as, “In a crossing between pure
(homozygous) organisms for contrasting pair of traits, only one of the traits appear in
the first filial generation”. It is also known as First law or Law of Dominance.
• Mendel commented the transmission of some discrete factors, from parents to
progeny were responsible for expression of the traits.
• Cytological investigations later identified these factors as chromosomes which we
now know as condensed and tightly packed DNA.
• Each diploid cell has two sets of chromosomes obtained from two different parents,
via their gametes.
• Occasionally the traits lack a clear ‘dominant-recessive’ relationship and leads to
creation of variation from either parental type, and are studied as incomplete
dominance and co-dominance.
• In, incomplete dominance, under heterozygous condition the dominant allele is
unable to mask the recessive allele completely leading to formation of an
intermediate trait.
• In co-dominance both the alleles are capable of some degree of phenotypic
expression.

13. Incomplete Dominance
• Repetition of experiments similar to peas
using other traits in other plants, yielded
different result and the F1 progeny had a
phenotype that did not resemble either of
the two parents and was in between the
two, e.g. inheritance of flower colour
snapdragon(Antirrhinum sp.).
• In a cross between true-breeding red-
flowered (RR) and truebreeding white-
flowered plants (rr), the F1 progeny was
observed to be pink.
• When the F1 was self-pollinated the F2
progeny demonstrated genotypic ratios
exactly as expected in any Mendelian
monohybrid cross, but the phenotype ratios
had changed from the 3:1 dominant :
recessive ratio.

14. Co-dominance
• It is closely related to incomplete dominance,
as the one allele is unable to mask the
expression of other allele.
• In co-dominance, both alleles are
simultaneously expressed in a heterozygote
individual.
• For example The MN blood group system of
humans. A person's MN blood type is
determined by presence of allele LM or LN,
(letter L is assigned in honor of its discoverer
Landsteiner and Levine).
• The three blood groups, M, N and MN
depend on the presence of antigen on the
surface of RBC, and can be detected by their
agglutination reaction with the
corresponding antisera.
• Homozygotes as seen here have only M or an
N markers, respectively, on the surface of
their red blood cells. However, heterozygotes
have both types of markers in equal numbers
on the cell surface.
Genotype Reaction with
antisera M
Reaction
with
antisera N
Blood Group
(Phenotype)
LMLM + - M
LMLN + + MN
LNLN - + N

15. Laws of Segregation
• Mendel’s second law or law of segregation is
also known as the law of purity of gametes.
• The law states that, A heterozygous diploid
organism passes an allele for a trait randomly
to its offspring, such that the offspring
receives one allele from each parent. The
alleles though remain together in the parent,
segregate independently at the time of
gametogenesis.
• Each gamete acquires only one of the two
alleles, as chromosomes separate into
different gametes during meiosis. For example
the F1 individual having genotype (Tt), during
gametogenesis produces two types of
gametes, where one gamete receives the T
allele while the t allele is received by a
another gamete.

16. Dihybrid Crosses
• To study how different traits would behave in relation to each other when
being inherited from one generation to another, Mendel performed crosses
of pea plants which were differing by two pairs of contrasting traits.
• The crosses yielded dihybrid, hence those crosses were termed as Dihybrid
crosses.
• In one of his dihybrid crosses Mendel crossed a homozygous pea plant
having yellow round seeds with a homozygous pea plant having green
wrinkled seeds.
• When Mendel performed this cross and observed the offspring, he found
that there were four different categories of pea seeds: yellow and round,
yellow and wrinkled, green and round, and green and wrinkled.
These phenotypic categories appeared in a ratio of approximately 9:3:3:1.
• Based upon the observations of his dihybrid crosses Mendel formulated
the third law or Law of Independent Assortment.

18. Law of Independent Assortment
• The F1 hybrids have four types of alleles R,r,Y,y and during gametogenesis
these four alleles may combine in the following four combinations: RY, rY,
Ry, ry, producing four types of gametes which may unite randomly at the
time of fertilization producing sixteen type of individuals in the F2
generation.
• It was observed that each pair of contrasting character behaves
independently and bears no permanent association or relation to a
particular character.
• Based upon these observation Mendel stated that: when the parents differ
from each other in two or more pairs of contrasting characters or factors,
then the inheritance of one factor is independent to that of the other pair
of factor. This observation came to be known as Law of Independent
Assortment.

19. Reason for independent assortment
• The two copies of a gene carried by an organism (such as a Y and a y allele)
are located at the same spot on the two chromosomes of a homologous
pair.
• Homologous chromosomes are similar but non-identical, and an organism
gets one member of the pair from each of its two parents. Thus, the
physical basis for the law of independent assortment lies in meiosis-I of
gamete formation, when homologous pairs line up in random orientations
at the middle of the cell as they prepare to separate.
• Consequently we get gametes with different combinations of "mother"
and “father" homologues in a random orientation.
• There are, however, gene pairs that do not assort independently. When
genes are close together on a chromosome, the alleles on the same
chromosome tend to be inherited as a unit more frequently than not. Such
genes do not display independent assortment and are said to be linked.
• Linked genes result in deviation from Mendelian inheritance and thus
studied separately.

20. Testing the genotype of an individual
• In genetics, test crosses are used to test an
individual's genotype and involves breeding the
individual in question with another individual
that expresses a recessive version of the same
trait.
• Recessive individuals are selected because
individuals that show the recessive phenotype
are known to have a homozygous recessive
genotype.
• Analyzing the proportions of dominant and
recessive offspring reveals the genotype of the
individual in question.
• If all offspring from the test cross display the
dominant phenotype, the individual in question
is homozygous dominant, while, if half the
offspring display dominant phenotypes and half
display recessive phenotypes, then the
individual is heterozygous.
Unknown
individual
Known recessive
Unknown
individual
Known recessive
Phenotypic ratio: 50% Yellow
Phenotypic ratio: 100%
Yellow
Unknown individual is:
Heterozygote
Unknown individual is:
Homozygote

21. Don’t forget to like and subscribe to our channel