Mendellism: Mendel’s law of Dominance

1. Mendellism: Mendel’s law of
Dominance
Vaishali S.Patil
Assosiate Professor, Department of Botany
Shri Shivaji College of Arts, Commerce & Science Akola

2. Introduction
• Gregor Johann Mendel was a meteorologist, mathematician,
biologist, Augustinian friar and abbot of St. Thomas‘
Abbey in Brno, Margraviate of Moravia.
• Mendel was born in a Austrian Empire (today's Czech Republic) and
gained founder of the modern science of genetics. He is known as
the "father of modern genetics"
• Mendel's pea plant experiments conducted between 1856 and 1863
established many of the rules of heredity.
• In 1856 and 1863 Mendel cultivated and tested some 28,000 plants,
the majority of which were pea plants (Pisum sativum).
• Mendel chose to work with pea plants because they have easily
identifiable traits, they can either self-pollinate or be cross-
pollinated.

3. • Mendel worked with seven characteristics of pea plants: plant height,
pod shape and color, seed shape and color, and flower position
and color.
• This study showed that, when true-breeding different varieties were
crossed to each other, in the second generation, one in four pea
plants had purebred recessive traits, two out of four were hybrids,
and one out of four were purebred dominant. His experiments led
him to make two generalizations, Law of Dominance, the Law of
Segregation and the Law of Independent Assortment, which later
came to be known as Mendel's Laws of Inheritance.

4. He published his work in 1866, demonstrating the actions of
invisible "factors"—now called genes—in predictably
determining the traits of an organism.
The profound significance of Mendel's work was not
recognized until the turn of the 20th century (more than
three decades later) with the rediscovery of his laws.
Erich von Tschermak, Hugo de Vries and Carl
Correns independently verified several of Mendel's
experimental findings in 1900, ushering in the modern age of
genetics.
Gregor Mendel, through his work on pea plants,
discovered the fundamental laws of inheritance.
He deduced that genes come in pairs and are inherited as
distinct units, one from each parent. ... Offspring therefore
inherit one genetic allele from each parent when sex cells
unite in fertilization.

5. Terminology in genetics
1.Gene:
Gene is the shortest segment of DNA responsible for the expression of
its specific character.
Gene is also called as the basic unit of heredity. It is small DNA
segment that determines a biological character of an organism. Gene is
situated on chromosomes.
Mendel used gene as the term ‘elementi’ for the genes. The term ‘factor’
was actually given by Carl correns.
2. Genome:
It is the total genetic composition of an organism.
3. Alleles (Allelomorph):
An Allele is one of a particular form of a gene. Allele is short form
allelomorph. The term was coined by Bateson and Saunders (1902) for
characters which are contrasting or differing from one another.
Allele is an alternate form of the same gene in which one is parental and
other is maternal. In case of plant height, tallness and dwarfness are the
two alleles of a gene.

6. A pair of alternative corresponding genes that are occupying the same position or locus of the
homologous chromosomes are called allele or allelomorph.
4. Locus:
The point on a chromosomes where an allele is located is called locus.
5. Dominant allele:
Out of two alleles, the one that is capable of expressing itself by hiding or suppressing its
contrasting allele is known as dominant allele.
Dominant allele is an allele that expresses itself in the same form whether present in the
homozygous or in the heterozygous condition.
It is represented by capital letter ‘T’ is the dominant allele for tallness.
6. Recessive allele:
Out of two alleles, the one that is being suppressed by its alternative allele is called as recessive
allele.
Recessive is an allele that does not express itself when present with the dominant allele (in
heterozygous form).
It expresses itself only in the homozygous form. It is represented by small letter as small ‘t’ is the
recessive allele for dwarfness.
7. Homozygous:
A diploid individual carrying two identical alleles is known as homozygous.
It is pure for a trait or character
Capital T and T are the two identical alleles for tallness. Similarly, small t and t are the two
identical alleles for dwarfness.
The homozygous organisms when self-pollinated or inbred always gives rise to true breeding
homozygous forms.

7. 8. Heterozygous:
A diploid individual carrying two different alleles is known as
heterozygous or hybrid.
It is impure for a trait (T t).
This term was proposed by Bateson and Saunders in 1902 for a zygote or
a diploid individual developed from it (zygote) which carries both factors
of a pair of alleles.
9. Hemizygous:
When a genotype is determined by a single gene/factor.
10. Genotype:
A genotype is a genetic expression of an organism. The term was
proposed by Johannason 1909 for hereditary or genetic constitution of an
individual.
Genotype is the genetic constitution of an organism.
For plant height, TT, Tt and tt are the different genotypes.
The genotype ratio of F2 in monohybrid cross is 1:2:3. It is observable
phenomenon.

8. 11. Phenotype (Trait):
It is the physical or the observable expression of an organism.
The term was coined by Johannason 1909 for the visible characters of external appearance of an
organism with respect to particular character or a group of characters.
It is the observable characteristic of an individual.
Though, the phenotype is determined by the genotype, but certain genes do not express for
number of reasons. So genotype and phenotype of an organism may not be the same.
12. Character:
It refers to a general feature of an organism such as eye color, plant height etc.
13. Dominant character:
The character which express itself in all the members of first filial generation from a cross
between two pure breeding parents differing in respect of this character is called dominant
character
14. Recessive character:
The suppressed character which does not appear in first filial generation is called recessive.
15. Parental generation:
The plants used as parents in a cross are said to represent parental generation, designed by ‘P’.
16. Hybrid:
The product of a cross between two genetically different individual belonging to the same genus
or species or between two different genera is called hybrid.
It is product of cross in between two dissimilar parents having contrasting characters.
17. Hybridization:
It is a phenomenon which speaks about the mixing of two unlike genetic constitutions.

9. 18. Monohybrid cross:
A cross between the two parents that are differing in one pair of alternating (contrasting)
character is called monohybrid cross.
In this type, single character is used.
19. Dihybrid cross:
A cross between the two parents that are differing in two pairs of alternating character is called
dihybrid cross.
In this type, two contrasting characters are used. For example, seed shape and seed color (round
yellow versus wrinkled green).
20. Polyhybrid cross:
This is a cross between two parents differing in more than two sets of alternating characters.
21. Filial generation:
The progeny obtained as a result of crossing between parents is a hybrid progeny and is called
First filial generation, represented by F1 (filial=offsprings).
The progeny obtained as a result of self-fertilization among F1 plants represents second filial
generation represented as F2.
22. Pure line:
It is the variety (line) which is homozygous in condition. It is called as true breeding varieties.
23. Reciprocal crosses:
A set of two reciprocal crosses means that the same two parents are used in two experiments in
such a way that in one cross one individual is used as female parent and other as male parent and
in second cross of same genotype sexes are reversed. For example, cross between a tall male and
dwarf female is reciprocal of a cross between a tall female and dwarf male.
The purpose of the reciprocal crosses is to find out whether both parents are making equal
contribution.

10. 24. Back cross:
It is a cross between F1 hybrid with one of its parents (P1 or P2)
Backcross is often used in genetics for the analysis of genetic
constitution of the offsprings.
It is quick way to make desirable homozygous individuals.
25. Test cross:
In this cross, the individual of an unknown genotype is crossed with
homozygous recessive parents.
This cross is made in order to determine the genotype of the unknown
individual.
A test cross is conducted to know whether an individual is homozygous
or heterozygous for a dominant character.
The off springs will be 100% dominant, if the individual is homozygous
dominant. But the ration will be 50% dominant and 50% recessive in
case of hybrid or heterozygous individual.

11. Law of Dominance
Mendel’s law of dominance states that:
“When parents with pure, contrasting traits are crossed together, only
one form of trait appears in the next generation. The hybrid offsprings
will exhibit only the dominant trait in the phenotype.”
Law of dominance is known as the first law of inheritance. In this
law, each character is controlled by distinct units called factors, which
occur in pairs. If the pairs are heterozygous, one will always dominate
the other.
Law of dominance explains that in a monohybrid cross between a
pair of contrasting traits, only one parental character will be
expressed in the F1 generation and both parental characters will be
expressed in the F2 generation in the ratio 3:1.
The one which is expressed in the F1 generation is called the
dominant trait and the one which is suppressed is called a recessive
trait.
In simple words, the law of dominance states that recessive traits are
always dominated or masked by the dominant trait. This law can be

13. A monohybrid cross is a cross between the two monohybrid traits
(TT and tt). Here plants which have the same characters, but differ
in only one character were crossed.
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 and the
offspring were called F1 progeny. The trait which is expressed in the
phenotype is called the dominant trait while the one that is not is
called the recessive trait.
He then continued his experiment with self-pollination of F1 progeny
plants. This resulted in both tall and short plants in the ratio of 3:1.

14. Parents(P1)- Tall plants X dwarf plants
TT x tt
F1 generation- Tt XTt
Tall
F2 generation- T t
T
t
TT
Tall
Tt
Tall
Tt
Tall
tt
dwarf
Genotypic ratio -1:2:1 i.e. 1=TT, 2=Tt, 1=tt
Phenotypic ratio-3:1 i.e.3= Tall, 1=dwarf