Mendelism: The Basic
Principles of Inheritance
Asmarinah
Gregor Johann Mendel (1822-1884)
explained how the characteristics of organism are
inherited
His ideas was published under: “Experiment in Plant
Hybridization”
His discoveries coined a new term to describe the study of
heredity: genetics (Greek word, meaning: to generate
Mendel’s Law
(consist of 3 part)
1. The Principle of Dominance
“In heterozygote, one allele may conceal the
presence of another”
-Dominant and recessive forms are called alleles
(Greek word meaning: of one another)
- Alleles are alternate forms of a gene
2. The Principle of Segregation
“In a heterozygote, two different alleles segregate
from each other during the formation of gametes”
Both of Mendel’s law I and II, it can be explained by
monohybrid crosses
Monohybrid crosses
Mendel crossed
tall vs dwarf pea plant:
-Tall male vs dwarf female
-Dwarf male vs tall female
Explanation:
- Two reciprocal crosses gave same result: tall
plant
- Dwarf characteristic seemed disappeared in the
progeny
-After self-fertilization, result: tall and dwarf plant
(with ratio of approximately 3 : 1)
-Hybrids carried a latent factor for dwarfness and
expression factor for tallness
latent factor: recessive
expression factor: dominant
- Each trait is controlled by a hereditary factor that
existed in two forms, one dominant and the other
recessive
- These factor are now called genes, a word coined by
Wilhelm Johannsen (1909)
-Each of reproductive cell (or gamete) contain only
one copy of a gene for each trait. A particular gamete
could have either the recessive or dominant allele for a
given trait, but not both.
-Consequently, one of the alleles that governed each
trait is inherited from female parent and the other
allele is inherited from male parent
Important conclusion from Mendel’s experiment:
Genes come in pairs
-The parental strain carried two identical copies of a
gene diploid
- During the production of gametes, these two copies
are reduced to one, so that its carry a single copy of a
gene  haploid
- The hybrid between father factor and mother factor
(zygot) would inherit two different alleles, called
heterozygot
3. The Principle of Independent Assortment
The alleles of different genes segregate or assort,
independently of each other
it can be explained by dihybrid crosses
Aim of dihybrid crosses
To see if the two seed trait (color and texture) are
inherited independently
Each trait is controled by a different gene
segregating two alleles, and two genes are inherited
independently
Application of Mendel’s Principles
It can be used to predict the outcome of crosses,
by 3 alternative methods:
1. The Punnet Square Method
2. The Forked-line Method
3. The Probability Method
Mendelian Principle in Human Genetics
Application of Mendel’s principle in human genetics is
limited
The genetic analysis of human heredity depends on
family records pedigree
Pedigree Convention