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 LawMendel’s Law(consist of 3 part)(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

Mendel crossed

tall vs dwarf pea plant:

-Tall male vs dwarf female

-Dwarf male vs tall female

Monohybrid crosses

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 PrinciplesIt 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