The First Law of Heredity

Mendel’s Experiment

An Austrian monk known as Gregor Mendel is considered as the Father of Genetics because of his pioneering work on inheritance. He carried out various breeding experiments and observed the variations in the different characteristics of the garden pea.

Mendel selected a group of dwarf plants & self-pollinated them by dusting mature pollen grains onto stigmas of the same plant. He then collected the resulting seeds and planted them. He then noted that these seeds germinated and grew into dwarf plants only.

He repeated these experiments for several generations and observed the same results. This showed that dwarf garden pea plants could only produce only their own type i.e. they were pure breeds/homozygous.

In another experiment, Mendel selected tall plants and self-pollinated them. He then planted the resulting seeds and observed that they grew into a mixture of tall and dwarf plants.

He took seeds from the tall offspring only and repeated the experiment, for many generations until he obtained tall plants only.

He then cross-pollinated purebred tall garden pea plants with the purebred dwarf variety. He planted the resulting seeds and observed that the offspring were all tall plants e.g.

He crossed two of these tall offspring and planted the resulting seeds. He observed that this 2nd generation consisted of a mixture of tall and dwarf plants. He counted these plants and noted that the ratio of the tall to dwarf plants was approximately 3: 1 respectively.

Mendel concluded that the characteristics of an organism are determined by hereditary factors, which occur in pairs. Only one of a pair of such factors can be represented in a single gamete. This is known as Mendel’s First law, the Law of Segregation.

The other characteristics include;

• Height of the stems – tall or dwarf
• Texture of seed coat- smooth or wrinkled
• Colour of seeds-z purple or white
• Texture of pods- smooth or wrinkled
• Colour of pods – green or yellow
• Position of flower- axial or terminal

Mendel’s First Law

States that the characteristics of an organism are determined by internal factors (genes) which occur in pairs. Only one of a pair of such factors (genes) can be represented in a single gamete.

Also called law of segregation.

It states that genes are responsible for the development of individual characters

These characters are transmitted individually without any alterations.

Only one character from a contrasting pair can be carried in a gamete, hence only one character can be inherited.

Monohybrid inheritance involves the transmission of just one pair of contrasting characteristics (traits).

Reasons why pea plant was suitable for genetic studies

• Has many observable distinct and contrasting traits.
• Has a short life cycle.
• Has few ecological requirements.
• Can be crossed or selfed.