MENDEL'S WORK IN GENETICS: His Experiments, His Reasons, His Laws

Gregor Mendel (1822-1884) is often regarded as the father of genetics.This is because his work formed the foundation for scientific study of heredity and
variation. He was a monk in an Augustinian monastery in Australia.

Read also: Genetics: Biology of Heredity


MENDEL'S EXPERIMENTS
Series of experiments were carried by Gregor Mendel to determine how traits are transferred from one generation to another. He used garden pea (Pisum sativum). He wanted to find out the pattern of different traits inheritance of the pea plant.

Reasons Mendel Chose a Pea Plant
Mendel chose to use pea plant for experiment because of the following:

1. Pea plants usually self-pollinate and could be pollinated by him
2. Pea plant was known for its unique traits which occur in contrasting pairs
3. Pea plants are annual plants, hence have a short life span

Examples of the unique traits, which were known to be exhibited by pea plants include: 

• some flowers were red while some were white
• some flowers were axial while others were terminal
• some pods were yellow while some were green
• some seeds were round while others were wrinkled
• some plants were short while others were tall
• some pods were rough while others were smooth
• some seeds were green while others were yellow

METHODS GREGOR MENDEL USED IN HIS EXPERIMENT

Two general methods were used by Mendel in his experiments. They are monohybrid inheritance and dihybrid inheritance.

1) MONOHYBRID INHERITANCE

 Mendel crossed two different two different pea plants with different pair of contrasting traits, for instance, short and tall plants, artificially. It was an example of complete dominance, The procedure was called a monohybrid inheritance. 

The order by which Mendel carried out the experiment is as follows:

1. He planted short pea plants for a series of generation, as well as tall pea plants. He noticed that the plants produced by the short plants were all short and those produced by the tall plants were all tall.
2. He then planted short plants and tall plants and waited for the flowers to be produced. He then collected the pollen grains of the plant, tagged the male, and pollinated the stigma of the short plant tagged the female. Also, he collected the pollen grains of the short plant and placed them on the stigma of the tall plant.
3. He then covered the flowers that he pollinated artificially with small paper bags. This was to prevent the chance of natural pollination by insects.
4. He plucked the seeds produced after the cross, planted them, and noticed that the plants produced were all tall plants. He called them first filial generation 
5. He then crossed the first filial generation plants. He collected their seeds and then sowed them. The plants produced by them were tall and short plants in the ratio 3:1 respectively. He named them second filial generation (F₂ generation)

The conclusion made from this experiment gave rise to Mendel's first law of inheritance



From above sketch, the genotype of all F₁ generation plants is Tt and the phenotype is all tall plants, though heterozygous.

For the F₂ generation to be obtained, the F₁ plants as self-pollinated, as seen below.

From the above picture(diagram):
 The genotype of the F₂ generation plants are: 

• 1 TT 
• 2 Tt 
• 1 tt

Therefore, the genotypic ratio is 1:2:1

Also, their phenotypes are:

• 3 Tall plants
• 1 short plant

Hence, the phenotypic ratio is 3:1

MENDEL'S FIRST LAW OF INHERITANCE

This states that genes are responsible for the development of the individual and are they are independently transmitted from one generation to another, without undergoing any alteration. 

This law (mendel's first law of inheritance) is also known as law of segregation of genes.

Explanation of the above diagrams:

As seen above, all the F₁ generation offspring are tall. This means, the gene for tallness (TT) is dominant over the recessive genes (tt). In the F₂ generation, out of the four offspring, three are tall and one is short.

According to Mendel's first law, the actual segregation helps in F₂ generation.
In F₂ generation, it can be summarized that:

1. Phenotypic ratio = 3:1 (3 for tall and 1 for short)
2. Genotypic ratio = 1:2:1(1 TT, 2 Tt and 1 tt)

Note: Genetypes of traits are represented by letters. In the aspect of complete dominance, the capital letter form of the first letter of the dominant trait is used to represent the dominant gene.

Since tallness in the plant is dominant over shortness:

1. T stands for gene for tallness.
2. TT stands for genotype of the pure breeding tall plants, homozygous for tallness.
3. t stands for gene for shortness.
4. tt stands for genotype of the pure breeding short plant, homozygous for shortness.
5. multiplication sign stands for a cross between two organisms.
6. Encircled letter, i.e (T) or (t), depending on the trait being discussed, stands for each gamete. This is according to Mendel's law of segregation of germinal units.
7. One dominant gene and one recessive gene, i.e Tt, stands for a heterozygous individual.Typical individuals are called carriers of a trait.

Recommended: Genetics: Biology of Heredity

2) DIHYBRID INHERITANCE

A series of experiments were also carried out by Mendel in which he crossed plants with two different pairs of contrasting traits e.g seed shape (round and wrinkled seeds) and seed color (yellow and green seeds). Gregor Mendel referred to the whole setup as dihybrid inheritance. 

When he crossed plants with wrinkled and green seeds with plants with round and yellow seeds, all the F₁ offspring(plants) produced round and yellow seeds. When the F₁ plants were self-pollinated, the F₂  plants produced were of four types:

1. round and green seeds
2. round and yellow seeds
3. wrinkled and yellow seeds
4. wrinkled and green seeds

All the above types were in the ratio 9:3:3:1 approximately.

Mendel's Conclusion

Mendel's conclusion after the series of experiments was that the above types could result if the contrasting traits of yellow and green seeds, and round and wrinkled seeds were inherited independent of each other.

This conclusion led to the Mendel's second law of inheritance.

MENDEL'S SECOND LAW OF INHERITANCE

Mendel's second law of inheritance is also know as the law of independent assortment of genes. 

It states that each trait behaves as a separate unit and is inherited independently of any other trait.

BACK CROSS AND TEST CROSS

Black cross is the crossing of an organism with the homozygous recessive organisms, from the original parental generation

Test cross is the crossing of an organism with the homozygous recessive organism.

The genotype of organisms showing dominant phenotype are determined with the combination of back cross and test cross. 

Categories: Biology, Biology of Heredity, Genetics