MENDEL’S WORK IN GENETICS

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

 

Mendel’s Experiments

Gregor Mendel carried out several experiments on how hereditary characters are transmitted from generation to generation. He worked with garden pea (Pisum sativum). His major aim was to find out the pattern of inheritance of different characteristics on the pea plant.

 

Methods used by Gregor Mendel in his experiment

Gregor Mendel used two major methods in conducting his experiments. These methods were grouped into monohybrid inheritance and dihybrid inheritance.

 

Reasons for Mendel’s Choice of Pea Plant

Gregor Mendel decided to use the pea plant for his experiment because of the following reasons:

1. Peas are usually self-pollinating and he could pollinate them by himself.
2. They have a very short life span because they are annual plants.
3. The pea plant was known to have several unique characteristics which exist in contrasting pairs such as:

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

 

Methods Used by Mendel in His Experiment

Gregor Mendel used two major methods in conducting his experiments. These methods were grouped into monohybrid inheritance and dihybrid inheritance.

 

Monohybrid Inheritance

Mendel used artificial method to cross two different plants at a time, which differed in one pair of contrasting characters, e.g. tall and short plants. This procedure was called a monohybrid inheritance and it was an example of complete dominance.

He carried out the experiment in the following order:

1. He planted tall plants for several generations and discovered that the plants produced were all tall plants. In the same way, he planted short plants for several generations and discovered that the plants produced were all short.
2. He proceeded to plant tall plants and short plants. By the time the flowers were produced, he collected the pollen grains of the tall plants tagged the male and pollinated the stigma of the short plant tagged the female. He also collected the pollen grains of the short plant and place them on the stigma of the tall plant. Mendel then covered the artificially pollinated flowers with small paper bags to prevent the chance of natural pollination by insects.
3. Mendel once again picked the seeds formed after the cross. When he planted the seeds, the plants obtained were all tall plants. These he referred to as the first filial generation or F1.
4. Mendel then crossed the F1 plants, collected their seeds and sowed them. The plants he got from these were tall and short plants in a ratio of 3:1 respectively. He then called this stage the second filial generation or F2.

 

The outcome of this experiment led to Mendel’s first law of inheritance.

 

Mendel’s First Law of Inheritance

This first law is also called the law of segregation of genes. The law states that genes are responsible for the development of the individual and that they are independently transmitted from one generation to another without undergoing any alteration.

 

Explanation

All the offspring in the F1 generation are all tall. It shows that the genes for tallness (TT) is dominant over the recessive genes (tt).

In the F2 generation, three of the offspring are tall while only one is short (tt).

From Mendel’s first law of segregation of genes, the actual segregation occurs in the F2, generation. The phenotypic and genotypic ratios in F, generation can be summarised as follows:

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

Note: Letters are used to represent the genotypes of the traits. In the case of complete dominance, the capital letter form of the first letter of the dominant trait is used to denote the dominant gene. The small letter form of it is used to represent the recessive gene.

Since tallness in the plant is dominant over shortness,

1. T represents gene for tallness.
2. TT represents genotype of the pure breeding tall plants. Such a plant is described as homozygous for tallness.
3. t represents gene for shortness.
4. tt represents genotype of the pure breeding short plant, homozygous for shortness.
5. A cross between two organisms is shown by a multiplication sign x.
6. Each gamete is represented by only one encircled letter, i.e. (T) or (t) depending on the trait being discussed. This is in compliance with Mendel’s law of segregation of germinal units.
7. A heterozygous individual is represented by one dominant gene and one recessive gene, i.e; Tt. Such individuals are called carriers of a trait.

 

Dihybrid Inheritance

Gregor Mendel also carried out several experiments in which he crossed plants which differed in two pairs of contrasting characteristics such as seed shape (round and wrinkled seeds) and seed colour (yellow and green seeds). Mendel therefore called the whole set up as dihybrid inheritance because two pairs of contrasting characters are involved.

When Mendel crossed plants which had round and yellow seeds with those which had wrinkled and green seeds, all the F1 plants produced round and yellow seeds. However, when the F, plants were self pollinated, the F, plants were of four types:

• plant that produced round and yellow seeds,
• wrinkled and yellow seeds,
• round and green seeds,
• wrinkled and green seeds.

All these were in the ratio of approximately 9:3:3:1.

Mendel then concluded that this could result if the contrasting characteristics of round and wrinkled seeds and the contrasting characteristics of yellow and green seeds were inherited independent of each other.

The outcome of this experiment led to Mendel’s second law of inheritance.

 

Mendel’s Second Law of Inheritance

This second law is also called the law of independent assortment of genes. Mendel’s second law of independent assortment of genes states that each character behaves as a separate unit and is inherited independently of any other character.

Mendel’s work can be represented by letters and their explanations as below: Parents Round yellow x wrinkled green.

The four phenotypes which appear in the ratio 9:3:3:1 are as follows:

• 9 round yellow r1, 2,3,4,5,7,9,10,13
• 3 round green r 6,8,14
• 3 wrinkled yellow r 11, 12, 15
• 1 wrinkled green r 16

The 9 genotypes which include 4 homozygous and 5 heterozygous conditions are:

• 1 is homozygous for both round and yellow (1).
• 1 is homozygous for both round and green (6).
• 1 is homozygous for both wrinkled and green (16).
• 1 is homozygous for both wrinkled and yellow (11).
• 2 are homozygous for round and heterozygous for yellow (2, 5).
• 2 are heterozygous for round and homozygous for yellow (3,9).
• 2 are heterozygous for round and homozygous for green (8, 14).
• 2 are homozygous for wrinkled and heterozygous for yellow (12, 15).