Mendelism- Reasons for success, Characters selected by Mendel and Mendels laws

                                                                 MENDELISM

The contribution of Mendel to Genetics is called Mendelism. It includes all concepts brought out by Mendel through his original research, on plant hybridization. Genetic laws proposed by Mendel are universal to most genes in microbes, plants and animals. Mendelian genetic concepts are basics to modern geneticists. Therefore, Mendel is called the Father of Genetics.

 

Mendel:

Mendel is the Father of Genetics. He was born in a peasant family in 1822 in Austria. He worked as a teacher. He passed his later life as an abbot. He died in 1884.

Mendel was fond of gardening and fruit-culture from his boy hood. When he was working as teacher, he performed a series of experiments with pea plants in the garden. His work contains inheritance of characters in 22 varieties of garden peas. His papers were published in 1866 and 1867 in the proceedings of the Natural History Society of Brunn.

The work out of Mendel remained unnoticed to the world for 33 years. In 1900, the principles of Genetics worked out by Mendel were rediscovered by three botanists, namely Correns, De Vries and Tshermark. The unrecognized papers of Mendel were taken out from the grave and made known to the scientific world. When Mendel's work was recognized and appreciated, he was no more.

 

Reasons for Mendel's Success:

 Mendel did his work by collecting several varieties of garden pea Pisum sativum from salesmen and studied the differences among them. Then he did hybridization experiments with these plants differing in certain characters. The secret of Mendel's success laid in his wise character selection.

The following are the reasons for the success of Mendel:

 1. The flowers of pea plants are normally self-fertilized.

2. The pea plant shows a number of clear-cut contrasting characters.

3. The hybrids of garden pea are perfectly fertile.

4.Cross pollination is not very difficult in pea plants.

5. Artificial fertilization is almost always successful.

6. The genes for the seven pairs of characters are located seven separate homologous pairs of chromosomes.

7. Many pure breeding varieties are available for the experiments.

8. It is very easy to cultivate the pea plants in open ground.

9. They have a short growth period and a short life cycle.

10. Mendel studied the inheritance of only one character at time. This made the complex problem simple.

11. He maintained statistical records of the results. It helped Mendel to derive numerical ratios of significance.

Characters Selected by Mendel:

 The pea plant contains a number of contrasting characters. of these contrasting characters Mendel selected only seven characters. Each of these seven characters has two varieties or alternatives. The seven characters and their contrasting alternatives are given below.

Mendel's Laws:

Based on Mendel's experimental results certain principles are framed. These principles are called Mendel's laws. They are as follows:

 1. Law of dominance.

2. Law of segregation or law of purity of gametes.

3. Law of independent assortment.

 

1. Law of Dominance:

Each organism is formed of a bundle of characters and each character is controlled by a pair of factors or genes (T or t). Each of the paired factors (Tr) is responsible for the expression of a particular character (height). Mendel's law of dominance states that one factor in a pair may mask or prevent the expression of the other. He called the character that appeared in the F, generation of his monohybrid cross as dominant and those which did not appear in the F, generation as recessive. A recessive factor freely expresses itself in the absence of its dominant allele. This law is formulated based on the monohybrid experiment.

2. Law of Segregation:

Each organism is formed of a bundle of characters. Each cha acter is controlled by a pair of genes. The two genes of a particular character remain uncontaminated when they are inside the organism During gamete formation, the paired genes segregate and enter different gametes. Hence each gamete contains only one of the paired gene which are responsible for a particular character. During gamete formation the genes of a particular character separate and enter different gametes. This is the law of segregation. This law is also called law of purity of gametes. This law is also formulated base on monohybrid experiment.

Example: During gamete formation the paired factors (T present in the F, plant segregate independently and enter different gametes. So each gamete receives either Tort from the paired fac tors Tr, which are responsible for the expression of a single character

 3. Law of Independent Assortment:

This law is based on dihybrid experiment. According to this law, the genes for each pair of characters separate independently from those of other characters during gamete formation.

Example: During gamete formation of a dihybrid cross, the independent assortment of genes from a dihybrid. factors for yellow colour assort out independently of the factors for round shape. The gene Y may combine with the dominant gene. R& the recessive gene r of the other character and enter a gamete. In the same way, the gene y may combine with the dominant gene R or the recessive gene r and enter a gamete. So the F, dihybrid plants produce four types of gametes and they are YR, Yr, yR and yr.