Here are your brief notes on Mendel’s Breeding Experiment

For his experiment, Mendel selected the same garden pea plant which knight and many others had studied earlier. This choice was good for several reasons:

1. Pea plants are small and easy to grow. They have relatively short generation time.

2. Flowers are bisexual and self pollinated. The sex organs are very well enclosed within the inflorescence. He could allow the flowers either for self pollination or for cross-pollination as per

3. Many varieties of pea plants showing alternative forms of characters,

(Tall plant and dwarf plant, Yellow or green seeds) were naturally available. Mendel carefully selected only seven pairs of contrasting or alternative characters.

4. Many earlier breeders had produced hybrids by crossing different varieties of pea plants with alternative characters. Hence from the very beginning Mendel expected separation of characters in the offsprings.

Working Methods:

(i) Mendel carefully carried out preliminary investigations to familiarize himself with experimental specimens.

(ii) I Ie considered only one or a few specific differences between the plants used at a time and ignored countless other differences. He only considered the differences that could be compared easily like the height of the plant or colour of the cotyledons etc.

(iii) He took special care to avoid undesirable cross- pollination.

(iv) He kept statistical records of all his findings.

(v) He collected sufficient data for analysis. Mendel usually carried out his breeding experiments in three stages:

1. Obtaining Purc-lincs:

He allowed pea plants of a given variety (say tall plants) to produce progeny by self pollination for several generations. By doing so, he ensured that the progeny produced by a tall plant arc all tall plants.

Such plants which produce similar progeny for a particular character are called pure-lines or true breeding or pure breeding for that particular character for example, a plant can be pure breeding or true breeding tall or pure-line for round seed.

2. Hybridization:

Mendel performed crosses between two varieties of plants showing contrasting or alternative forms of characters such as tall plant and dwarf plant. For this he removed male parts from the flowers of one plant which then was used as female plant.

The pollens from the other plant were then dusted on the stigmas of the female plants. After pollination all the flowers of the female plants were covered to prevent unwanted cross-pollination.

The pollen contributing plant was considered as male plant. Then he repeated his experiment by reciprocal cross where he used the pollen contributing plant as female and the other plant as male. For example, in the first case he used tall plant as male and dwarf plant as female and then in the reciprocal cross he used tall plant as female and dwarf as male. In both the crosses, he obtained the same results.

The offspring and seeds of such crosses (hybridizations) constituted the first filial generation or Fi-generation.

3. Selling:

In the third and final stage, Mendel allowed the hybrids of Fi-generation to self pollinate. The offsprings and seeds produced from the Fi-generations constituted the second filial generation or Fa-generation. The original plants used in the hybridization were denoted as Pi and P2.

Mendel’s Findings:

The alternative forms of characters studied by Mendel had only two variants those were easy to identify and score. In one set of experiments he considered only one pair of contrasting characters and ignored all other differences.

The cross in which only one pair of alternative characters is taken into consideration is known as monohybrid cross. In another set of experiment he considered the inheritance pattern of two pairs of contrasting characters. This type of cross where two pairs of alternative characters taken into consideration is known as dihybrid cross.

Monohybrid cross:

When Mendel crossed a true breeding tall plant with the dwarf plant the Fi-plants were all tall.

Then he subjected the Fi-tall plants to self pollination and in the F2- generation the tall and dwarf plants appeared in the ratio of almost 3:1. Then individual plants of F2-generation were self pollinated and F3-generation was raised. In F3 all the dwarf plants produced only dwarf plants; the dwarf plants were true breeding. Out of the tall plants two third tall plants produced tall and dwarf plants in the ratio of 3:1. Hence two third tall plants were not true breeding.

The rest one third tall plants of F2 produced only tall plants; which means that they were true breeding. The same results were obtained in reciprocal crosses and also with all the seven pairs of alternative forms of characters.

Explanation:

Mendel’s monohybrid cross suggested that, in Fi-generation out of the two alternative characters (Tall & dwarf) only one (Tall) is expressed and the other(dwarf) is masked.

The character which appeared in the Fi-generation (Tall) is called dominant and the character which was masked or suppressed (dwarf) is called reccssive. In I-‘2-generation the dominant and recessive (Tall & dwarf) appeared in the ratio of 3:1.But in the I^;3-gcncration it became clear that the F2 ratio of 3:1 is actually 1:2:1 as one third of the total population of F2 arc pure tall, two third are hybrid tall like that of Fi and rest one third are pure dwarf.

In order to keep the records of the crosses Mendel used certain symbols for each pair of alternative characters. He used English capital letters for dominant characters and small letters for recessive characters as given below:

According to Mendel a character is inherited by “elemente” or factors. Thus a diploid organism contains a pair of factors for a character. A pure breeding tall plant, therefore, contains two similar factors for tallhess as “TT’and a pure breeding dwarf contains “tt”. Such offsprings containing two similar factors for a character are called homozygous.

The hybrid tall is “Tt” and is called heterozygous tall. The morphological expression of a character is called phenotype and the make-up of the factors (now known as genes) responsible for the phenotype is called genotype. A tall phenotype can have TT or Tt genotypes. One can now represent Mendel’s monohybrid cross as follows:

Phenotypically tall: dwarf:: 3:1 ; genotypically pure tall: hybrid tall: dwarf :: 1:2:1

The two Mendelian factors of a character that a diploid organism contains arc now known as alleles

Thus a homozygous tall plant carries two similar alleles (TT) and heterozygous tall plant carries two different alleles (Tt).Basing on the observations of Mendel, the German scientist Carl Correns formulated certain principles of heredity. These are as follows:

1. Principle of dominance.

2. Principle of segregation or purity of gametes.

3. Principle of independent assortment.

Out of these three principles the first two arc based on monohybrid crosses and the last one is based on dihybrid crosses. Principle of dominance:

When two homozygous parents for two contrasting characters arc crossed a hybrid results in Pi -genera tion.This hybrid shows only one of the two alternative characters in its phenotype. Hence
in a heterozygous, having a copy each of both types of alleles, only one is able to express its phenotypic effcct.This is known as the dominant factor or dominant allele. The other factor or allele whose effect is completely masked or suppressed is known as recessive factor or allele.

The phenotypic effect of recessive allele is seen in the F2-generation when the recessive alleles occur in homozygous form. Thus characters pass from parents to offsprings in particulate forms as factors without mixing or amalgamation.