Laws of Mendel
Table of Contents
LAWS OF MENDEL
Introduction about Mendel
- Original name (until 1843) Johann Mendel
- Born July 22, 1822, Heinzendorf, Austria [now Hynčice, Czech Rep.]
- Died Jan. 6, 1884, Brünn, Austria-Hungary [now Brno, Czech Rep.]
- Austrian botanist, teacher, and Augustinian prelate, the first to lay the mathematical foundation of the science of genetics, in what came to be called Mendelism.
Education and early career
- Born to a family with limited means in German-speaking Silesia, Mendel was raised in a rural setting.
- His academic abilities were recognized by the local priest, who persuaded his parents to send him away to school at the age of 11.
- His Gymnasium (grammar school) studies completed in 1840, Mendel entered a two-year program in philosophy at the Philosophical Institute of the University of Olmütz (Olomouc, Czech Rep.), where he excelled in physics and mathematics, completing his studies in 1843.
- His initial years away from home were hard, because his family could not sufficiently support him.
- He tutored other students to make ends meet, and twice he suffered serious depression and had to return home to recover.
- As his father’s only son, Mendel was expected to take over the small family farm, but he preferred a different solution to his predicament (difficult situation), choosing to enter the Altbrünn monastery as a novitiate (religious learner) of the Augustinian order, where he was given the name Gregor.
LAW OF SEGREGATION:
Definition:
- Each organism contains two factors for each characteristic and the factors segregate (separate) during the formation of gametes so that each gamete contains only one of each pair of factors. OR
- If a cross is made b/w two contrasting characters, the characters will segregate in the ratio of 3:1 in the F2 generation.
- According to this law during the gametes formation the pairs of factors separate from each other, so that each gamete receives only one of the pair of the factors of the pairs. At the time of fertilization, the offspring receives one factor from the father and other forms the mother to have a complete set of factors.
Mendel’s Experiment:
- (Cross b/w tall and dwarf pea plants)
- Mendel took two true breeding of pea plants: 1. Pure tall Pure dwarf
- He called this parental generation as the P1 generation.
- He plucked the stamens form the flower of one plant and carpels from the flower of the other plant. In this way one flower became male while other as female. He wanted to prevent the self-fertilization because the pea plant is bisexual.
- He took the pollen grains from the anthers of the male flower and transferred them to the stigma of carpel of the female flower. He collected seeds which were reduced as a result of this cross and grew them in the soil.
- When the plants became mature then he observed that all the plants in this generation were tall.
- There were no plants intermediate or dwarf in size. This clearly showed that tall character was dominant while the dwarf character was recessive.
- Mendel called this generation as the first filial generation of F1.
- Mendel now allowed the F1 plants for self-fertilization. He took the seeds and grew them. When the plants became mature he observed that both types of plants have appeared in this generation. He called second filial generation or F2 to this generation.
- In F2 he found tall plants and also dwarf plants which were disappeared in F1. He counted 1064 plants in F2.
- In these plants 787 were tall and 277 were dwarf. He calculated nearly 3:1 ratio b/w tall and dwarfs.
- In order to make this study more meaningful, Mendel carried his experiments further and raised F3 generation from the seeds of F2. He found two types of tall plants in F3.
- Pure Tall:
In F3 1/3 of the F2 generation produced only tall plants. This showed that these were pure tall.
- Hybrid Tall:
In F3 1/3 of the F2 generation produced tall and dwarf plants. This showed that these were hybrid tall.
He also raised F1 generation from the seeds of dwarf plants of F2. He found that all the plants from these seeds were dwarf. This showed that these were the pure dwarf.
| GENOTYPE | PHENOTYPE |
| T T | Tall |
| T t | Tall |
| T t | Tall |
| t t | Dwarf |
LAW OF INDEPENDENT ASSORTMENT:
Definition:
When two traits (or characters) are together followed in the same cross then the distribution of the alleles for one trait into the gametes does not affect the distribution of the alleles for the other trait.
Explanation:
When there are two pairs of contrasting characters in a cross (dihybrid cross), then the genes of these contrasting characters are independent in their transmission to gametes and the gametes are free to unite with one another according to the law of chance. In other words, we can say when a pair of contrasting characters is crossed then the characters are assorted (fit into the group) independent of each other.
Examples:
Seed Shape and Seed Color in Pea Plants:
- Mendel took two pure varieties of pea plants having the following characteristics:
- Round and yellow seeds (both dominant characteristics)
- Wrinkled and green seeds (both recessive characters)
- He crossed (dihybrid cross) these plants. In F1, he found all the plants with round and yellow seeds. This clearly showed that round character was dominant over wrinkled and yellow color was dominant over green. Mendel allowed the F1 plants for self-fertilization and obtained F2 generation.
- In F2 he found two types of combinations:
Parental Combination:
- Plants with round and yellow seeds
- Plants with wrinkled and green seeds.
Both these showed parental combination.
Recombinants:
- Plants with round and green seeds
- Wrinkled and yellow seed plants.
The appearance of new characters showed that the genes for seed color and seed shape did not necessarily stay together in the combination in which they occurred in the parental generation. Instead, they assorted independently of each other.
The actual number of seed plants according to seed shape and seed color obtained by Mendel in F2 is as follows:
- Round yellow = 315
- Round green = 108
- Wrinkled yellow = 101
- Wrinkled green = 32
From these results, Mendel made the following ratio among the four types of phenotypes in F2.
9 : 3 : 3 : 1
(Round yellow) (Round green) (Wrinkled yellow) (Wrinkled green)
Graphical Representation:
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