Mendel's Laws of Inheritance

Biography of Mendel

Gregor John Mendel was born in 1822 in Moravia of Czech Republic. Due to poor family status, he joined an Augustinian monastery at Brunn in Austria in 1843, where he studied maths, physics, and science and developed a great interest in plant hybridization. Then he spent his life making crosses in between plants.

He studied various characters of plants and performed experiments on Pisum sativum(common garden pea plant). With the help of his experiments on garden pea, he was able to formulate the laws and process of inheritance of characters. He published his results in "The Annual Proceedings of Natural History Society" a local natural history society.

Although Mendel described his results in 1860, no importance was given to his work until the death of Mendel in 1844. His work was recognised in 1900 when his work was discovered by Hugo de Vries (Dutch botanist), Carl Corens (German botanist), Erich von Tschermark (Austrian botanist) who then found Mendel to be correct and published Mendel's work. The rediscovery of Mendel's work was widely accepted and thus, was Mendel named 'Father of Genetics.'

Mendel's work-

Mendel did his experiment in 5 steps:

1)Selection of material

2)Selection of parent plant

3)Hybridization

4)Self-pollination of F1 hybrids

5)Mendel's results

1)Selection of material-

Mendel chose garden pea (Pisum sativum)as plant material for his experiment due to following reasons:

•Pea plant is normally self-fertilising due to closed petals. Self-fertilisation helps to produce the pure line with the constant trait.

•Pea plant is easy to cultivate and generations can be produced within a year.

•It has several pairs of contrasting characters.

•The cross-pollination and fertilisation can also be achieved easily.

•The flowers are bisexual and hermaphrodite.

•Hybrids of pea plants are fertile.

2)Selection of parent plant-

Mendel chose pure breed(homozygous) plants for his experiment. He obtained pure breed by self-pollinating pea plants for many years. Then Mendel performed 100s of crosses. Seven pairs of contrasting characters were chosen for his study.

3)Hybridization-

The process of production of hybrids by crossing two contrasting homozygous parents is called hybridization. To produce hybrids(heterozygous), monohybrid and dihybrid crosses were done by him.

Monohybrid cross is the cross in which one pair of the contrasting trait is taken. For this, he took one of the seven contrasting traits. He removed anthers, as the garden pea is self-fertilizing. The process of removal of anthers before their maturity is called emasculation.For example- He took pure tall (TT) and pure dwarf( tt) as parents.

Fig: Formation of hybird
Fig: Formation of hybrid

Dihybrid-cross os the cross in which two pairs of contrasting traits are taken. For this, he took any two traits out of 7. For example- Pure tall with red flowers (TTRR) and a pure dwarf with white flowers (ttrr) as parents. He further made reciprocal process.

4)Self-pollination of F1hybrids-

The last stage of Mendel's experiment was to let the F1 hybrids pollinate themselves. Plants thus produced were called second filial generation or F2 plants. Similarly, F3, F4 e.t.c. were also obtained.

5)Mendel's results-

Mendel concluded his experiments by giving the following results-

−All the plants in F1 generation were of the same type in all reciprocal crosses.

−Oneof the trait was seen dominant of the alternate trait.

−Two alleles were not mixed in the hybrid.

−In F2generation, both the parental forms were reappeared.

−In dihybrid crosses, the characters were found assorted independently.

−The phenotypic and genotypic ratio in F2 generation was found as follows:

Phenotypic ratio Genotypic ratio
Monohybrid cross- 3:1 1:2:1
Dihybrid cross- 9:3:3:1 1:2:1:2:4:2:1:2:1

Differences between phenotype and genotype

Phenotype Genotype
1) It is the morphological appearance of an organism. It is the genetic makeup of an organism.
2) By the help of phenotype, the genotype cannot be determined. By the help of genotype, the phenotype can be decided.
3) It remains the changed with time and environment. It remains same throughout the life of an individual.
4) It can be known by the morphological study. It cannot be studied by the morphological study.
5) Individuals usually having different phenotypes have different genotype. Individuals with different genotypes may have a similar phenotype.
6) In the monohybrid cross, the ratio of phenotype is 3:1. In the monohybrid cross, the ratio of genotype is 1:2:1.

Differences between monohybrid cross and dihybrid cross

Monohybird cross Dihybrid cross
1) The cross between homozygous parents differing in only one pair of contrasting character is called monohybrid cross. The cross between homozygous parents differing in two pairs of contrasting character is called dihybrid cross.
2) The phenotypic ratio in the F2 generation is 3:1 and the genotypic ratio is 1:2:1. The phenotypic ratio in the F2 generation is 9:3:3:1 and genotypic ratio is 1:2:1:2:4:2:1:2:1.
3) It involves cross with only one character. It involves cross with two characters.

Principles(laws) of Inheritance given by Mendel

The following principles of Inheritance were given by Mendel-

A) Law of dominance

B) Law of segregation / purity of gametes

c) Principle of independent assortment

A) Law of dominance-

Out of two contrasting allelomorphic factors, only one expresses itself in an individual. The factor that expresses itself is called dominant and the other which has not shown its effect is known as recessive character,

When homozygous tall pea plants are crossed with homozygous dwarf pea plants, the plants appeared in F1 generation are tall, although they have received a factor from the dwarf plant. However, this recessive hidden trait reappears, unchanged in F2 generation.

Following conclusions can be made for law of dominance-

»Characters are controlled by discrete units called factors(gene).

»Factors occur in a pair.

»In a dissimilar pair of factors, one member of pair dominates and the other fails to appear.

♦ Importance of law of dominance-

The phenomenon of dominance is of practical importance as the harmful recessive characters remain hidden by the normal dominant character in hybrids. In humans, a form of idiocy, diabetes, and haemophilia are recessive characters.

B) Law of segregation / purity of gametes-

The law of segregation states that when a pair of contrasting factors or alleles are brought together in a hybrid, these alleles don't blend or mix-up but simply associate themselves and remain together and separate during gamete formation.

OR

This can also be defined as-"Allele pair separate during gamete formation and the paired condition is restored by random fusion of gametes during fertilisation.

This law is also called First law of Mendel.

Fig: Representation of principle of segreration
Fig: Representation of principle of segregation

C) Principle of independent assortment-

This law states that the genes of different characters located in different pairs of chromosomes are independent of one another in their segregation during gamete formation. It means if we consider the inheritance of two or more genes at a time, then their distribution in the gametes and progeny of subsequent generation is independent of each other.

Fig: Dihybrid cross showing independent assortment
Fig: Dihybrid cross showing independent assortment

Reasons for Mendel's success-

1) His choice of plants as pea plant was an excellent choice.

2) He selected only pure varieties for his experiment.

3) Mendel kept the complete record of every cross.

4) He also used statistical methods and laws of probability for finalizing the results.

5) Mendel was fortunate also that the characters which he selected didn't show linkage, incomplete dominance, gene interaction e.t.c.

6) He took utmost care to check contaminations from foreign pollen at the time of cross-breeding.

7) He formulated theoretical explanations for interpreting his results.

8) He didn't attempt to solve all the variations of his experiments where he was not clear.

Reference

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram. Principle of biology. 2nd edition. Kathmandu: Asmita, 2068,2069.

Jorden, S.L. principle of biology. 2nd edition . Kathmandu: Asmita book Publication, 2068.2069.

  • Gregor John Mendel was born in 1822 in Moravia of Czech Republic.
  • Mendel joined an Augustinian monastery at Brunn in Austria in 1843, where he studied maths, physics, and science and developed a great interest in plant hybridization. Then he spent his life making crosses in between plants.
  • Mendel  published his results in "The Annual Proceedings of Natural History Society" a local natural history society.
  • Although Mendel described his results in 1860, no importance was given to his work until the death of Mendel in 1844. His work was recognised in 1900 when his work was discovered by Hugo de Vries (Dutch botanist), Carl Corens (German botanist), Erich von Tschermark (Austrian botanist) 
  • Mendel did his experiment in 5 steps
  • Mendel formulated 3 laws.
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deacribe the mendal's mono hybrid n dihybrid cross n their laws.

BABURAM