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CLASSICAL OR MENDELLIAN GENETICS

Definition : "Genetics is the study and understanding of the phenomena of heredity and variation."

The term ’genetics’ was first coined by Bateson in 1906. In Latin, it means genesis or origination of organisms.

Heredity is the transmission of characters from one generation to the next, i.e., from parents to their offspring. Because of heredity, the offspring resemble their parents. Heredity is the essence of self-reproduction. It is owing to heredity or self-reproduction that we commonly observe the phenomenon of "like begets like", i.e., a seed of mango develops into a mango tree, or the offspring of a dog is a puppy, and that of human beings is a human being only.

Variations are the visible differences between the parents and the offspring, or between two offsprings of the same parents.

An offspring receives all the characters from its parents and yet, an offspring is never an exact copy of its parents. Similarly, no two offsprings of the same parents are identical (exception : identical twins).

In order to understand the principles of inheritance and to discover the reasons for the variations, Mendel began a systematic search during the second half of the nineteenth century. For this, Mendel experimented on garden pea plants and performed various crosses with great precision, care and objectivity. He carefully counted the plants resulting from such crosses and kept statistical records of successive generations with the accuracy of a mathematician.

Gregor Mendel

Gregor Johann Mendel was born on July 22, 1822 in Moravia, Austria. He had his early education in a monastery in Brunn, Austria (now Brno in Czechoslovakia) and later studied science and mathematics at the University of Vienna. He graduated in 1840. Mendel returned to the monastery in Brno as a monk. He worked as a teacher of physics and natural science in a Higher Secondary School of Brno during 1854 to 1868. He was appointed abbot of the monastery in 1868 and held this post untill his death.

Mendel carried out his legendary experiments on garden pea plants in the monastery garden from 1857 to 1865. He had a clear perspective and worked on the experiments with precision and thoroughness. He published his research paper containing his observations and conclusions in 1866 in the annual proceedings of the Natural History Society of Brunn. These conclusions are now known as Mendel’s Laws. This work is a classic in biology for its elegance and simplicity and ranks amongst the most outstanding biological contributions of all times.

Sixteen years after Mendel's death, in 1900, Hugo de Vries (Holland), Karl Korrens (Germany) and Von Tschermark (Austria) independently arrived at similar conclusions as those of Mendel. De Vries rediscovered the research paper of Mendel and it was published again in 1901. The experiments on heredity of plants and animals since then have confirmed that Mendel's laws of heredity are applicable to other organisms as well. They form the basis of modern genetics. Hence, Mendel is called The Father of Genetics.

Mendel’s Experiments on Sweet Pea

Selection of the material : Mendel selected garden sweet pea (Pisum sativum) for his hybridization experiments for of the following reasons : (i) Plants are annual and easy to cultivate. (ii) Peas have many distinct, well-defined and easily observable morphological characteristics (traits). (iii) Flowers are bisexual and naturally self-fertilizing, but they can also be easily cross-fertilized. (iv) The offspring of cross-fertilized plants are fertile. (v) Flowers are sufficiently large for easy emasculation (removal of stamens) and artificial cross-pollination.

Selection of characters (traits) : Mendel selected 14 different varieties of the pea and grouped them into seven pairs. Each pair was considered for a specific trait (characteristic) such as flower color or seed shape or stem length, etc. The two members of each pair showed contrasting forms of the chosen trait, ,e.g., in a pair selected for stem length, one variety had a tall stem (6-7 feet tall) while the other had a dwarf stem . These seven pairs of contrasting traits are shown in Table 1.1.

Table 1.1 : Showing seven pairs of contrasting characters selected by Mendel

Sr. No.

Characters

Contrasting pairs (Allelic pairs)

 

(Dominant)

(Recessive)

1.

Form of seed

Round (R)

Wrinkled(r)

2.

Color of cotyledons

Yellow(Y)

Green(y)

3.

Color of seed coat

Colored(C)

White(c)

4.

Form of pod

Inflated (I)

Constricted(i)

5.

Color of pod

Green(G)

Yellow(g)

6.

Position of flower

Axial(A)

Terminal(a)

7.

Height of plant (Length of stem)

Tall(T)

Dwarf(t)

Nature and procedure of the experiments : Mendel had very clear concepts of what he was doing and what requirements were necessary if he had to arrive at the conclusions accurately and successfully. He realized the necessity of: (i) using pure breeding parent plants; (ii) considering only one character at a time during the breeding experiments; (iii) always keeping the generations separate; (iv) and using statistical and mathematical principles to compute the obtained results properly.

Mendel conducted his experiments in three stages.

Stage-1 : It involved selecting a pair of parents with contrasting characters and obtaining each parent plant in pure condition ,i.e,. breeding true for the characters selected.

For example, Mendel ensured that the plant selected for round seeds produced only round seeds on self-fertilization and the plant selected for wrinkled seeds produced only wrinkled seeds . Such pure, true-breeding parents were obtained by Mendel through repeated self fertilizing, generation after generation.

Stage -2 : It involved crossing of the selected parents. Of the pair, one plant was used as the male parent and the other as the female parent. Pollen from the male was dusted on the stigma of the female parent for cross-pollination. Mendel conducted reciprocal crosses also.

For example : in one cross, the round seeded variety was used as the male parent and the wrinkled seeded variety as the female parent, while in the reciprocal cross of the same parents, the wrinkled variety was used as the male parent and the round seeded variety as the female parent. This may be represented as follows :

                                   Male Parent          Female Parent
                                    
                                    Cross - I	                Rounded seeded      Wrinkled seeded   
                                    Reciprocal Cross   Wrinkledd seeded     Rounded seeded

Such a cross between two parents representing contrasting forms of a single character is called monohybrid cross and the offspring is called a hybrid. The hybrid represents the first filial generation or F1 generation.

Stage - 3 : In the third stage, Mendel allowed each F1 hybrid to self-pollinate and produce the next, i.e., Second Filial generation or F2 generation.

Mendel conducted a similar type of hybridization experiment separately for each of the seven pairs. He meticulously maintained a complete record of the actual number of each type of offspring in every generation (i.e., data of qualitative as well as quantitative results).

Mendel observed that in each of these crosses, all the F1 hybrids resembled only one parent, while the character of the other parent was not seen in the F1 hybrid. For example, in a cross between round X wrinkled type, the F1 were all round seeds only. The wrinkled character was not seen. The character which appears in the F1 hybrid was termed dominant and the other as recessive by Mendel. Thus, in each of the seven pairs, one form is dominant and the other is recessive (See Table 1.1). The F2 progeny showed presence of both parental forms which always appeared in the ratio of 3 dominants : 1 recessive. This 3:1 F2 ratio was termed as the monohybrid ratio.

Reasons for Mendel’s success:

i. Mendel concentrated on the results of one trait at a time.
ii. When the behavior of one trait was established, only then he considered two characters together.
iii. He conducted a large number of crosses (2000-3000) to eliminate the chance factor and to obtain a valid and accurate explanation
iv. Most of all, he actually counted the number of offsprings of each category and maintained accurate records for each generation in each experiment.
v. However, luck played a major role in his success (though Mendel did not know this fact) in the selection of pea plants as well as in the selection of those particular seven pairs of contrasting characters. Because, luckily for Mendel, in each pair, one form of the character is completely dominant over the other. Moreover, these seven characters are present on seven different chromosomes in the pea plant. However, Mendel was unaware of this fact.

Terminology Used

  1. Factor : A particle or unit in the organism which is responsible for the inheritance and expression of a particular character.
  2. Gene : Mendel’s factor is now known as gene. A gene is a particular segment of a DNA molecule which determines the inheritance and expression of a particular character.
  3. Alleles or Allelomorphs : Two or more alternative forms of a gene are called alleles or allelomorphs. For example in pea, the gene for producing seed shape may occur in two alternative forms: round (R) and wrinkled (r). Round and wrinkled forms of the gene are alleles of each other. Alleles occupy same locus on homologous chromosomes.
  4. Dominant : Of the two alternating forms (allomorphs) of a trait, the one which appears in the F1 hybrid is called the dominant trait (Dominant Allele).
  5. Recessive : Of the two alternating allomorphs of a trait, one which is suppressed (does not appear) in the F1 hybrid is called the recessive trait (recessive allele).
  6. Genotype : The genetic make-up or genic constitution of an individual (which he/she inherits from the parents ) is called the genotype, e.g., the genotype of pure round seeded parent will be RR.
  7. Phenotype : The external (morphological) appearance of an individual for any trait or traits is called the phenotype, e.g. for seeds, round shape or wrinkled shape is the phenotype.
  8. Homozygous : An individual possessing (receiving from parents) identical alleles for a trait is said to be homozygous or pure for that trait, e.g. plant with RR alleles is homozygous for the seed shape. A homozygous always breeds true for that trait.
  9. Heterozygous : An individual receiving dissimilar alleles for a trait is said to be heterozygous or impure for that trait, e.g. a plant with Rr alleles is heterozygous for the seed shape. Heterozygous is also called a hybrid.
  10. Parent generations : The parents used for the first cross represent the parent (or P1) generation.
  11. F1 generation : The progeny produced from a cross between two parents (P1) is called First Filial or F1 generation.
  12. F2 generation : The progeny resulting from self hybridization or inbreeding of F1 individuals is called Second Filial or F2 generation.
  13. Monohybrid cross : The cross between two parents differing in a single pair of contrasting characters is called monohybrid cross and the F1offspring as the hybrid(heterozygous for one trait only).
  14. Monohybrid ratio : The phenotypic ratio of 3 dominants : 1 recessive obtained in the F2 generation from the monohybrid cross is called monohybrid ratio.
  15. Dihybrid cross : The cross between two parents in which two pairs of contrasting characters are studied simultaneously for the inheritance pattern. The F1 offspring is described as dihybrid or double heterozygous (i.e. with dissimilar alleles for two characters).
  16. Dihybrid ratio : The phenotypic ratio obtained in the F2 generation from a dihybrid cross is called dihybrid ratio. In Mendelian experiments, this ratio is 9:3:3:1.
  17. Homologues or Homologous chromosomes : The morphologically similar looking chromosomes in a diploid cell (one chromosome coming from the male parent and the other from the female parent) are called homologous chromosomes. They have identical gene loci bearing alleles.