NEET Biology Principle Of Inheritance And Variation Mendelism

Mendelism

Introduction: Gregor Johann Mendel (1822-1884) first "geneticist", also known as father of genetics was born in 1822 in Silisian, a village in Heizendorf (Austria). In 1843, he joined Augustinian monastry at Brunn (then in Austria, now Brno Czechoslovakia). In 1856, Mendel got interested in breeding of Garden pea (Pisum sativum). He selected pure breeding varieties or pure lines of pea. Breeding experiments were performed between 1859 - 1864. The results were read out in two meetings of Natural History Society of Brunn in 1865 and published in 1866 in "Proceedings of Brunn Natural History Society" under the topic "Experiments in Plant Hybridisation". Mendel died in 1884 without getting any recognition during his lifetime.

In 1900, Hugo de Vries of Holland, Carl Correns of Germany and Erich von Tshermak of Austria came to the same findings as were got by Mendel. Hugo de Vries found the paper of Mendel and got it reprinted in ‘Flora’ in 1901. Correns converted two of the generalisations of Mendel into two laws of heredity. These are law of segregation and law of independent assortment.

(1) Reasons for Mendel’s success: The reasons of his success can be discussed as follows:

(i) Method of working: He maintained the statistical records of all the experiments and analysed them. He selected genetically pure (pure breed line) and purity was tested by self-crossing the progeny for several generations.

(ii) Selection of material: Mendel selected garden pea as his experimental material because it has the following advantages.

It was an annual plant. Its short life-cycle made it possible to study several generations within a short period and has perfect bisexual flowers containing both male and female parts. The flowers are predominantly self-pollinating because of self-fertilization, plants are homozygous. It is, therefore, easy to get pure lines for several generations and also easy to cross because pollens from one plant can be introduced to the stigma of another plant by removing anthers (emasculation) and bagging. In addition to that there was one reason more for his success. He studied seven pairs of characters which were present on four different pairs of chromosomes.

(iii) Selection of traits: Mendel selected seven pairs of contrasting characters as listed in the table. Luckily all were related as dominant and recessive.

(2) Mendel's experiments

(i) Monohybrid cross: Experiments with garden pea for single pair of contrasting characters.

(a) Procedure: Mendel crossed pure tall and dwarf plants. The plants belonged to \[{{\mathbf{F}}_{\mathbf{1}}}\] generation all tall were self-pollinated. The plants of \[{{\mathbf{F}}_{\mathbf{2}}}\] generation were both tall and dwarf, in approximate \[3:1\] ratio phenotypically and \[1:2:1\] genotypically. On, self-pollination, the tall plants of \[{{\mathbf{F}}_{\mathbf{2}}}\]only 1/3-rd breed true for tallness, the rest 2/3-rd produced tall and dwarf in the ratio of \[3:1\] (\[{{\mathbf{F}}_{\mathbf{3}}}\]generation). It means \[{{\mathbf{F}}_{\mathbf{2}}}\]generation consisted of three types of plants (instead of apparent two types) -

Tall homozygous (Pure)   

1          25%      TT

Tall heterozygous (Hybrid)

2          50%      Tt

Dwarf homozygous (Pure)

1          25%      tt

Hence it is to be said that in \[{{\mathbf{F}}_{\mathbf{2}}}\] generation 50% plants passes parental combination while 50% are new combination.

(b) Mendel’s explanation: Mendel explained above results by presuming that Tallness and darkness are determined by a pair of contrasting factors or determiners (now these are called genes). A plant is tall because it possesses determiners for tallness (represented by T) and a plant is dwarf because it has determiners for darkness (represented by t). These determiners occur in pairs and are received one from either parent. On the basis of this behaviour the tallness is described as dominant character and dwarfness as recessive (law of dominance). The determiners are never contaminated. When gametes are formed, these unit factors segregate so that each gamete gets only one of the two alternative factors. When \[{{F}_{1}}\] hybrids (Tt) are self-pollinated the two entities separate out and unite independently producing tall and dwarf plants (law of segregation).

(ii) Dihybrid cross (Crosses involving two pairs of contrasting characters)

(a) Procedure: Later on Mendel conducted experiments to study the segregation and transmission of two pairs of contrasting characters at a time. Mendel found that a cross between round yellow and wrinkled green seeds \[({{P}_{1}})\] produced only round and yellow seeds in \[{{F}_{1}}\] generation, but in \[{{F}_{2}}\] four types of combinations were observed. These are

Round yellow 9 Parental combinations

Round green 3 Non-parental combinations

Wrinkled yellow 3 Non-parental combination

Wrinkled green 1 Parental combination

Thus the offsprings of F₂ generation were produced in the ratio of 9 : 3 : 3 : 1 phenotypically and 1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1 genotypically. This ratio is called dihybrid ratio.

The results can be represented as follows:

Mendel represented round character of seed by R and wrinkled by r. Similarly he designated the yellow character by Y and green by y. Therefore, it was a cross between RRYY and rryy.

(b) Mendel’s explanation : Mendel explained the results by assuming that the round and yellow characters are dominant over wrinkled and green so that all the \[{{F}_{1}}\] offsprings are round yellow. In \[{{F}_{2}}\]-generation since all the four characters were assorted out independent of the others, he said that a pair of alternating or contrasting characters behave independently of the other pair i.e., seed colour is independent of seed coat.

Therefore, at the time of gamete formation genes for round or wrinkled character of seed coat assorted out independently of the yellow or green colour of the seed. As a result four types of gametes with two old and two new combinations i.e., RY, ry Ry, rY are formed from the \[{{F}_{1}}\] hybrid. These four types of gametes on random mating produced four types of offsprings in the ratio of 9:3:3:1 in \[{{F}_{2}}\] generation (Law of Independent Assortment).

(iii) Trihybrid cross : The offsprings shows 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1 ratio is found in trihybrid cross. This suggests that a di, tri, or polyhybrid cross is actually a combination of respectively two, three or more monohybrid crosses operating together.

(iv) Mendel's laws of inheritance : Mendel's law are still true because these take place in sexually reproducing organisms or parents are of pure breeding. He enunciated two major laws of inheritance i.e., law of segregation and law of independent assortment.

(a) Law of segregation (Purity of gametes): The law of segregation states that when a pair of contrasting factors or genes or allelomorphs are brought together in a heterozygote (hybrid) the two members of the allelic pair remain together without being contaminated and when gametes are formed from the hybrid, the two separate out from each other and only one enters each gamete as seen in monohybrid and dihybrid cross. That is why the law of segregation is also described as law of purity of gametes.

(b) Law of independent assortment: If the inheritance of more than one pair of characters (two pairs or more) is studied simultaneously, the factors or genes for each pair of characters assort out independently of the other pairs. Mendel formulated this law from the results of a dihybrid cross.

Important Tips

• Cytogenetics is Integrated study of cytology and genetics to find cytological basis for various events of genetics. This term was coined by Muller.
• J. Hammerlings proved that nucleus controls the heredity by a experiment on acetabularia (A unicellular green algae)
• Only sexually derived organisms are called offspring or siblings (offsprings at different births) e.g., brother and sister.
• Variations due to environment are known as ecophenotypes.
• Every test cross is back cross but every back cross is not a test cross.
• Back cross is used by breeders as a rapid method of making homozygous.
• When the two genetic loci produce identical phenotypes in cis and trans position they are considered to be pseudoalleles.
• Somaclonal variations are produced in tissue culture during differentiation of callus.
• In thalassemia, the b chain of haemoglobin is changed due to frame shift mutation as a result, bone marrow is not formed.
• Bateson coined the term Genetics, allele, F₁, F₂, homozygous heterozygous and epistasis. He is also known as father of animal genetics.
• The basis of genetic counseling is mendelism.
• In heredity, the genes are obtained from father and mother.
• Nucleus and chromosomes are stained by hematoxylin.
• Johannsen coined the term genotype, phenotype, pure line.
• The term hybrid vigour (heterosis) given by
• In mitosis, the daughter cells resemble each other and also the parent cell, in meiosis they differ not only from parent cell in having half the number of chromosomes, but also differ among themselves qualitatively in genetic constitution due to crossing over, independent assortment and segregation.
• Mendelian genetics is also called as forward genetics.
• Mendel either avoided the result or could not conduct independent assortment between pod form and stem length.
• Mendel also observed that flower colour and colour of the seed coat may not assort independently.
• Mendel failed to produce same results of his experiments of pea in Hawkweed (Hieracium) and Beans.
• Mendel's typical monohybrid phenotypic ratio was 3 : 1 which was in reality a hidden 1 : 2 : 1 ratio of genotypes.
• Mendelism gave well-defined principles even in early stage in compare to Darwinism.
• Mendel did not recognize the linkage phenomenon in his experiments because characters, he studied were located on different chromosomes.
• Mendelian factor are separated during Anaphase-I in Meiosis-I.
• If Mendel had studied the seven traits using a plant with 12 chromosomes instead of 14, He would not have discovered the law of independent assortment.
• Law of filial regression was postulated by Galton.
• Mendel didn't imagine of linkage.
• Mendel in his experiments on pea considered quantity in relation to quality.
• In bound seeds (RR/Rr) starch branching enzyme (SBE -1) is found but it is absent in wrinkled seeds or in rr seeds.
• It was thought previously that seven traits in pea studied by Mendel were located on seven different chromosomes but recent studies proves that these are on four chromosomes.
• The genes for seed form in pea was present on chromosomes no. 7.
• Independent assortment is shown by the allels present on different loci.