MENDEL LAWS OF INHERITANCE

                                                          MENDELISM

Gregor Johann Mendel, known as father of Genetics. He was the first person who represented the rule of  inheritance in 1865. He was born in a farmer family near Brunn in Austria in 22 july 1822. His father had a great  interest in plants which influence Mendel. In 1847 he become to Brunn to study for priesthood in St. Augustinian Monastery . Later he went to University of Vienna for studying  science. Mendel taught in Brunn Modern School for about 14 years. He began his Genetic experiment with garden pea (Pisum sativum) in 1856 in a garden at Monastery. the experiment took about 8 years to complete. Result of his work was published in a paper entitled “Experiments in plant hybridization” .

MENDEL’S EXPERIMENTAL MATERIAL & CHOSEN CHARACTERS:

Why Mendel choose pea plant for his experiments ?

Mendel took garden pea (Pisum sativum) as his experimental material due to certain suitable reasons:

·       It shows number of well defined contrasting characters

·       Its flowers are bisexual (easy for self fertilization)

·       The plant have shorter life span (with in short period of time study many generations)

·       Large number of offspring’s

·       Hybridization and crossing is easy in pea plant

Mendel selected seven pairs of characters for his experiments 

·       Stem height – tall and dwarf.

·       Seed shape – round and wrinkled

·       Flower position – axial and terminal

·       Seed colour – yellow and green

·       Flower colour – violet-red and white

·       Pod shape – inflated and constricted

·       Pod colour – green and yellow

MENDELS EXPERIMENT AND LAWS OF INHERITANCE

Mendel conducted many experiments on the pea plants and observed their pattern of inheritance from one generation to the next generation.

He perform both monohybrid and dihybrid  cross, observe/investigate and gave few principles, famously known as Mendel’s Laws of Inheritance.

MONOHYBRID CROSS AND LAWS DERIVED FROM IT :

A monohybrid cross is a genetic mix between two individuals who have homozygous genotypes, or genotypes that have completely dominant or completely recessive alleles.

Or

A cross made between single pair of contrasting characters.

To describe monohybrid cross, Mendel made a cross between a pair of pea plants with two contrasting traits i.e., one pure tall(TT) and another pure dwarf(tt). All the hybrid plants were tall in F₁ generation. Then he self pollinate the F₁ plants to obtain F₂ progeny. He observed all the three plants were tall and one dwarf out of four plants. The phenotypic ratio is 3(tall):1(dwarf). And the genotypic ratio is 1:2:1, one pure tall(TT), two heterozygote tall(Tt) and one pure dwarf(tt) respectively.

Principle of unit character:

Mendel assumed that the unit of hereditary characters are the factor(gene) occurs in pairs. One factor of the pair comes from male and the other factor comes from female.

Law of Dominance:

When a pair of contrasting characters(The pair of contrasting characters means a single character exhibit two opposite different character. For example if we considered height, the contrasting trait will be short and tall.) are present together in a hybrid, only one of them express in the next generation called dominant character and the other one remain suppressed or hidden called recessive character.

Mendel cross between pure tall(TT) and pure dwarf(tt) and observed that all the F₁ plants were tall(Tt) . here the tall character is dominant over the dwarf. Hence tall is the dominant character and dwarf character is the recessive character.

Law of segregation:

Law of segregation stated that when a pair of contrasting characters are present together in a hybrid, the alleles pair do not fuse with one another and segregate or separate during gametes formation and re-united randomly during fertilization in the next generation.

Here in monohybrid cross (TT x tt),  allele ‘T’ and ‘T’ do not fuse  and allele ‘t’ and ‘t’ do not fuse but segregate at the time of gamete formation. Here  ‘T’ is cross with ‘t’ and vice versa.

DIHYBRID CROSS AND LAW DERIVED FROM IT:.

A dihybrid cross is a cross between two organisms, with both being heterozygous for two different traits. The individuals in this type of trait are homozygous for a specific trait.

                                                        Or

A cross made between two pair of contrasting characters

To describe dihybrid cross Mendel  chose to cross a pea plant that was homozygous and dominant for round (RR), yellow (YY) seeds with a pea plant that was homozygous and recessive for wrinkled (rr), green (yy) seeds, represented by  RRYY(round yellow ) x rryy (wrinkled green).

After the crossing the parent plants (RRYY x rryy) He obtained only heterozygote round-yellow (the is genotype RrYy) seeds in the F1 generation. This indicated that round shape and yellow colour of seeds are dominant in nature.

Then he self pollinated the F₁ plants (RrYy) x (RrYy) to obtained the F₂ generation. In F₂ generation Mendel observed that an allele of dominant or recessive of a given character freely combines with either one of the allele of another character. Hence dominant allele of a character combines not only with the dominant but also recessive of another.

(Fig showing alleles of one gene is assorted independently with the alleles of other gene in F₂  generation)

In F₂ generation there are four different combinations of plants were observed. They were round-yellow, wrinkled-yellow,  wrinkled -green  and round-green in the phenotypic ratio of 9:3:3:1.   The genotypic ratio is 1:2:1:2:4:2:1:2:1.

Law of independent assortment:

The law state that When two  or more pairs of contrasting characters are taken into consideration in a cross, the factor or gene of each pair of allele assort independently of the other pairs

 From the dihybrid cross Mendel observed that , the alleles of two more genes get sorted into gametes independent of each other. The allele received for one gene does not influence the allele received for another gene.

                                                                             Or

The Law of Independent  Assortment states that different genes and their alleles are inherited independently within sexually reproducing organisms. During meiosis, chromosomes are separated into multiple gametes. Genes linked on a chromosome can rearrange themselves through the process of crossing-over. Therefore, each gene is inherited independently.