HISTORY OF MENDEL

History of Mendel:

    Gregor Mendel was a monk in Australia. He selected common garden pea (Pisum sativum) for research work in the year 1856- 1864. He presented several principles of inheritance. For his valuable work in genetics, Mandel is regarded as the 'Father of genetics'.

    Genotype can determine phenotype.

Why Mendel selected pea plant for his experiment work?

ANSWER

     Mendel selected pea plant for his research study. As pea has the following features. Alleles can be dominant T or receive t.

1. Pea has a short life cycle of 3 months.

2. It can be grown in a limited area.

3. Pea plants have distinct tall and dwarf plants, have red and white colour flowers.

4.Each pea plant contains both male and female reproductive parts.

5. It is naturally self-pollinated but can easily be cross pollinated.

6. It produces a large number of seeds which can be used for statistical study.

Seven characters studied by Mendel in pea plants:

    Mendel studied seven contrasting traits in pea plant. The following chart clearly shows the dominant and recessive characters. He called "P" (parental) to the original true breeding (homozygous). The first set of offspring is represented by F-1 generation (first filial' from the Latin word filius meaning Son).

     Mendel studied the inheritance of seed shape. He crossed a true breading round v seeded plant with a true breeding wrinkle seeded plant. All seeds in F, generation were found to be round. Mendel declared the trait 'round seed dominant while wrinkled seed as recessive.

Accidental or sudden changes in DNA produce mutations.

    Characters.     Dominant.      Recessive 

1  Seed shape .          Round.           Wrinkled

2. Seed flower.           Yellow.           Green

3. Flower color.         Violet.            White

4. Pod shape.              Full.               Constricted

5. Pod color.               Green             yellow

6. Flower position.     Axial.            Terminal

7. Stem height.           Tall.                Dwarf

LAW OF DOMINANCE

Law of dominance:

    According to this law "at the time of fertilization the offspring receives one factor from the father and the other factor from the mother. In the presence of dominant allele the recessive allele is not expressed". This is known as law of dominance.

     The dominant factor masks (hides) the recessive factor and that is not expressed, being recessive.

LAW OF SEGREGATION

Law of HISTORY OF MENDEL (monohybrid cross):

     A cross in which only one trait is studied at a time is called monohybrid cross.

     In each organism (2N), the genes are present in pairs that are in the form of two alleles. But during gamete (IN) formation, the two alleles separate (segregate) from each other. Each gamete has one of the two alleles. It is called law of segregation (separation). When the male gamete (1N) unites with a female gamete (1N), the resulting offspring again gets 2 alleles in the gene (2N). Two alleles remain together in one generation (F) but segregate (separate) in the next generation (F2). Gametes have only one gene for any characteristics.

Example:

     Mendel crossed a true breeding tal pea plant with a true breading dwarf plant. He named them P-l (first parents). All the offsprings of F1 generation were tall plants. The tall character which appeared in F1 generation was dominant, the other character dwarf was recessive.

     When the F1 offsprings were self-crossed, the F2 generation was obtained with 75% dominant trait and 25% recessive trait, This 3:1 ratio of dominant to recessive remained consistent in all of the F2 offsprings. This ratio is called monohybrid ratio.

Monohybrid ratio

Monohybrid ratio:

     The phenotypic ratio in the F2 generation of the monohybrid cross is called the monohybrid ratio. In the Mendelian monohybrid experiment, this ratio was always 3:l (75% is dominant and 25% is recessive).

    Mendel selected true breeding homozygous parents showing contrdsting characters for the height of the plant. The result obtained is shown in the diagram.

MENDEL OF OF INDEPENDENT ASSORTMENT

Mendel law of independent assortment (Dihybrid cross):

     Mendel at this time, decided to examine the two contrasting traits at the same time. Such crosses are called dihybrid crosses. According to the law of independent assortment, "different traits are inherited independently of one another. It means alleles of one gene pair segregate (separate) independent from the alleles of another gene pair. Mendel crossed true-breeding plants having round yellow seeds (RRYY) with plants having wrinkled green seed (rryy).

i) In Fi generation, all the seeds were round and yellow.

ii) When Fi seeds grow into plants. This generation was self crossed.

iii) Four different types of seeds were obtained. The ratio of these were

a) Round yellow seeds = 315

b) Round green seeds = 108

c)Wrinkled yellow seeds 101

d) Wrinkled green seeds = 32

The ratio of these phenotypes was 9:3:3:1

● 9/16 round and yellow.

● 3/16 wrinkled and yellow.

● 3/16 round and green.

● 1/16 wrinkled and green.

(9:3:3:1) phenotype

     From above crosses Mendel explained that the two trains (i) seed shape and (ii) seed colour are independent of each other. Thus he made his law of independent assortment which states, the alleles of a gene pair segregate (get separated) independently from the alleles of other gene pairs". That is alleles separate independently  during gamete formation.