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Wednesday, 6 June 2012

Medelian Genetics:Genetic Permutation Combination of Pea and Flowers

Gregor Johann Mendel

Gregor Johann Mendel
(born 22nd July 1822, died 6th January 1884)

Born to well off peasants, he entered into the 'Koniginklosser', an Augustinian foundation in Altbrunn in 1843 and was ordained a priest in 1847. He traveled to Vienna where he studied physics and natural science from 1851-1853. On returning to the cloister he taught in the Realschule in Brunn where he later became Abbot. It was in 1856 and the following years that he undertook his investigations into hybridization of plants in the garden of the convent.

Through the careful study of experimental crosses between pure breeding pea lines varying for different characters he was able to establish the mathematical basis of the inheritance of characters.

Gregor Johann Mendel

He published his work in the seminal paper of 1865  but, due to the relative obscurity of the journal, the work remained unnoticed for over three decades. Plant hybridization was an extremely active area of scientific and horticultural endeavor across Europe and North America at this time well illustrated by the International Conference of Hybridization staged by the RHS in July 1899 . In the following year De Vries and others, who were close to answers themselves, came across Mendel's work and word reached William Bateson who reported on the paper during a presentation to the Royal Horticultural Society in May of that year. Bateson was commissioned to translate Mendel's paper which appears the following year

Why Mendel Use Pea:

Picture of Different types of Pea which are crossed By Mendel

Mendel was clearly well aware that there were certain preconditions that had to be carefully established before commencing investigations into the inheritance of characters. Firstly the parental plants must known to possess constant and differentiating characters. He undertook a preliminary trial a year in advance of the crosses to establish that the lines were indeed true breeding and the characters constant. Secondly, the flowers of the hybrid plants should be protected from any foreign pollen. The special shape of the flower of species of the Leguminosae with their enclosed styles drew his attention and on trying several, finally selected pea as his species of choice.

Picture of White and Purple Colored Flower

Some Definitions For Mendelian Cross:
Pair of Chromosomes

Genes are units of instructions for producing or  influencing a specific trait in offspring.

Each gene has its own locus, or particular location on a chromosome.

Although the two genes of a pair deal with the same trait, they may vary in their information about it.This happens when they differ inform, as when a gene for flower color specifies "red" and a different molecular form of that gene specifies "white" . The various molecular form of a gene are called alleles.

The gene shuffling during meiosis and fertilization can put together different mixes of alleles in offspring.Thus a pair of alleles at homozygous loci might be the same molecular form or different from each other.If they are the same, this is the  homozygous condition; if different, this is a heterozygous condition.

Often one allele is "Dominant" meaning its effect on a trait actually masks the effect of a "Recessive" allele that occupies the homologous locus.We normally use capital letters to indicate dominance and lowercase letters to indicate recessiveness(for example A and a).

To keep the distinction clear between genes and the traits they specify, we use the word Genotype for the sum total of an individual`s genes(or even for one pair at a time). And we use Phenotype for an individual`s traits that is the observable aspects of its structure, physiology, and behavior.

Diploid organisms inherit a pair of genes for each trait (On a pair of homologous chromosomes).The two genes segregate from each other during meiosis, so that each gamete formed will end up with one or other gene, but or both.

Purple Colored Flower (Dominant Trait) (Aa)

White-flowered (aa) Recessive Trait
Mendel gained support for his concept of segregation through the testcross, in which first generation hybrids are cross to an individual known to be true breeding for the same recessive trait as the recessive parent.
         In one case, purple-flowered F1 plants were back-crossed with true-breeding, white flowered plants.If Mendel`s idea were not correct then only the dominent form of the trait  would show up in the test-cross offspring.If his idea were correct,though, there would have to be about as many recessive as dominant plants in the offspring from the testcross:

 Is F1 plant:                             Aa                                                                      aa

Gametes:                           A          a                                                             a             a

Offspring:                             1/2 Aa                                                                    1/2 aa

This is exactly what happened in the testcrosses. As predicted, about half the testcross offspring were purple-flowered(Aa) and half were white-flowered (aa).

Each gene pair tends to assort into gametes independently of other gene pairs located on non homologous chromosomes.

 Let`s now turn to Mendel`s first series of experiments, which we would call "Monohybrid crosses" in this type of cross, two parents that are true-breeding for constrasting forms of a single trait give rise to heterozygous offspring.Mendel studied 7 different traits, one at a time.
2nd Generation of Seven Types of Monohybrid Crosses on Basis of 1) Seed shape, color 2) Pod Shape and color 3) Flower color and Position 4) Stem Length, Where Dominant and Recessive trait were expressed in 3:1 Ratio

          In one monohybrid cross, a purple-flowered plant and a white-flowered plant yielded offspring with purple flowers only.What happened to the white-flowered trait? had it disappeared? may be not.Mendel allowed the purple flower offspring to self fertilize and produce seeds,rather than crossing them with new plants. Later, some plants grown from the seeds had white flowers ; the white flower trait had not been lost, after all.

Cross Between Green and Yellow Pea
          Results were much the same for all of Mendel`s monohybrid cross.One form of the trait seemed to disappear in the hybrid offspring of the first generation plants, only to show up again in some of the second-generation plants.
Cross Between Smooth and Wrinkled Yellow Pea

           To explain those results,Mendel assumed that each plant had two "unblended" units of hereditary material for the trait.When it came time to produce sperm, the two unit somehow were moved apart and ended up in separate gametes.

Cross Between Smooth and Wrinkled green Pea Pod

Mendel reasoned the one parent had two "dominant" unit and other had two "recessive" unit why? The hybrid offspring which received one unit from each parent had purple flower: the unit specifying purple was being expressed, but the one specifying white was not.

Cross Between Green and Yellow Pea Pod

           Let`s recast Mendel`s conclusion in light of what we know about meiosis and fertilization.Pea plants are diploid with pair of homologous chromosome.Assume one parent is homozygous dominant (AA) for flower color and the other, homozygous recessive(aa) .Following meiotic cell division, a chromosome carrying the gene for flower colour will be present in each gamete.When gametes from the parents combine at fertilization.Only one outcome is possible: A+a = Aa. With homozygous parents, all gametes will have the same genotype. With heterozygous parents, they can only have one of two genotypes.

 2nd Generation of Seven Types of Monohybrid Crosses on Basis of 1) Seed shape, color 2) Pod Shape and color 3) Flower color and Position 4) Stem Length, Where Dominant and Recessive trait were expressed in 3:1 Ratio respectively.

Cross Between Purple Colored Tall Plant and White Colored Dwarf Plant (Dihybrid Cross)

In another series of experiments, Mendel crossed true breeding pea-plants having contrasting forms of two traits. Here is one such cross:

                  Purple-flowered,                                                White-flowered,   
                  Tall plant                                     *                      Dwarf plant

we would call this a "Dihybrid cross" because the heterozygous offspring inherit two gene pairs, neither of which consists of identical alleles. We can show this by letting A and B stand for dominance in flower color and height, with a and b standing for recessiveness: Mendel anticipated that all the F1 offspring would be purple-flowering and tall. But what would happen when it was time for those AaBb offspring to form sperm and eggs of their own? Would a gene for flower color and gene for height travel together or indipendently of each other, then four allelic combinations in gametes were possible:

All Purple colored tall Plant with Heterozygus type of plant produced in F1 Generation

   1/4 AB                    1/4 Ab                      1/4 aB                  1/4 ab

This is indeed what happens in pea plants. As we now know, a gene pair tends to segregate into gametes independently of other gene pairs- when the others are located on non homologous chromosomes. Remember the random alignment of maternal and paternal chromosomes during metaphase 1 of meiosis ? by  depicting just two of the pea plant`s seven pairs of chromosomes, we can readily see how the four different combinations of alleles can arise.
             Now think of the variation that can arise at fertilization.Simple multiplication shows that 16 allelic combinations are possible in hybrid F2 plants. When we add up the possible combinations we get 9/16 tall purple-flowered, 3/16 dwarf purple-flowered, 3/16 tall white flowered, and 1/16 dwarf white-flowered plants.That is a phenotypic ratio of 9:3:3:1.
            The results were close to a 9:3:3:1 ratio in all of Mendel`s dihybrid crosses. Results of this sort led to the formulation of another principle:
Four Different types of plant produced in 9:3:3:1 Ratio in F2 generation.

Summery of Mendel`s Di Hybrid Cross

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