INTRODUCTION:

Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets relating to the transmission of hereditary characteristics from parent organisms to their children; it underlies much of genetics. They were initially derived from the work of Gregor Mendel published in 1865 and 1866 which was "re-discovered" in 1900, and were initially very controversial. When they were integrated with the chromosome theory of inheritance by Thomas Hunt Morgan in 1915, they became the core of classical genetics.

MENDEL'S LAW OF SEGREGATION:

Definition:

The principles that govern heredity were discovered by a monk named Gregor Mendel in the 1860's. One of these principles, now called Mendel's law of segregation, states that allele pairs separate or segregate during gamete formation, and randomly unite at fertilization.

Mendel's Law of Segregation Mendel's first law of inheritance, the law of segregation or law of purity of gametes states that in a heterozygote a' dominant and a recessive allele remain together throughout the life (from the zygote to the gametogenesis stage) without contaminating or mixing with each other and finally separate or segregate from each other during gametogenesis, so that, each gamete receives only one allele either dominant or recessive. For example, the F1 hybrids (Tt) of a monohybrid cross between tall (TT) and dwarf (tt) pea plant have one dominant allele (T) for tallness and one recessive allele (t) for dwarfness.

This genotype of F1 hybrids remains the same from the unicellular zygote stage to the gametogenesis stage of multicellular adult plant.
These F1 hybrids by self-fertilization produce tall and dwarf plants in the ratio of 3 : 1, It means that tall and dwarf alleles though, remain together for long time but does not contaminate or mix with anyone and both alleles segregate to produce gametes which either having dominant allele T or recessive allele t. These gametes unite to produce the 3 : 1 phenotypic ratio in F2.

MENDEL'S LAW OF INDEPENDENT ASSORTMENT:

DEFINITION:

The principles that govern heredity were discovered by a monk named Gregor Mendel in the 1860's. One of these principles, now called Mendel's law of independent assortment, states that allele pairs separate independently during the formation of gametes. This means that traits are transmitted to offspring independently of one another.

Mendels Law of Independent Assortment - The third law states that Alleles for different traits are distributed to sex cells (offspring) independently of one another.Mendel noticed during all his work that the height (tall or short), of the plant and the shape (round or wrinkled) of the seeds and the colour (green or yellow), of the pods had no impact on one another.In other words, being tall did not automatically mean that the plants had to have green pods, nor did green pods have to be filled only with wrinkled seeds, the different traits seem to be inherited independently.It involves what is known as a dihybrid cross, meaning that the parents are hybrid for two different traits.

The genotypes of the parent pea plants will be: RrGg x RrGg whereR = dominant allele for round seeds r = recessive allele for wrinkled seedsG = dominant allele for green pods g = recessive allele for yellow podsThus we are dealing with two different traits: (1) seed texture (round or wrinkled) and (2) pod colour (green or yellow). Also each parent is hybrid, for each trait (one dominant and one recessive allele for each trait).The results from a dihybrid cross are always the same: 9/16 boxes (offspring) show dominant phenotype for both traits (round and yellow), 3/16 show dominant phenotype for first trait and recessive for second (round and green), 3/16 show recessive phenotype for first trait and dominant form for second (wrinkled and yellow), 1/16 show recessive form of both traits (wrinkled and green).

So, as can be seen from the results, a green pod can have round or wrinkled seeds, and the same is true of a yellow pod.The different traits do not influence the inheritance of each other. They are inherited independently.Interesting to note is that if we consider one trait at a time, we get the usual 3: 1 ratio of a single hybrid cross (like we did for the Law of Segregation).For example, let us compare the colour trait in the offspring; 12 green and 4 yellow (3: 1 dominant recessive) and the same for the seed texture 12 round and 4 wrinkled (3: 1 ratio). The traits are inherited independently of each other.

BACKGROUND:

The reason for these laws is found in the nature of the cell nucleus. It is made up of several chromosomes carrying the genetic traits. In a normal cell, each of these chromosomes has two parts, the chromatids. A reproductive cell, which is created in a process called meiosis, usually contains only one of those chromatids of each chromosome.

By merging two of these cells (usually one male and one female), the full set is restored and the genes are mixed. The resulting cell becomes a new embryo. The fact that this new life has half the genes of each parent (23 from mother, 23 from father for total of 46) is one reason for the Mendelian laws. The second most important reason is the varying dominance of different genes, causing some traits to appear unevenly instead of averaging out (whereby dominant doesn't mean more likely to reproduce - recessive genes can become the most common, too).

There are several advantages of this method (sexual reproduction) over reproduction without genetic exchange:

#Instead of nearly identical copies of an organism, a broad range of offspring develops, allowing more different abilities and evolutionary strategies.

#There are usually some errors in every cell nucleus. Copying the genes usually adds more of them. By distributing them randomly over different chromosomes and mixing the genes, such errors will be distributed unevenly over the different children. Some of them will therefore have only very few such problems. This helps reduce problems with copying errors somewhat.

#Genes can spread faster from one part of a population to another. This is for instance useful if there's a temporary isolation of two groups. New genes developing in each of the populations don't get reduced to half when one side replaces the other, they mix and form a population with the advantages of both sides.

#Sometimes, a mutation (e. g. sickle cell anemia) can have positive side effects (in this case malaria resistance). The mechanism behind the Mendelian laws can make it possible for some offspring to carry the advantages without the disadvantages until further mutations solve the problems.