Genetics is the branch of science that deals with genes, heredity, and variation in living organisms. It is the process of trait inheritance from parents to offspring. Further, the scope of genetics is also extended to study gene behaviour in the context of a cell or organism (e.g. dominance and epigenetics), gene distribution, and variation and change in populations and the molecular structure and function of genes.
The father of genetics is Gregor Mendel. He studied 'trait inheritance', how traits are transmitted from parents to offspring. He observed that organisms (pea plants) inherited traits by way of discrete "units of inheritance".
1.1.    Mendelian and classical genetics 
Genetics is rooted in the research of Gregor Mendel, a monk who discovered how traits are inherited. Mendel presented his paper "Versuche über Pflanzenhybriden" ("Experiments on Plant Hybridization"), to the Naturforschenden Vereins (Society for Research in Nature).  Mendel described mathematically the inheritance patterns of certain traits in pea plants.  Mendel published his results in 1866 just six years after the publication of Charles Darwin’s "Origin of Species" but no one paid attention to the Mendel. In 1900 Hugo de Vries, Carl Correns, and Erich von Tschermak, each working separately, published results of experiments similar to Mendel's. Mendelian inheritance is the inheritance of biological features that follow the laws proposed by Gregor Johann Mendel. When Mendel's theories were integrated with the chromosome theory of inheritance by Thomas Hunt Morgan in 1915, they became the core of classical genetics.
1.2.    Mendelian Experiments on pea plants
Inheritance is governed by information stored in discrete factors called genes. 
Mendel selected 7 (Seven) contrasting characters in pea plant (Pisum sativum). The characters chosen by Mendel were in two alternating forms i.e. either dominant or recessive. These contrasting characters are:-


Mendel selected pea plant because of these reasons:-
1.    Mendel noticed contrasting characters in pea plant which are in two forms i.e. dominant and recessive.
2.    Pea plant has a short life cycle i.e. annual. 
3.    Pea plant produces many offspring within a year.
4.    Pea plant can be easily raised and handled conveniently.
5.    Pea plants reproduce by self-pollination. 
6.    Pea plant can easily be cross-pollinated to produce hybrids, Mendel performed emasculation i.e. remove the immature anther before they start shedding pollen and spray the pollen taken from the stigma of another plant.
7.    It grows to maturity in a single season.
According to Mendel, the character refers to the distinctive qualities which make up an individual. Characters need to be built by a person. For example; eye colour, plant height etc.
Trait: Trait refers to an individual's feature which is inherited. The trait is a part of the character. For example; blue or brown eyes etc.
1.3.    Explanation of Mendelian Genetics:-
1.     Genes come in pairs, that means a cell or individual has two copies of each gene. Mendel called genes as 'factors'.
2.     The different forms of a particular gene are called alleles. 

In the given figure of the allele, R allele is given for round seeds and r for wrinkled seeds. R and rare alleles because they are alternative forms of the gene for seed shape. Alternative alleles are typically represented by the same letter or combination of letters, distinguished either by uppercase and lowercase or by means of superscripts and subscripts or some other typographic identifier. All alleles for any particular gene are found at a specific place on a chromosome called the locus for that gene. (The plural of the locus is loci.) Thus, there is a specific place—a locus—on a chromosome in pea plants where the shape of seeds is determined. This locus might be occupied by an allele for round seeds or an allele for wrinkled seeds. An allele is an alternative form of the gene which is found at the same locus on a chromosome.
The allele may also be defined as an alternate form of the gene. i.e. one form of the gene for height, allows pea plants to grow more than 2 meters tall and another form of this gene limits their growth to about half a meter. Organisms are diploid, carrying two alleles for each gene. If the two alleles are identical then the organism is said to be homozygous (WW,ww); if the two alleles are different then the organism is heterozygous (Ww). The two alleles interact with each other to generate the phenotype of the cell in which they are found; the way in which they interact depends on their dominance relationship.
The frequency of all the alleles of all the genes in a population constitutes the gene pool of that population. Changes in the gene pool, via changes in allele frequency, are the basis of evolution.
Allele frequency - Allele frequency or gene frequency is the proportion of the copies of a gene at a particular locus in a population. This can be expressed as a fraction, decimal or percentage. For a given allele X, found at a locus, Y, the allele frequency of allele X would mathematically be represented as: 

So, for instance, if in a population of 300 diploid organisms, a single genetic locus has three alleles; allele a, allele b and allele c. Allele a appears 174 times then, given that there must be 600 copies of all the alleles for this locus in total (300 x 2 loci, due to diploidy), the allele frequency of allele a is 174/600 or 29%.
Changes in allele frequency are the basis of evolution. Such changes are initially caused by random mutation, which may either eliminate existing alleles or introducing new ones, and the effect of those mutations may be shaped by any combination of natural selection, genetic drift or gene flow.
3.     Gamete produced by an individual contains only one allele of a gene (that is, either allele W or allele w).
4.     In the formation of gametes, any particular gamete is equally like to include either allele from a heterozygous Ww genotype in which half the gametes contain allele W and the other half contain allele w.
5.     The union of male and female gametes is a random process that restores the alleles in pairs.
1.4.    Homozygous and heterozygous
A diploid organism that possesses two identical alleles of a gene is homozygous for that locus. A homozygous organism may be homozygous dominant (RR) or homozygous recessive (rr). A diploid organism that possesses two non-identical alleles of a gene is heterozygous  for that locus. 

1.5.    Genotype:- The genotype is the genetic constitution of an individual. Genotype means the set of alleles that an individual possesses. With respect to seed shape in pea, genotype RR, genotype Rr, and genotype rr are examples of the possible genotypes for the allele R and allele r.
1.6.  Phenotype:- A phenotype is the physical, physiological, biochemical, or behavioural characteristic display by the organism.
Thus, the tall plant height is a phenotype, a body weight of 50 kg is a phenotype, and having sickle-cell anaemia is a phenotype. 
The molecular basis of phenotype
Scientist later identified the gene which determines the seed shape. The wrinkled phenotype of seed is due to the absence of amylopectin. Amylopectin is a branched chain form of starch which is not synthesized in wrinkle seed due to a mutation in the gene coding for a starch-branching enzyme (SBEI).
WW :- Two copies of SBEI allele are present.
Ww :- Only a single copy of SBEI allele is present. Which is sufficient enough to produce round seed shape phenotype.
ww :- Both the copies of SBEI allele are mutated that produce wrinkle phenotype.
Mendel begins with pure-breeding pea plants because they always produced progeny with the same characteristics as the parent plant. Mendel cross-bread these pea plants and recorded the traits of their progeny over several generations. Mendel proposed three basic laws of genetics.

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