Genetics

Genetics

Genetics is the study of biologically inherited traits, including traits that are influenced in part by the environment.
Gregor Mendel - Father of genetics
Gregor Mendel (Original name - Johann Mendel) was an Austrian monk.
The observations he made while growing peas in his monastery's garden became the foundation of modern genetics and the study of heredity.

Through the selective cross-breeding of common pea plants (Pisum sativum) over many generations, Mendel discovered that certain traits show up in offspring without any blending of parent characteristics. For instance, the pea flowers are either purple or white, intermediate colours do not appear in the offspring of cross-pollinated pea plants.
Mendel observed 7 traits that are easily recognized and apparently only occur in one of two forms.
His ideas had been published in 1886 but largely went unrecognized until 1900, which was long after his death.

Terms used often and its definition

Allele 

One alternative form of a given allelic pair; tall and dwarf are the alleles for the height of a pea plant; more than two alleles can exist for any specific gene, but only two of them will be found within any individual

Allelic pair 

The combination of two alleles which comprise the gene pair

Homozygote 

An individual which contains only one allele at the allelic pair; for example DD is homozygous dominant and dd is homozygous recessive; pure lines are homozygous for the gene of interest

Heterozygote  

An individual which contains one of each member of the gene pair; for example the Dd heterozygote

Genotype  

The specific allelic combination for a certain gene or set of genes 

Phenotype 

The way an organism looks (outer appearance)
genotype + environment + random variation = phenotype.

Polygenic inheritance

A pattern of inheritance where the trait is controlled by many genes and each dominant allele has an additive effect.

Variations - differences amongst the individuals of the same species and the offspring of the offspring of the same parents. Variations can be due to mutations.
Gene 

It is the functional unit of heredity.
Heredity 

This includes those traits or characteristics which are transmitted from generation to generation and it is therefore fixed for a particular individual.

Laws of Mendel
Law of segregation (Law of purity of gametes)
A gamete may carry either the dominant or the recessive factor but not both as we find in f1 individuals. This suggests that there is no blending of mendelian factors in f1, but that they stay together and only one is expressed. The gametes which are formed are always pure for a particular character. This is why it is called either as principle of segregation or law of purity of gametes.

Law of independent assortment (Mendel's second law)
The two character under consideration are assorting (give rise to different combination) in an independent manner. This principle is known as independent assortment.

Law of dominance
Out of the 2 alleles, the one which is able to express itself is the dominant allele while the other is the recessive allele. In the presence of recessive allele and dominant allele, only dominant allele will express itself.

For example:

Here plant height is the trait and the two alleles responsible for the traits are Tall (T) and short (t). 

When both allels responsible for Tall (T) and short (t)  are present irrespective of the presence of recessive allele (t), only dominant allele (T) is always expressed resulting in heterozygous tall plants. 

Law of alleles
Mendel proposed that each genetic character is controlled by a pair of unit factor, now commonly called alleles or allelomorphic factor.

Allelomorphs or alleles indicate alternative forms of the same genes.
Backcross 

 The f1 individuals obtained in a cross are usually selfed to get f2 progeny. They can also be crossed with the one of the other two parents from which they were derived; such a cross of f1 individual with either of the two parents is known as a backcross.
Test cross 

When f1 is backcrossed with recessive parent is called testcross.

The crossing of an organism, with an unkown genotype, to a homozygous recessive organism (tester). A cross between an individual of unknown genotype or a heterozygote (or a multiple heterozygote) to a homozygous recessive individual. 

A cross between an individual exhibiting the dominant phenotype of a trait and an individual that is homozygous recessive for that trait in order to determine the genotype of the dominant individual.

a cross between an organism whose genotype for a certain trait is unknown and an organism that is homozygous recessive for that trait so the unknown genotype can be determined from that of the offspring