Law of Segregation

Mendel crossed pure tall and pure short (dwarf) plants. The offspring were all tall. When the offspring of the F1 generation were crossed he got the ratio of 3:1.

The reason for the occurrence of short character is that the parents were pure homozygous tall and homozygous short. F1 generation were all tall but heterozygous, when two F1 offspring were crossed Mendel got three tall and one short, because one tall plant was homozygous, two tall plants were heterozygous and one short plant was homozygous. The gene for short character is recessive. The recessive character, occurs only when it is homozygous i.e. when this gene is given by both the parents. So out of every four plants (meiosis), three were tall and one was short. From these experiments law of segregation has been formed known as Mendel's law of segregation.

Monohybrid Inheritance
“Each organism contains two factors for each characteristics and the factors segregate during the formation of gametes so that each gamete contains only one of each pair of factors” or in other words “if a cross is made between two contrasting characters, the characters will segregate in the ratio of 3:1 in the F2 generation”. In the F2 generation Mendel found that:
Tall Plants: (i) 1/3 of the tall plants produced only tall plants.
(ii) 2/3 of the tall plants produced both tall and short plants.
Short Plants: The F2 short plants produced only short plants.
Gene pool: It is the total collection of genes in a population at any one time. The gene pool is the reservoir from which the members of the next generation of that population derive their genes; it consists of all alleles (alternate form of genes) in all the individuals making up a population.

Some Definitions
Gene: In Mendelian Genetics a gene determines the nature of a phenotypic character.
Genome: The total complement of genetic information contained in a cell or organism.
Locus: The position of a gene on the chromosome is called its locus (plural .loci).
Alleles: Alternative forms of genes are called alleles, or allelomorph.
Gene pool: Total of all the genes of all the individuals in a population.
Dominant: When expression of only one allele in a heterozygous individual is observed, this allele is called a dominant allele.
Recessive: The allele whose expression is suppressed in a heterozygous condition is called recessive allele.
Phenotype: The external appearance of the character (allele, gene) is called phenotype.
Genotype: The genetic make-up of the character i.e. homozygous or heterozygous.
Homozygous: Having same type of allele e.g. TT, tt.
Heterozygous: Having different types of alleles e.g. Tt.
First filial generation: The offspring of the parent is known as the first filial generation.

Mendel's Seven Pairs of Contrasting Traits in Garden Peas

Mendel chose those characters that had different forms of expression. He selected seven pairs of contrasting characters. These characteristics were easily, distinguishable and bred true, generation after generation. A true breeding variety is one which produces offspring identical to the parents.


Pure Tall Plants
One of Mendel's varieties of peas grew about a meter tall, as contrasted with another that grew as a short. He crossed tall plants with tall plants for many generations and in this way obtained pure tall plants.
Mendel’s seven pairs of contrasting traits in garden peas

Pure Short (dwarf) Plants
Simultaneously he crossed the short (dwarf) plants with short plants for many generations and obtained pure short plants.

Crossing of Tall and Short Plants
Then he crossed the pure tall and pure short plant. The offspring were all tall plants. When the two tall offspring were crossed the result was different. He had grown 1064 plants and found that 787 of them were tall and 277 were short. The numbers struck him. They showed ratio of almost 3:1. It was certainly close to 798 to 266 have been exactly 3:1. Secondly Mendel was surprised at the occurrence of short plant as the offspring of two tall plants. This lead to a series of experiments. On the basis of his breeding experiments Mendel was able to propose that “each character occurs in pair” One part is given by the male and other by the female. When the characters (or factors as called by Mendel) are the same it is called homozygous and when the characters are different it is called heterozygous. When the tall plant get the character of tall from male and female, it will be homozygous tall. When it receives tall character from one parent and short character from another parent it will be heterozygous. In heterozygous condition only one character appears. The character which is seen is called a dominant character and the character which though remains but does not appear is called a recessive character.

In genetics the dominant character is shown by capital alphabet or by + and the recessive character is shown by small alphabet or by letters of the character e.g. vestigial (Vg, vg), arginine (Arg, arg) etc.

TT: It means the plant has one tall character from father (male) and another tall character from mother (female). It is homozygous tall.

Tt: It means that the plant is tail but it has tall character T and another short character t. Though the short character t is present but it is not exhibited. So the tall T character is dominant and short character t is a recessive one. The characters are therefore in the form of unit. The unit is now called pair of genes. (The term, gene, was introduced in 1909 by Wilhelm Johansson, then in 1912 Bateson introduced the word Genetics, Mendel used the word factor for gene). So the gene for tall is T and gene for short is t. Alternate forms of a genes are called alleles. Thus, in this example there is an allele for tallness (T) and an allele for shortness (t).

Genotype and Phenotype
The character seen externally is called phenotype e.g. tall and short. The genetic make-up of individual character is called genotype. The genotype may be homozygous or heterozygous e.g. tall homozygous TT, tall heterozygous Tt. Mendel did not use words phenotype and genotype, but he obviously understood the basic concept of visible and hidden characteristics.

The parents are shown by notation P, the offspring of the parents are known as first filial generation. (GK: Filial; offspring). It is represented as F. The offspring of this generation are called second filial generation. It is represented as F2.

Gregor Johann Mendel (1822-1884)

Mendel was born in a small village in Austria. His father was a professional gardener. Mendel had a very strong desire for learning and because of poverty he could not continue his studies. At the age of 21 he joined the Augustinian (relating to St. Augustine), Briinn in Austria (now Briino). At the age of 25 Mendel was made priest and then in charge of the church. From 1851 to 1853 Mendel was a student at the University of Vienna, where he studied botany, mathematics, and physics.
Gregor Johann Mendel 

Mendel began his breeding experiments on the common garden peas (Lathyrus odoratus, note: Pisum sativum is the sweet pea) for which he was allowed a limited space in the monastery garden. In 1865 he presented the results of his experiments to the Briinn society of Natural Science and published his conclusion in the proceeding of the society in 1866.

Mendel knew nothing about chromosomes or the process of mitosis and meiosis. His reasoning was based entirely upon his observation and experiments. Observation, assumption, experimentation and creativity, all of them are evident in Mendel's work. The experiments performed by Mendel were elegant and his conclusion constitute foundation of the modern science of Genetics, Mendel is therefore appropriately called, the father of Genetics.

Definition of Genetics

Genetics is the science of heredity. The word genetics, was first introduced by Bateson, but genetics began with the work of Mendel, who was trying to understand some of the basic laws of inheritance.

Apoptosis and Necrosis

Ultrastructural features of cell death by apoptosis
The first cell of a new multi-celled individual contains marching orders that will guide its descendants along a program of growth, development, and reproduction, then on to death. As part of that program, many cells proceed to self-destruction when they complete a prescribed function. If they become altered in ways that might pose a threat to the body as a whole, as by infection or cancer, they can execute themselves. Apoptosis (Greek: dropping off or falling off) is the name for this form of cell death. It starts with molecular signals that activate and unleash lethal weapon of self-destruction, which were stockpiled earlier within the cell. Protein cleavage enzymes are such weapons. Think of them as folded pocket knives. When popped open, they chop apart structural proteins, including the building blocks of cytoskeleton elements and nucleosomes that organize the DNA.

The dying cells shrink, condense and ultimately split up, thus release small membrane bounded apoptotic bodies, which are generally phagocytized by other cells. Intracellular constituents are not released freely in extracellular atmosphere which otherwise might have deleterious effects. In contrast to suicide the cell death due to tissue damage is called necrosis during which the typical cell swells and bursts, releasing the intracellular contents, which can damage neighboring cells and cause inflammation.