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.
“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.
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.