The Mendelian principles are named after the Augustinian monk Gregor Johann Mendel (1822-1884, middle of the 19th century). He was considered as father of genetics due to his work on study of inheritance. He conducted cross-breeding experiments on garden peas in his monastery garden in Brunn (Czech Republic) in 1865. Mendel recognized that heredity is based on individual factors that are independent of each other. These factors are transferable from one generation to another in a particular pattern, each factor being responsible for an observable trait. The observable morphology of trait is known as phenotype and underlying genetic information is known as genotype. Mendel chose the garden pea, Pisum sativum as an experimental organism. Mendel choose seven traits as markers- flower color, flower position, seed color, seed shape, pod shape, pod color and height of the plant.
A genetic cross between parents that differ in the alleles they possess for one particular gene, one parent having two dominant alleles and the other two recessives. All the offsprings (called monohybrids) have one dominant and one recessive allele for that gene (i.e. they are hybrid at that one locus). Crossing between these offspring yields a characteristic 3:1 (monohybrid) ratio.
A genetic cross between parents that differ in two characteristics, controlled by genes at different loci. Mendel performed a dihybrid cross using pea plants and the characteristics of seed color and texture: the parental plants had either smooth yellow seeds (SSYY) – the dominant characteristics – or wrinkled green seeds (ssyy) – the recessive characteristics. All the offspring had smooth yellow seeds, being heterozygous (SsYy) for the two alleles. Crossing between these offspring produced an F2 generation of plants with smooth yellow, smooth green, wrinkled yellow and wrinkled green seeds in the ratio 9:3:3:1.
Mendel used these results as the basis for his Law of Independent Assortment.
Monohybrid and dihybrid crosses proved three principles of genetics:
• First is the law of dominance, according to which offspring will have traits of one parent because of dominance.
• Second is the law of segregation, two traits separate or segregate during reproduction and then combine in new combination in offspring.
• Third is law of independent assortment, according to which factors of characters assort independently without influencing others
Law of Dominance
Firstly the progeny which he got in F1 generation had the appearance of one parent only, this is because of dominance of one trait over the other one. This led him to propose law of dominance. According to this law when two dissimilar unit factors are present in an individual then only one expresses itself while other was completely masked. The one which expressed itself is said to be dominant and other which do not expressed itself is known as recessive.
For example when pea plants with round seeds (RR) are crossed with plants with wrinkled seeds (rr), all seeds in F1 generation were found to be round (Rr). When these round seeds were self fertilized, both the round and wrinkled seeds appeared in F2 generation in 3:1 ratio. Hence, in F1 generation, the dominant character (round seeds) appeared and the recessive character (wrinkled seeds) got suppressed, which reappeared in F2 generation
Law of Segregation
Mendel recognized that in order to get 3:1 ratio in the second generation each parent must possess two copies of the trait. This ratio makes him understand that the pea traits he was working with, has distinct units of heredity material. He proposed that these two traits separate or segregate during reproduction i.e. at the time of gamete formation and then combine in new combinations in the offspring. This is known as law of segregation. Genotypic ratio is 1:2:1
For example The cross between true breeding tall (TT) pea plant and pure breeding dwarf (tt) plant.
Law of Independent Assortment
In the di-hybrid cross, Mendel followed the same pattern as he did in the single-trait cross. The ratio 9:3:3:1 he obtained from di-hybrid cross was explained by him in the form of law of independent assortment. According to this law, the factors of different traits are not influenced by each other‘s presence and each of the two factors of character can assort with either of two factors of another character. Genotypic ratio 1:2:2:1:4:1:2:2:1. The two characters considered were round and yellow (dominant) and wrinkled and green (recessive).
For example, consider the characteristics of seed color and seed texture for two pea plants: one that has green, wrinkled seeds (ggww) and another that has yellow, round seeds (GGWW). Because each parent is homozygous, the law of segregation indicates that the gametes for the green/wrinkled plant all are gw, while the gametes for the yellow/round plant are all GW. Therefore, the F1 generation of offspring all are GgWw. For the F2 generation, the law of segregation requires that each gamete receive either an W allele or an w allele along with either a G allele or a g allele. The law of independent assortment states that a gamete into which an w allele sorted would be equally likely to contain either a G allele or a g allele. Thus, there are four equally likely gametes that can be formed when the GgWw heterozygote is self-crossed as follows: GW, Gw, Wg and gw. From the cross, a phenotypic ratio of 9 round/yellow: 3 round/green: 3 wrinkled/yellow : 1 wrinkled/green and genotypic ratio i.e. 1:2:2:1:4:1:2:2:1 is obtained.