Some defined terms:
To understand monohybrid and dihybrid cross, we required to know about some terms related to cross, which are expressed below-
Factor:-Unit of heredity which is responsible for inheritance and appearance of characters. These factors were referred as genes by Johannsen (1909) Mendel used term “element” for factor, Morgan first used symbol to represent the factor .Dominant factor are represented by capital letter while recessive factor by small letter.
Allele :- Alternative forms of a gene which are located on same position (loci) on the homologous chromosome is called Allele. Term allele was coined by Bateson.
Homozygous:- A same allelic genes of a pair is called homozygous. Ex. TT, RR, tt.
Heterozygous:- A different allelic genes of a pair is called heterozygous. The term homozygous and heterozygous are coined by Bateson.
Hemizygous:- If individual contains only one gene of a pair then individual said to be Hemizygous. Male individual is always Hemizygous for sex linked gene.
Phenotype:- It is the external and morphological appearance of an organism for a particular character.
Genotype:-The genetic constitution or genetic makeup of an organism for particular character. Genotype &phenotype terms were coined by Johanson.
A back cross is a cross in which F1individuual are crossed with any of their parents. When F1individual is crossed with dominant parent then it is termed out cross the generation obtained from this cross all possess dominant character so the any analysis cannot possible in F1 generation.
When F1 progeny is crossed with recessive parent the process is called test cross. The total generation obtained from this cross 50% having dominant character and 50% having recessive character (monohybrid test cross) test cross help to find out the genotype of dominant individual.
What is Monohybrid Cross:
When we consider the inheritance of one character at a time in a cross this is called monohybrid cross.
Conclusion of monohybrid cross
Ist conclusion: According to Mendel each genetic character is character if controlled by a pair of unit factor .It is known as know as conclusion of paired factor or unit factor.
IInd conclusion: This conclusion is based on F1 generation .When two different unit factor are present in Single individual, only on unit factor is able to express itself and knows as dominant unit factor Another unit factor fail to express is the recessive factor. It is known as conclusion of dominance.
IIInd conclusion: During gamete formation ; the unit factors of pair segregate randomly and transfer inside different gamete. Each gamete receives only one factor of a pair so gametes are pure for a particular trait it is known as conclusion of purity of gametes or segregation.
Checker Board method
First time it was used by Reginald C punnett (1875-1967). The representation of generations to analyze in the form of symbols of squares, male gametes lie horizontally and female gametes lie vertically
T T=tall (dominant homozygous)
T t=Tall(dominant heterozygous)
tt= Dwarf(recessive homozygous)
The ratio of characters (traits )appear/visible morphologically is phenotypic ratio it is 3:1. Genetic constitution is called Genotype (using symbols for genes) it is 1:2:1.
Fork line method: To find out the composition of factors inside the gamete we use fork line method
AaBb=4type of gamete
Type of gamete / phenotype category = 2n
N= NO. of hybrid character or heterozygous pair.
Type of genotype = 3n
E.g. in di-hybrid cross =32 9 genotype.
No of zygote produced by self pllination of a genotype = 4N