Dihybrid cross is a cross in which we do study of inheritances of two pairs of contrasting traits. Mendel wanted to observe the effect of one pair of heterozygous on other pair. Mendel selected traits for dihybrid cross for his experiment as follows:-
(1) Color of cotyledons Yellow (Y) green (y)
(2) Seed form Round (R) and wrinkled (r)
Yellow and round characters are dominant and green and wrinkled are recessive characters.
Mendel crossed yellow and round seeded plants with green and wrinkled seeded plants. All the plant in F1-generation had yellow and round seeds. When F1plant were self pollinated to produce four kinds of plants in F2 generation such as yellow round, yellow wrinkled, green round and green wrinkled there were in the ratio of 9:3:3:1 this ratio is known as dihybrid ratio.
Expression of yellow round (9) and green wrinkled (1) traits shows as their parental combination Green round and yellow wrinkled wrinkled type of plant are produced by the results of new combination.
The F2 generation plant produce two new phenotypes so inheritance of seed coat colour is independent from the inheritance of shape of seed Otherwise it can not possible to obtain yellow wrinkled and green round type of seeds.
This observation leads to the Mendel conclusion the different type of characters present in plant assorted independently during inheritance. This is known as Conclusion of independent assortment it is based on F2 generation of dihybrid cross.
The non-homologous chromosome show random distribution during anaphase I of meiosis.
Demonstration by checker board method
Yellow Round = 9/16
Yellow wrinkled = 3/16
Green Round = 3/16
Thus phenotypic Ratio = 9 : 3 : 3 : 1
Homozygous yellow &Homozygous Round-YYRR = 1
Homozygous yellow &Heterozygous Round-YYRr = 2
Heterozygous yellow &Homozygous Round-YyRR = 2
Heterozygous yellow & Heterozygous Round-YyRr =4
Homozygous yellow& Homozygous wrinkled–YYrr = 1
Heterozygous yellow &Homozygous wrinkled-Yyrr = 2
Homozygous Green &Homozygous Round-yyRR = 1
Homozygous Green & Heterozygous Round-yyRr = 2
Homozygous Green & Homozygous wrinkled-yyrr = 1
Thus genotypic Ratio =1:2:2:4:1:2:1:2:1
Monohybrid test cross:- The progeny obtained from the monohybrid test cross are in equal proportion it can be represent in symbolic forms as follows. Monohybrid test cross ratio is 1: 1.
Dihybrid test cross:- The progeny is obtained from dihybrid test cross are four types and each of them is 25%. The ratio of di-hybrid test cross =1:1:1:1
Conclusion:- In test cross phenotypes and genotype ration are same.
Reciprocal cross:- When two parents are used in two experiment in such way that in one experiment A is used as the female parent and “B” is used as the male parent in the other experiment “A” will be used as the male parent and “B” as the females parent such type of set two experiments is called reciprocal cross.
Characters which are controlled by karyogene are not affected by reciprocal cross in case of cytoplasmic inheritance result change by reciprocal cross.
Also Read: History of Genetics