In the dihybrid cross, the parent plants having the phenotype round-yellow seeds have the factors of inheritance or gene combination RRYY (in which RR are the dominant genes for round shape whereas YY are the dominant genes for yellow colour).
On the other hand, the parent plants having the phenotype wrinkled-green seeds have the factors of inheritance or gene combination rryy (in which rr are the recessive genes for wrinkled shape and yy are the recessive genes for green colour).
Keeping these points in mind, we can now show the dihybrid cross by drawing a chart as we did in the case of a monohybrid cross. The chart showing the dihybrid cross between pea plants having round-yellow seeds and wrinkled-green seeds is given below.
An amazing thing about Mendel’s work is that he worked out the underlying rules of inheritance before any knowledge of DNA, chromosomes or genes became available. Let us answer one question now.
(a) What do the progeny of a tall plant with round seeds and a short plant with wrinkled seeds look like? Why?
(b) What happens when the F1 progeny obtained above are used to produce F2 progeny by self pollination?
(a) The progeny of a tall plant having round seeds crossed with short plants having wrinkled seeds are all tall plants having round seeds. This is because ‘tallness’ and ’round shape’ of seeds are dominant traits. On the other hand, ‘shortness’ and ‘wrinkled shape’ of seeds are recessive traits.
(b) When F1 progeny are cross-bred by self-pollination, then we will get four types of progeny in the F2 generation. Of these four types of progeny, two types will have traits like parents and the other two will have new combinations of traits. Thus,
(i) Some F2 progeny will be tall plants with round seeds (9).
(ii) Some F2 progeny will be tall plants with wrinkled seeds (3).
(iii) Some F2 progeny will be short plants with round seeds (3).
(iv) Some F2 progeny will be short plants with wrinkled seeds (1).
Please note that though Mendel studied the inheritance of characteristics by using plants (or rather pea plants) but the rules for the inheritance of traits given by Mendel are also applicable to the inheritance of traits in animals (including human beings). Thus, human genetic follows Mendelian principles.