(Cross including two pairs of contrasting characters)
Mendel too experimentally studied the transmission and segregation of two pairs of contrasting characters at a time. This was termed as the Dihybrid cross or two-factor crosses. He took up the round and wrinkled characters of seed coat all along with green and yellow color of seeds.
Mendel found that a cross among round, yellow and wrinkled green seeds (P) produced only round yellow seeds in the F1 generation, however for in the F2 generation four kinds of combinations appeared of which two were distinct from that of the parental combinations, in the ratio given below:
Round Yellow: 9 ParentalRound Green: 3 New combinationsWrinkled Yellow: 3 New combinationsWrinkled Green: 1 ParentalHence the offsprings of the F2 generation in a dihybrid cross were generated is a ratio of 9:3:3:1. This ratio is termed as the dihybrid ratio.
In F2 generation, as all the four characters assorted out independent of the others, he stated that a pair of contrasting characters behaves separately of the other pair that is; seed color is independent of seed coat. At the time of gamete formation in the F1 dihybrid, genes for round or wrinkled character of seed coat assorted separately of the green or yellow color of seed coat. As a consequence four kinds of gametes with two old and two combinations were formed that is, RY, Ry, rY and ry. Such four types of gametes on arbitrary mating generated 16 offsprings in the ratio of 9:3:3:1. The real numbers of individuals got by Mendel for each of the four classes were:
i. 108 round green seedsii. 315 round yellow seeds iii. 32 wrinkled green seedsiv. 101 wrinkled yellow seeds Mendel symbolized round character of seed as R and wrinkled as r, and yellow character as Y and green character as y. Therefore the dihybrid cross was among parents containing factor constitution as RRYY x rryy. This cross might be symbolized as follows:
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