In 9:3:3:1 dihybrid phenotypic ratio, the number of hybrid dominant of both traits is______.
a. 9
b. 6
c. 5
d. 4

Hint: Mendel, on the basis of the hybridization experiment on Pisum sativum, obtained remarkable results and proposed laws of inheritance. These laws were made on the crossing between a monohybrid cross (involves only one trait) and a dihybrid cross(involves two traits). The ratio of dominant and recessive traits obtained in both the crosses was different.

Complete answer:
A cross that involves the analysis of two independent traits (height of plant and color of flower) is termed as a dihybrid cross. This proves the law of independent assortment.
Example- Suppose crosses are made between a pea plant with yellow and round seeds and another one with wrinkled and green seeds. The outcome will be as below-

Parent generation- YYRR (Yellow round seeds, Dominant) and yyrr (Green wrinkled seeds, recessive)
Gametes- YR and yr
F1 generation- YyRr (Yellow and round seeds)
Then, self-fertilization of F1 generation occurs.
Gametes- YyRr- YR, Yr, yR, yr and YyRr- YR, Yr, yR, yr

Male gametes


Female gametes
So the above question demands a hybrid outcome that is one allele will be dominant while the other recessive. So, there are four such cases. The above highlighted four progenies will show dominant as well as a hybrid case.

Hence, the correct answer is option (D).

Note: Some important points-
- The hybrid dominant case will involve both recessive as well as dominant alleles and still it is called dominant because the dominant allele will mask the expression of the recessive allele.
- The frequency is calculated using a punnett square. The outcome of the dihybrid cross makes it clear that segregation of the seed color is independent of the seed shape, and both paternal and new combinations of certain characters appear in the F2 generation.