Question

When a dihybrid cross is fit into a Punnett Square with 16 boxes, the maximum number of different phenotypes available are

• A. 8
• B. 4
• C. 2
• D. 16

Answer 1

Answer: (B)

4

Total number of types of gamete produce by an organism is 2${}^n$, where n is the number of heterozygous genes present. As we know that monohybrid plant is heterozygous for one gene, thus total possible gametes by it = 2${}^1$=2. During fertilization, any of the 2 types of male gamete can fuse with any of the 2 types of female gamete to produce 4 possible combinations of zygote. Similarly, dihybrid, trihybrid and tetrahybrid plants are heterozygous for two, three and four genes respectively, thus total possible gametes= 2${}^2$, 2${}^3$ and 2${}^4$ = 4, 8 and 16 respectively. Thus total possible genotypes by dihybrid, trihybrid and tetrahybrid cross are 16, 64 and 256 respectively. The total number of boxes in a Punnett square equals the number of rows times the number of columns and represents the total number of genotypes formed by gametes. Thus, monohybrid, dihybrid, trihybrid and tetrahybrid cross have 4, 16, 64 and 256 boxes respectively.

The phenotypic ratio for a dihybrid cross is 9 : 3 : 3 : 1; thus total 4 types of genotypes can be formed; option B is correct. Phenotypic ratio for a monohybrid cross is 3 : 1; thus total 2 types of genotypes can be formed; option C is incorrect for a dihybrid cross. Phenotypic ratio for a trihybrid cross is 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1; thus total 8 types of genotypes can be formed; option A is incorrect for a dihybrid cross. Phenotypic ratio for a tetrahybrid cross is 81 : 27 : 27 : 27 : 27 : 9 : 9 : 9 : 9 : 9 : 9 : 3 : 3 : 3 : 3 : 1; thus total 16 types of genotypes can be formed; option D is incorrect for a dihybrid cross.