Question

In dihybrid cross, out of 16 plants obtained, the maximum number of different genotypes possible are
A. 4
B. 9
C. 16
D. 12

Answer 1

Mendel proposed the principles of inheritance, which are today referred to as ‘Mendel’s Laws of Inheritance’. According to him, some characters in an organism are dominant over others. The Law of Dominance states that the dominant characters are expressed when factors are in heterozygous condition. In homozygous conditions, recessive characters are expressed. The characters never blend in heterozygous condition. During heterozygous condition, a recessive character that was not expressed in many generations may be expressed again when it becomes homozygous. Thus the characters segregate while formation of gametes which satisfies the Law of Segregation.

Complete answer:
Let’s consider an illustration to understand the question the properly:
According to the observations on dihybrid crosses (crosses between plants differing in two traits), Mendel proposed a second set of generalisations that we refer to as the Mendel’s Law of Independent Assortment. In a hybrid, the two pairs of traits are combined, and segregation of one pair of characters is completely independent of the other pair of characters.
Let’s consider a case where the phenotypes of the parents are Round yellow (RR, YY) and Wrinkled green (rr, yy). The seeds produced are heterozygous for both the traits. We get gametes in the F1 generations as RY and ry. These gametes cross and produce an offspring of genotype RrYy i.e. round yellow. Then selfing is done which produces 9 round yellow, 3 round green, 3 wrinkled yellow and 1 wrinkled green seeds.
3/4 of all the offspring will have round seeds, 3/4 of all the offspring will have yellow seeds and 9/16 will have round yellow seeds. After considering the punnett square we get the maximum number of genotypes are: RRYY, RRYy, RrYy, RrYY, Rryy, RRyy, rryy, rrYy and rrYY, i.e. 9 genotypes.
The maximum number of different genotypes possible are 9. So, the correct answer is option (B).

Note: The Mendel’s laws are extended as Chromosomal Theory of Inheritance. But later it was found that there were some exceptions to the Mendel’s law of independent assortment for the genes that were located on the same chromosomes. They are referred to as linked genes. The genes located closely on a chromosome assort together and the genes located at distant places, due to recombination, assort independently.