Question

Phenotypic dihybrid ratio is
(a)9:3:3:1
(b)15:1
(c)9:6:1
(d)1:2:1

Answer 1

A dihybrid cross suggests that there is a study of two different traits involved which is represented by two different alleles each. Each trait is represented by a different characteristic and each characteristic is divided into two respective forms. Thus we get four expressions of a dihybrid cross.

Complete answer:
When Mendel's law of segregation and law of independent assortment is applied to the first filial generation, all the four offsprings are seen to have identical characters. When these are further crossed in the second filial generation, the phenotypic ratio seen is 9:3:3:1

Additional Information: A dihybrid cross can be visualized by a 4×4 Punnett square which gives 16 individual results.
Let's take an example of F1 generation as 'RRYY' and 'rryy', the F2 generation is genotypically 'RrYy' and phenotypically expressed dominant genes only. When the RrYy generation is crossed the with RrYy the
phenotypic ratio of 9:3:3:1 and a genotypic ratio of RRYY 1: RRYy 2: RRyy 1: RrYY 2: RrYy 4: Rryy 2: rrYY 1: rrYy 2: rryy 1.
Here in 9:3:3:1 the first 9 Represent phenotypically dominant individuals with both the traits dominant. The first 3 represents individuals with the first trait dominant and second recessive. The second 3 represents the first trait recessive and second dominant, and the final 1 is the individual with both recessive traits.
So, the correct answer is '9:3:3:1'

Note: Mendelian laws of inheritance are
-The Law of Segregation: 'Each inherited trait is defined by a gene pair. Parental genes are randomly separated into the sex cells so that sex cells contain only one gene of the pair. Offspring, therefore, inherit one genetic allele from each parent when sex cells unite in fertilization.'
-The Law of Independent Assortment: 'Genes for different traits are sorted separately from one another so that the inheritance of one trait is not dependent on the inheritance of another.'
- The Law of Dominance: 'An organism with alternate forms of a gene will express the form that is dominant.'