Question

In Duroc-Jersey hog, the coat colour is dependent on two pairs of alleles, R and r and S and s. Any genotype containing at least one R-gene and at least one S-gene results in red coat colour. The double recessive genotype results in white coat colour. All other genotypes result in sandy coat colour. If one hog with genotype "RrSs" mated with another hog with genotype "rrss", what kind of offspring will be produced by the above cross?
A)9 red : 6 sandy : 1 white
B)9 red : 3 sandy : 4 white
C)12 red : 3 sandy : 1 white
D)1 red : 2 sandy : 1 white

Answer 1

Dihybrid cross is a cross between two separate genes/lines that vary in the two features found. According to Mendel's argument, there is a relationship with fully dominant - recessive characteristics between the alleles of both these loci.

Complete answer:
The non-allelic gene association is recessive epistasis where the presence of one recessive gene masks the expression of another recessive gene, the dihybrid ratio becomes 9: 3: 4. The non-allelic gene association where the involvement of one dominant gene hides the expression of another dominant gene is dominant epistasis, the dihybrid ratio becomes 12: 3: 1. The existence of one gene does not obscure the influence of another gene in the question, but creates a combined effect; therefore, it is not epistasis that makes options B and C wrong. A cross between two dihybrids gives a ratio of 9: 6: 1 when two separate genes produce a similar phenotypic outcome, but when present together, they produce a cumulative phenotype.

The existence, depending on the issue, of a single copy of at least one R-gene and at least one S-gene results in a combined phenotype; the colour of the red coat, while the colour of the white coat results from the double recessive genotype.
Thus, it is an example of cumulative-effect duplication chromosomes. All other genotypes result in sandy coat colour. A ratio of 1: 2: 1. is given by the cross-analysis between dihybrid and double recessive. On the offspring, there are only dominant alleles of R and S, which will have a red coat colour. For the other two, there would be a sandy coat colour for one dominant allele and a white coat colour with recessive alleles for one offspring.

Hence, the correct answer is option (D)

Note: The easiest way to produce such an example is by a dihybrid test cross, which, during a cross between two heterozygous parents, calls two separate genes. This phenotypic ratio of 9:3:3:1 is the classical Mendelian ratio for a dihybrid cross in which gametes independently assort the alleles of two separate genes.