Modification of Dihybrid Phenotypic Ratio

The classical or Mendelian phenotypic ratio of 9:3:3:1 is obtained only when the alleles at both gene loci display dominant and recessive relationships. If one or both gene loci have incompletely dominant alleles, or codominant alleles or lethal alleles, the dihybrid ratio becomes modified variously, such as follows: 1. 3: 6 : 3 : 1 : 2 : 1 ratio When the dihybrid parents have dominant and recessive alleles at one gene Iocus and codominant alleles at second gene locus, the F2 9 : 3 : 3 : 1. phenotypic ratio becomes 3 : 6 : 3 : 1 : 2 : 1.

Example:

In cattles. hornless or polled (P) condition is dominant to horned (p) condition, and white (W) coat colour is codominant to red coat colour (w). The mating of a homozygous white, polled (hornless) cattle (PP WW) with a homozygous red horned cattle (pp ww) will produce F1 heterozygotes with the phenotype of hornless or polled, roan and genotype of Pp Ww. These hornless or polled roan F1 heterozygotes produce a F2 progeny in the ratio of 3 : 6 : 3 : 1 : 2 : 1 as follows:

The ratio of 3: 6: 3: 1: 2: 1 also can be get by cross-multiplying the F2 monohybrid phenotypic ratios of dominant recessive alleles (3: 1) and codominant alleles (l : 2 : 1), i.e., (3 : 1) X (1 : 2 : 1) =3 : 6 : 3 : 1 : 2 : 1. F2 analysis of figure:

2. 1: 2: 1: 2: 4: 2: 1: 2: 1 ratio

When each parent of a dihybrid cross has incompletely dominant alleles at both gene loci, then in F2 large number of phenotypic classes are produced. Example: In snapdragons, red flower colour (R) is incompletely dominant to white flower colour (r) and broadness of leaf (B) is incompletely dominant to narrowness of leaf (b). The dihybrid cross between red, broad plant (RR BB) and white, narrow plant (rr bb) produces F1 heterozygotes having pink flowers and leaves of intermediate width and genotype of Rr Bb. These F1 heterozygotes produced the F2progeny in 9 phenotypic classes as shown in Figure.The phenotypic ratio of 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 can also be obtained by cross multiplying the F2 monohybrid ratios of each gene (R or B), i.e., (1 : 2 : 1) X (1 : 2 : 1) = 1: 2 : I : 2 : 4 : 2 : 1:2:1. 3. 3: 1: 6: 2 Ratio

When the F1 hybrids have dominant recessive alleles at one gene locus and recessive lethal alleles at second gene locus, the F2 offsprings manifest the phenotypic ratio of 3: 1 : 6 : 2. This ratio can also be obtained by cross-multiplying the F2 monohybrid ratios of both types of alleles, i.e., (3: 1) X (2:1) = 3: 1 : 6 : 2. 4. 1: 2: 1: 2: 4: 2 Ratio When the F1 dihybrids contain codominant alleles at first gene locus and recessive lethal alleles at second gene locus, then their F2 progeny display the phenotypic ratio of I : 2 : 1 : 2 : 4 : 2. This ratio can also be deducted by cross-multiplying the F2 monohybrid ratios of both types of genes, I.e., (1: 2: 1) x (2: 1) =1: 2: 1: 2:4:2.

5. 4: 2: 2: 1 Ratio

This phenotypic ratio is achieved when at both gene loci of F1 hybrids occur the recessive lethal alleles. We can get this phenotypic ratio by multiplying the F2 monohybrid ratios of both types of lethal genes, i.e., (2:1) x (2:1) = 4: 2: 2: 1.