Question

In a dihybrid cross, if one character shows codominance and the other character shows incomplete dominance. What will be the genotypic ratio in its progenies?
A) 9:3:3:1
B) 3:1:6:2:3:1
C) 1:1:1:1:4:2:2:2:2
D) 1:1:1:1

Answer 1

Codominance is the type of inheritance in which both the genes in a heterozygote are fully expressed. The resultant is the combination of the phenotype of both the parents. Incomplete dominance also occurs in a heterozygous pair when the dominant allele is not completely dominant over the recessive.

Complete answer:
Dihybrid cross is the cross that involves the cross of the alleles basing on two characters. According to the law of Dominance by Mendel, it states that in a heterozygous pair one allele which is completely expressed is called the dominant and the other which remains unexpressed is the recessive trait. But in some cases, this law fails to prove its hold. There are cases in a heterozygous pair where both the alleles are expressed (codominance) and one allele is not completely dominant over the other as in the case of incomplete dominance.

Let us take the example of white coloured flowers, tall plants (WWTT) and red colour flowers, short plants (wwtt).

The gametes formed are WT and wt
When the cross is done WT X wt (F1 generation)
The offspring formed is WwTt. Considering white and red as codominance it will produce both the type and in case of incomplete, it will be intermediate that is medium height plant. The f2 generation cross will be represented as-

| WT| Wt| wT| Wt
---|---|---|---|---
WT| WWTT (white, tall)| WWTt (white, medium)| WwTT (pink, tall)| WWTt(white medium)
Wt| WWTt (white, medium)| WWtt (white, short)| WwTt(pink, medium)| WWtt(white, short)
wT| WwTT (pink, tall)| WwTt (pink, medium)| wwTT (red, tall)| WwTt (pink, medium)
Wt| WWTt (white, medium)| WWtt (white, short)| WwTt (pink, medium)| WWtt (white, short)

From the above table, it is evident that the ratio is 1:1:1:1:4:2:2:2:2. The correct answer is C.

Note: The dihybrid ratio is 9:3:3:1 in case of dominant genes or complete dominance. An example of codominance is ABO blood grouping. An example of incomplete dominance is the flower colour in mirabilis Jalapa.