Another type of monohybrid inheritance involves the expression of both phenotype
ID: 63282 • Letter: A
Question
Another type of monohybrid inheritance involves the expression of both phenotypes in the heterozygous situation. This is called codominance.
One well-known example of codominance occurs in the coat color of Shorthorn cattle. Those with reddish-gray roan coats are heterozygous (RR’), and result from mating between a red (RR) Shorthorn and one that’s white (R’R’). Roan cattle don’t have roan-colored hairs, as would be expected with incomplete dominance, but rather appear roan as a result of having both red and white hairs. Thus, the roan coloration is not a consequence of pigments blending in each hair. Because of R and R’ alleles are both fully expressed in the heterozygote, they are dominant.
If a roan Shorthorn cow is mated with a white bull, what will be the genotypic and phenotypic ratios in the F1 generation?
Genotypic ratio:
Phenotypic ratio:
List the parental genotypes of crosses that could produce at least some
White offspring:
Roan offspring:
Explanation / Answer
It is understood from the condition that roan coats has the genotype of RR’, while, the white bull should have R’R’ (since, they are codominant). Let us follow the cross based on this genotype.
Cross 1:
RR’ X R’R’
The gametes for these two genotypes would ultimately be R, R’ for the roan shorthorn cow, while, it would be R’ and R’ for the white cow. Based on Punnet Square the following are the genetic traits that could be a result of the cross between a roan shorthorn cow and a white bull.
F1 Generation
Genotypic ratio:
R
X
R’
Y
R’
X
RR’
XX
(Roan shorthorn cow)
R’R’
XY
White bull
R’
X
RR’
XX
(Roan shorthorn cow)
R’R’
XY
White bull
Phenotypic ratio - The cross would produce 50% of roan shorthorn cow and 50% of white bull.
Cross 2:
What happens if a roan shorthorn cow is crossed with a roan shorthorn bull?
RR’ (Roan shorthorn cow) X RR’ (Roan shorthorn bull)
R
X
R’
Y
R
X
RR
XX
red shorthorn cow
RR’
XY
Roan shorthorn bull
R’
X
RR’
XX
Red shorthorn cow
R’R’
XY
White bull
Thus it could be ascertained that this cross has produced an offspring that is homozygous from both red shorthorn and white animal.
Cross 3:
What happens if a white cow is crossed with a white bull?
R’R’ (White cow) X R’R’ (White bull)
R’
X
R’
Y
R’
X
R’R’
XX
White cow
R’R’
XY
White bull
R’
X
R’R’
XX
White cow
R’R’
XY
White bull
Thus it could be ascertained that in this cross the offspring produced would be completely white.
Cross 4:
What happens if a red cow is crossed with a red bull?
RR (Red cow) X RR (Red bull)
R
X
R
Y
R
X
RR
XX
Red cow
RR
XY
Red bull
R
X
RR
XX
Red cow
RR
XY
Red bull
Thus it could be ascertained that in this cross the offspring produced would be completely red.
R
X
R’
Y
R’
X
RR’
XX
(Roan shorthorn cow)
R’R’
XY
White bull
R’
X
RR’
XX
(Roan shorthorn cow)
R’R’
XY
White bull