Question

The allele for black fur $\left(F^B\right)$ is co-dominant with white fur $\left(F^W\right)$ in mice and when both the alleles are present together, the mice will have black and white spotted fur.
The allele for brown eyes (B) is dominant over blue eyes (b) in mice.
If two mice heterozygous for both traits are crossed, what will be the number of mice showing black and white spotted fur with blue eyes?

• A. Eight
• B. Three
• C. Two
• D. Four

Answer 1

The correct option is C Two

When two mice heterozygous for both genes ($F^B$ $F^W$ Bb) are crossed, the offspring are as follows:
$\begin{array}{ccccc}& F^{B}B& F^{B}b& F^{W}B& F^{W}b\\ & & & & \\ F^{B}B& F^B{F}^{B}BB& F^B{F}^{B}Bb& F^B{F}^{W}BB& F^B{F}^{W}Bb\\ & \text{(black, brown)}& \text{(black, brown)}& \text{(spotted, brown)}& \text{(spotted, brown)}\\ & & & & \\ F^{B}b& F^B{F}^{B}Bb& F^B{F}^{B}bb& F^B{F}^{W}Bb& F^B{F}^{W}bb\\ & \text{(black, brown)}& \text{(black, blue)}& \text{(spotted, brown)}& \text{(spotted, blue)}\\ & & & & \\ F^{W}B& F^B{F}^{W}BB& F^B{F}^{W}Bb& F^W{F}^{W}BB& F^W{F}^{W}Bb\\ & \text{(spotted, brown)}& \text{(spotted, brown)}& \text{(white, brown)}& \text{(white, brown)}\\ & & & & \\ F^{W}b& F^B{F}^{W}Bb& F^B{F}^{W}bb& F^W{F}^{W}Bb& F^W{F}^{W}bb\\ & \text{(spotted, brown)}& \text{(spotted, blue)}& \text{(white, brown)}& \text{(white, blue)}\end{array}$
Thus, 2 offspring are of the phenotype black and white spotted fur with blue eyes.