Question

suppose that you had a male mouse with black eyes and a female mouse with red eyes. you know that the allele for black eyes is dominant, and you want to determine whether the male is heterozygous. to do so, you would do a test cross. breed them and look for approximately a ratio of black eyes to red eyes. 100% to 0% 75% to 25% 50% to 50% 25% to 75% 0% to 100%

Explanation:

Step1: Define genotypes

Let $B$ = dominant black eye allele, $b$ = recessive red eye allele.
Male (black eyes): if heterozygous, genotype = $Bb$;
Female (red eyes, recessive phenotype): genotype = $bb$.

Step2: Set up test cross

Cross $Bb \times bb$. Use Punnett square:

Gametes	$B$	$b$
$b$	$Bb$	$bb$

Step3: Calculate phenotype ratio

$Bb$ (black eyes) = 2 out of 4, $bb$ (red eyes) = 2 out of 4.
Ratio = $\frac{2}{4} : \frac{2}{4} = 50\% : 50\%$

Answer:

50% to 50%