Final answer:
Both parents have the genotype Bb for hair color, where black (B) is dominant over brown (b). Using a Punnett square, 75% of their offspring are expected to have black hair as this is shown by three out of the four possible genotype combinations (BB, Bb, Bb).
Step-by-step explanation:
To determine the percentage of offspring with black hair from parents both with genotype Bb, we use a Punnett square. The alleles from one parent (B and b) are placed at the top, and from the other parent (B and b) on the left side of the square. After drawing the alleles down and across from each parent into the boxes, the resulting genotypes of the offspring can be as follows: BB, Bb, Bb, and bb.
Because black hair (B) is dominant over brown hair (b), offspring with either BB or Bb genotype will have black hair. Hence, the possibility of having black hair in the offspring is represented by BB, Bb, and Bb, which are three out of the four possible combinations. Therefore, the percentage of offspring with black hair would be 75%.
When both parents have the Bb genotype, there is a 75% chance that their offspring will have black hair, as three out of four genotypic combinations (BB, Bb, Bb) present the dominant phenotype.