Final answer:
When crossing BbMm with BbMm, a Punnett square can be used to determine the fraction of offspring expected to have magenta petals. The expected fraction is 1/4 or 25%.
Step-by-step explanation:
When crossing BbMm with BbMm, we can use a Punnett square to calculate the expected ratio of offspring with magenta petals. The B allele represents the dominant trait for magenta petals, and the b allele represents the recessive trait for white petals.
Similarly, the M allele represents the dominant trait for magenta petals, and the m allele represents the recessive trait for purple petals. In this case, since both parents are heterozygous (Bb and Mm), the Punnett square will have 4 possible combinations: BBMM, BBMm, BbMM, and BbMm. Out of these, only 1 combination (BbMm) will result in magenta petals. Therefore, the fraction of offspring expected to have magenta petals is 1/4 or 25%.