Final answer:
The probability of two heterozygous parents (Ww) having an offspring without a widow's peak (straight hairline) is 25%, as per Mendelian genetics and the outcomes illustrated by a Punnett Square.
Step-by-step explanation:
The question concerns the probability of an offspring inheriting a particular genetic trait, specifically a widow's peak, when both parents are heterozygous for that trait. When solving this type of problem, we use the principles of Mendelian genetics to determine the genotypic ratios of possible offspring. Each heterozygous parent (Ww) can pass on two possibilities for alleles: either the dominant allele (W) or the recessive allele (w). When we create a Punnett Square to represent the cross of two Ww parents, we find that there are four possible genotypic outcomes for the offspring: WW, Ww, Ww, and ww. The offspring with the ww genotype would be the only ones to exhibit a straight hairline, as the recessive trait is expressed only in the absence of the dominant allele. Since there is only one ww out of four possible outcomes, the probability of two heterozygous parents having an offspring with a straight hairline (no widow's peak) is 1 in 4, or 25%.