Question

A straight hairline is recessive. If two parents with a widow's peak have a child with a straight hairline, then what is the chance that their next child will have a straight hairline? a. no chance b. \(1 / 4\) c. \(3 / 16\) d. \(1 / 2\) e. \(1 / 16\)

Short answer

The probability that their next child will have a straight hairline is \(1/4\).

Step-by-step solution

Determine the parents' genotypes

Given that two parents with a widow's peak have a child with a straight hairline, it means that both parents carried the recessive allele for the straight hairline, making them heterozygous. Therefore, both parents will have the genotype Ww, where W represents the dominant widow's peak allele and w represents the recessive straight hairline allele.

Set up a Punnett square

Create a 2x2 Punnett square. Label one parent's alleles along the top of the square and the other's along the left side. ``` W w ``` ``` W ``` ``` w ```

Fill in the Punnett square

Fill in the boxes by combining the alleles in the corresponding rows and columns. ``` W w ``` ``` W WW Ww ``` ``` w Ww ww ```

Determine the probability of a straight hairline

Examine the Punnett square: the probability of a straight hairline (genotype ww) is the ratio of offspring with the ww genotype to the total number of offspring represented in the square. One out of the four possible offspring (ww) has a straight hairline, so the probability is 1/4. The correct answer is option (b): \(1/4\).