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): \(1/4\).

Key concepts

Punnett Square

A Punnett square is a simple, graphical tool used by geneticists to predict the possible genotypes of offspring from a particular genetic cross. It's a grid that helps visualize how alleles from two parents can combine.
First, list the alleles from one parent along the top and the other parent's alleles down the left-hand side of the grid. Each box within the square is filled in with one allele from each parent, showing all possible genetic combinations for their offspring.
This square can illustrate potential genetic outcomes, such as physical traits or inherited conditions, making it a helpful tool in understanding basic genetic inheritance.

Recessive Allele

A recessive allele is a version of a gene that must be inherited from both parents in order for the associated trait to manifest. It usually contains information for a trait that can be hidden by a dominant allele.

• In genetics, alleles are different forms of the same gene. They are located at the same position, or gene locus, on a chromosome.
• A recessive allele is represented by a lowercase letter, such as 'w' for a straight hairline.
• Recessive traits appear only when an individual has two recessive alleles, one from each parent.

Recessive alleles require specific conditions to express their traits, generally when the dominant allele is absent.

Heterozygous Genotype

When an individual is heterozygous for a gene, it means they have two different alleles for a particular gene. In the context of the exercise, each parent has one dominant allele for a widow's peak (W) and one recessive allele for a straight hairline (w).
This results in the genotype Ww, where the dominant allele masks the expression of the recessive allele, resulting in the physical appearance of a widow's peak.
Heterozygous genotypes are important in predicting genetic outcomes, as they can pass either allele to their offspring, influencing the genetic combinations and appearance of traits in future generations.

Dominant Allele

A dominant allele is an allele that is expressed in an individual even if only one copy is present. It overrules the effect of a recessive allele in determining the organism's traits.

• Dominant alleles are usually represented by uppercase letters, such as 'W' for a widow's peak.
• Only one dominant allele is needed to express the trait it codes for.
• If a dominant allele is present, it will mask the presence and effect of a recessive allele in the genotype.

Understanding dominant alleles helps in predicting how traits are passed through generations, crucial for genetic inheritance studies.