Question

In humans, the allele for black hair (B) is dominant to the allele to brown hair \((\mathrm{b})\), and the allele for curly hair \((\mathrm{C})\) is dominant to the allele for straight hair (c). When a person of unknown genotype is crossed against a straight-brown-haired individual, the phenotypic ratio is as follows: \(25 \%\) curly black hair \(25 \%\) straight black hair \(25 \%\) curly brown hair \(25 \%\) straight brown hair What is the genotype of the unknown parent? A. BbCC B. \(\mathrm{bbCc}\) C. Bbcc D. \(\mathrm{BbCc}\)

Short answer

D. \(BbCc\)

Step-by-step solution

Understand the given information

Given: The allele for black hair \((B)\) is dominant over brown hair \(b\), and the allele for curly hair \(C\) is dominant over straight hair \(c\). A straight-brown-haired individual has the genotype b\(bcc\).

Analyze the phenotypic ratio

The offspring phenotypic ratio is: 25% curly black hair, 25% straight black hair, 25% curly brown hair, 25% straight brown hair. This suggests that each phenotype appears with equal probability.

Set up a Punnett square

Set up a Punnett square by crossing the unknown parent with the genotype b\(bcc\). Since each phenotype is equally probable, the unknown parent must have one dominant and one recessive allele for each trait: \(BbCc\) to produce the given ratio.

Confirm the genotype

Cross \(BbCc\) with \(bbcc\): The Punnett square will show 4 possible combinations (BbCc, BbCc, bBCC, bbCc), each occurring with an equal probability, resulting in the 1:1:1:1 ratio observed in the offspring phenotypes.

Verify all phenotypes

Confirm that all offspring phenotypes (curly black, straight black, curly brown, and straight brown) match the phenotypic ratio given. Each phenotype should appear in 25% of the cases, verifying that the genotype of the unknown parent is indeed \(BbCc\).

Key concepts

Understanding Dominant and Recessive Alleles

In genetics, genes come in different forms called alleles. These alleles can be either dominant or recessive. Dominant alleles are represented by capital letters (e.g., B for black hair, C for curly hair), while recessive alleles are represented by lowercase letters (e.g., b for brown hair, c for straight hair).

When an individual has at least one dominant allele for a trait, the dominant allele will express itself. For example, a person with Bb (one black allele and one brown allele) will have black hair because the black hair allele (B) is dominant over the brown hair allele (b).

Conversely, a recessive trait manifests only when an individual has two recessive alleles (e.g., bb for brown hair). This leads to the different observable characteristics or phenotypes based on the combination of alleles, known as the genotype.

Utilizing a Punnett Square for Crosses

The Punnett square is a useful tool in genetics for predicting the genotypes and phenotypes of offspring from a genetic cross. It helps visualize how alleles from two parents might combine.

To create a Punnett square, place one parent's alleles on the top and the other parent's alleles on the side. Then, fill in the squares by combining the alleles from each parent. For example, when crossing a BbCc individual (unknown parent) with a bbcc individual (straight-brown-haired parent), the Punnett square will provide the possible genetic combinations of their offspring.

Here's how it looks when crossing BbCc with bbcc:
Parent 1: BbCc
Parent 2: bbcc

The possible offspring combinations will be BbCc, Bbcc, bbCc, and bbcc, which will help us determine what traits are expressed.

Decoding the Phenotypic Ratio

The phenotypic ratio in genetics is the ratio of different physical traits (phenotypes) seen in the offspring. In our example, the offspring phenotypic ratio is:

• 25% curly black hair
• 25% straight black hair
• 25% curly brown hair
• 25% straight brown hair

This 1:1:1:1 ratio suggests that each phenotype is equally probable.

From this information, we can deduce that the unknown parent must possess one dominant and one recessive allele for each trait, thus having the genotype BbCc. By crossing BbCc with bbcc and analyzing the Punnett square, we'll observe that all four phenotypic combinations occur in equal proportions.

Determining Genotype

Genotype determination involves figuring out the allelic composition of an organism for specific traits. In our example, we are given the phenotypic outcomes of a specific cross (curly black hair, straight black hair, curly brown hair, and straight brown hair) in a 1:1:1:1 ratio.

To determine the genotype of the unknown parent, we use the given phenotypic ratio and set up a Punnett square with the known genotype of the straight-brown-haired individual (bbcc).

By crossing different possible genotypes of the unknown parent with bbcc and comparing the resultant phenotypic ratios to the given data, we find that only the genotype BbCc produces the observed 1:1:1:1 phenotypic ratio.

Therefore, the correct genotype of the unknown parent, given the specific offspring phenotypes and ratios, is determined to be BbCc.