Question

Consider the following two traits in domestic rabbits, each controlled by a single pair of contrasting alleles exhibiting complete dominance: $$ \begin{array}{|l|l|} \hline \text { hair color } & \text { hair texture } \\ \hline \text { black hair = B (dominant) } & \text { straight hair = K (dominant) } \\ \hline \text { white hair = b } & \text { kinky hair }=\mathrm{k} \\ \hline \end{array} $$ Assume that each pair of genes is located in a different pair of chromosomes (i.e., independent gene pairs). For each of the crosses, below, predict what phenotypes will occur, and in what proportions they will occur, for progeny produced by each: (a) Bb Kk \times BBKk (b) Bb kk \times Bb Kk

Short answer

For cross (a) Bb Kk x BB Kk, the phenotypic ratio of the offspring would be 9:4:3, corresponding to Black straight hair, Black kinky hair, and White kinky hair, respectively. For cross (b) Bb kk x Bb Kk, the phenotypic ratio of the offspring would be 6:2:3:1, corresponding to Black straight hair, Black kinky hair, White straight hair, and White kinky hair, respectively.

Step-by-step solution

Identify the parental genotypes

The parents' genotypes are given as BbKk (Parent 1) and BBKk (Parent 2).

Determine the gametes produced by each parent

Parent 1 can produce the following gametes: BK, Bk, bK, bk. Parent 2 can produce the following gametes: BK, Bk.

Create a Punnett Square

To find the offspring genotypes, set up a Punnett Square with Parent 1's gametes on one axis and Parent 2's gametes on the other axis. Then, fill in the squares with the appropriate offspring genotypes. In our case, the Punnett Square will look like: $$ \begin{array}{|c|c|c|} \hline & \text { BK } & \text { Bk } \\ \hline \text { BK } & \text { BBKK } & \text { BBKk } \\ \hline \text { Bk } & \text { BBKk } & \text { BBkk } \\ \hline \text { bK } & \text { BbKK } & \text { BbKk } \\ \hline \text { bk } & \text { BbKk } & \text { Bbkk } \\ \hline \end{array} $$

Determine phenotypes and their proportions

From the Punnett Square, we can see the possible genotypes, which we can then use to determine phenotypes. We have: 1 BBKK: Black straight hair 2 BBKk and 2 BbKK: Black straight hair 4 BBkk and 4 BbKk: Black kinky hair 2 Bbkk: White kinky hair The phenotypic ratio is 9:4:3 (Black straight hair: Black kinky hair: White kinky hair). (b) Bb kk x Bb Kk:

Identify the parental genotypes

The parents' genotypes are given as Bbkk (Parent 1) and BbKk (Parent 2).

Determine the gametes produced by each parent

Parent 1 can produce the following gametes: Bk, bk. Parent 2 can produce the following gametes: BK, Bk, bK, bk.

Create a Punnett Square

To find the offspring genotypes, set up a Punnett Square with Parent 1's gametes on one axis and Parent 2's gametes on the other axis. Then, fill in the squares with the appropriate offspring genotypes. In our case, the Punnett Square will look like: $$ \begin{array}{|c|c|c|c|c|} \hline & \text { BK } & \text { Bk } & \text { bK } & \text { bk } \\ \hline \text { Bk } & \text { BBKk } & \text { BbKk } & \text { BbKk } & \text { Bbkk } \\ \hline \text { bk } & \text { BbKk } & \text { bbKk } & \text { Bbkk } & \text { bbkk } \\ \hline \end{array} $$

Determine phenotypes and their proportions

From the Punnett Square, we can see the possible genotypes, which we can then use to determine phenotypes. We have: 1 BBKk: Black straight hair 2 BbKk and 2 BbKk: Black straight hair 2 Bbkk: Black kinky hair 1 bbKk: White straight hair 2 bbkk: White kinky hair The phenotypic ratio is 6:2:3:1 (Black straight hair: Black kinky hair: White straight hair: White kinky hair).

Key concepts

Genetic Inheritance

Understanding genetic inheritance is fundamental to predicting how traits will be passed on to the next generation.
Inheritance is the process by which genetic information is passed on from parent to offspring. It's a cardinal principle of genetics, harkening back to the pioneering work of Gregor Mendel in the 19th century.

At the core of Mendel's discovery are genes, which are units of hereditary information. Each organism has two alleles for each gene, one inherited from each parent. These alleles can be dominant or recessive, with the dominant allele typically masking the effects of the recessive allele.

In the rabbit example from the exercise, the genes for hair color and hair texture are inherited separately, which is known as independent assortment. This means the traits are determined by genes located on different chromosomes. In such crosses, the Punnett Square is an invaluable tool for visualizing how alleles from each parent might combine in their offspring.

When a student faces this kind of problem, it's crucial to identify the genotypes of the parents. Once we have this, we can determine what gametes each parent can produce, and thus, predict the genotypes and phenotypes of the offspring.

Dominant and Recessive Alleles

Alleles are different forms of a gene that determine distinct traits. They can be dominant or recessive.

A dominant allele is expressed even if only one copy is present. It overrides the expression of the recessive allele, which requires two copies (one from each parent) to be expressed. For hair color in rabbits, 'B' represents the dominant allele for black hair, while 'b' is the recessive allele for white hair.

In the exercise, a rabbit with the 'BB' or 'Bb' genotype will have black hair since the presence of at least one 'B' allele is enough for the dominant trait to be expressed. Conversely, 'bb' results in white hair as it requires two copies of the recessive allele for the trait to manifest.

To successfully solve inheritance problems, one must correctly interpret the dominant and recessive relationships between alleles. Through practice, students can develop a keen eye for these patterns, which is essential for determining potential offspring characteristics.

Phenotypic Ratios

The phenotypic ratio is a critical concept that reflects the diversity of offspring traits resulting from genetic crosses.

A phenotypic ratio is an expression of the different types of offspring phenotypes (observable traits) that can result from a particular genetic cross. It is depicted as a simple ratio or fraction. The Punnett Square predicts the genotype of offspring, and from those, we infer the phenotype.

In our rabbit example, the phenotypic ratios for the different crosses illustrate how traits distribute among offspring. For instance, the first cross results in a phenotypic ratio of 9:4:3 for black straight hair, black kinky hair, and white kinky hair, respectively. This tells us that out of 16 total offspring, nine are expected to have black straight hair, four black kinky hair, and three white kinky hair.

Understanding these ratios is crucial in predicting the probability of each phenotype. It requires a clear grasp of genotype to phenotype mapping and dominance relationships. Enhancing a student's problem-solving skills through practice with Punnett Squares can greatly assist in grasping phenotypic ratio calculations.