Question

Colored aleurone in the kernels of corn is due to the dominant allele \(R\). The recessive allele \(r,\) when homozygous, produces colorless aleurone. The plant color (not the kernel color) is controlled by another gene with two alleles, \(Y\) and \(y\). The dominant \(Y\) allele results in green color, whereas the homozygous presence of the recessive \(y\) allele causes the plant to appear yellow. In a testcross between a plant of unknown genotype and phenotype and a plant that is homozygous recessive for both traits, the following progeny were obtained: $$\begin{array}{lc} \text { colored, green } & 88 \\ \text { colored, yellow } & 12 \\ \text { colorless, green } & 8 \\ \text { colorless, yellow } & 92 \end{array}$$ Explain how these results were obtained by determining the exact genotype and phenotype of the unknown plant, including the precise arrangement of the alleles on the homologs.

Short answer

Answer: The unknown parent's genotype is R_ Yy (where _ indicates either R or r, as we don't have enough information to identify the second allele in the R locus), and its phenotype is colored aleurone and green plant color.

Step-by-step solution

Understand the given information

First, let's summarize the given information: - Aleurone color: Dominant allele \(R\) produces colored aleurone, recessive allele \(r\) produces colorless aleurone when homozygous (rr). - Plant color: Dominant allele \(Y\) produces green color, recessive allele \(y\) produces yellow color when homozygous (yy). - Testcross progeny results: $$\begin{array}{lc} \text{colored, green} & 88 \\ \text{colored, yellow} & 12 \\ \text{colorless, green} & 8 \\ \text{colorless, yellow} & 92 \end{array}$$ We have to determine the genotype and phenotype of the unknown parent plant.

Understand the testcross concept and identify the homozygous recessive parent

In a testcross, one individual of unknown genotype is crossed with a homozygous recessive individual. This means the homozygous recessive parent has the genotype "rr yy."

Analyzing the progeny's phenotype ratios

Based on the given ratios of progeny phenotypes, let's analyze them and determine which alleles are likely to be present in the unknown parent: 1) \(\frac{88}{100}\) colored, green and \(\frac{12}{100}\) colored, yellow suggest that the unknown parent has the dominant color allele R. 2) Similarly, the ratio of green to yellow progeny, \(\frac{88+8}{100}\) green and \(\frac{12+92}{100}\) yellow, suggests that the unknown parent has a heterozygous genotype for the plant color, meaning they have one dominant Y allele and one recessive y allele.

Determine the exact genotype and phenotype of the unknown parent

Combining the results of steps 3 and 4, we can now determine the unknown parent's genotype and phenotype. Genotype: R_ Yy (where _ indicates either R or r, as we don't have enough information to identify the second allele in the R locus.) Phenotype: colored aleurone and green plant color

Explain the progeny results based on the determined genotype

Since we determined that the unknown parent's genotype is R_ Yy, we can now explain the results obtained in the testcross progeny. 1) The R_ Yy parent crossed with the rr yy parent could produce the following genotypes: - Rr Yy (colored, green): gives the highest probability of occurrence, 88 out of 100 progeny. - Rr yy (colored, yellow): least probability of occurrence, 12 out of 100 progeny. - rr Yy (colorless, green): least probability of occurrence, 8 out of 100 progeny. - rr yy (colorless, yellow): gives the highest probability of occurrence, 92 out of 100 progeny. This matches the observed progeny results. 2) The arrangement of alleles on homologs can be depicted as: Unknown parent: RY / Ry Homozygous recessive parent: ry / ry Considering these alleles and the progeny results, we can conclude that the unknown parent's genotype is R_ Yy and its phenotype is colored aleurone and green plant color.

Key concepts

Alleles

In the world of genetics, alleles are different forms of a gene found at the same place on a chromosome. Think of them as the "options" for a gene. For example, in the corn exercise, the alleles are represented by letters such as "R" and "r" for aleurone color. The dominant allele "R" gives a colored aleurone. Similarly, "Y" and "y" are alleles for plant color, with "Y" being dominant.

Alleles are important because they determine different traits in an organism. Each individual inherits two alleles for each gene, one from each parent. These can be:

• Homozygous: both alleles are the same (e.g., "RR" or "rr").
• Heterozygous: the alleles are different (e.g., "Rr").

In a genetic testcross, like the one in the exercise, identifying combinations of alleles helps in determining how traits are passed along and expressed.

Genotype

A genotype is the genetic makeup of an organism in terms of the alleles present. It can't be seen directly because it lies in the DNA, but it plays a crucial role in determining what traits an organism will express. For example, in the corn exercise, the genotype for aleurone color can be "RR", "Rr", or "rr".

The first two genotypes ("RR" and "Rr") result in the same phenotype due to the presence of the dominant allele "R". Meanwhile, the "rr" genotype, lacking the dominant allele, results in a colorless aleurone.

Genotypes are typically presented using letters, with each letter representing an allele:

• "RR" or "Rr" - indicates the presence of the dominant allele, resulting in a dominant trait expression.
• "rr" - indicates the absence of the dominant allele, leading to the expression of the recessive trait.

Understanding genotypes helps predict how traits will appear in offspring, as shown in the exercise where the unknown plant's genotype influenced the color of aleurone and plant.

Phenotype

Phenotype refers to the observable characteristics or traits of an organism, which emerge from the genotype. In simpler terms, it's "what you see." Phenotypes may include traits like size, color, or shape. For corn, as in the exercise, phenotype examples are the color of aleurone (either colored or colorless) and the plant color (green or yellow).

Phenotypes result from the interaction of the genotype with the environment. Hence, two organisms may have the same phenotype but different genotypes; for example, both "RR" and "Rr" genotypes result in colored aleurone.

Understanding the phenotype helps in studying how genetic traits are expressed in the real world. In a testcross:

• Colored, green (observed when the dominant alleles are present)
• Colorless, yellow (occurs when recessive alleles are present)

Analyzing phenotypes along with genotypic information can provide insights into how traits are passed down through generations, helping solve puzzles like predicting the traits of offspring when parents' phenotypes are known.