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 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 testross between a plant of unknown genotype and phenotype and a plant that is homozygous recessive for both traits, the following progeny were obtained: colored, green 88 colored, yellow 12 colorless, green 8 colorless, yellow 92 Explain how these results were obtained by determining the exact genotype and phenotype of the unknown plant, including the precise association of the two genes on the homologs (i.e., the arrangement).

Short answer

Answer: The genotype of the unknown plant is RyYy, and its phenotype is colored aleurone and green plant color.

Step-by-step solution

Determine the traits of each progeny and the expected ratio

From the given information, we know the dominant and recessive alleles: Colored aleurone (R), colorless aleurone (r), green plant color (Y), and yellow plant color (y). With the given data of the progeny, let's organize their genotypes and phenotypes: 1. Colored, green: 88 (Genotype: R_Y_) 2. Colored, yellow: 12 (Genotype: R_yy) 3. Colorless, green: 8 (Genotype: rrY_) 4. Colorless, yellow: 92 (Genotype: rryy) To make things easier, let's divide each number by 4: 1. 88/4 = 22 2. 12/4 = 3 3. 8/4 = 2 4. 92/4 = 23 These ratios indicate that the proportions of the different phenotypes are 22:3:2:23.

Determine the genotype of the unknown plant

In a testcross, the unknown plant is crossed with another plant that is homozygous recessive for both traits (rryy). Since we know that the proportions of the different phenotypes in the progeny are 22:3:2:23, we will use them to determine the genotype of the unknown plant. Let's analyze the second characteristic (yellow color): 2. colored, yellow: 12 (Genotype: R_yy) Since colored is dominant, the genotype cannot be rr and it will be Ry. Now, to determine the alleles for green or yellow color, let's analyze the other phenotypes: 1. Colored, green: 88 (Genotype: R_Y_) The only combination is RyY_. Looking at the other two phenotypes: 3. Colorless, green: 8 (Genotype: rrY_) 4. Colorless, yellow 92 (Genotype: rryy) Comparing the number of progeny with yellow and green plants, we can understand the genotype of the unknown genotype of the green color gene should be heterozygous. So the unknown plant genotype should be RyYy.

Determine the phenotype of the unknown plant

From the genotype RyYy, we can determine the phenotype of the unknown plant. Since it has the dominant R allele, it will have colored aleurone. Since it has the dominant Y allele, it will be green in color. Therefore, the phenotype of the unknown plant is colored aleurone and green plant color.

Determine the arrangement of the two genes on the homologs

The arrangement of the genes on the homologs can be determined using the ratios from the progeny. Given that the ratio is approximately 22:3:2:23, the two genes must be unlinked or very weakly linked; if they were closely linked or if there were complete linkage, the ratios would be different. Therefore, the arrangement of the two genes on the homologs must allow for independent assortment during gamete formation. The precise association is unknown based on the information given, but the unlinked or weakly linked arrangement explains the observed progeny ratios. As a result, through many steps of analysis, we obtained the exact genotype (RyYy) and phenotype (colored aleurone, green plant color) along with the arrangement of the genes for the unknown plant, explaining the results of the progeny as given in the exercise.

Key concepts

Mendelian Inheritance

Mendelian inheritance, a core principle of genetics, was first articulated by Gregor Mendel in the 19th century. It lays the foundation for understanding how traits are passed from one generation to the next. This concept introduces the idea of dominant and recessive alleles.

Each organism inherits two alleles for each trait, one from each parent. Dominant alleles mask the expression of recessive ones. In the exercise, the dominant allele "R" leads to a colored aleurone, while the recessive allele "r" results in a colorless aleurone when homozygous.

The plant color gene operates similarly, with "Y" producing green plants and "y" generating yellow plants in a homozygous recessive state. This interplay of alleles is what shapes the phenotype observed. Mendelian inheritance also introduces concepts like homozygosity, where both alleles are identical, and heterozygosity, where the alleles are different.

Through understanding Mendelian principles, we can predict and explain the traits seen in offspring based on the alleles present in their parents.

Testcross

A testcross is a useful method in genetics to determine the genotype of an organism exhibiting a dominant trait. To perform a testcross, the organism in question is crossed with another organism that is homozygous recessive.

This approach was employed in the exercise to discover the genotype of a plant with dominant traits. By crossing an unknown plant with recessive phenotypes ("rryy" in our scenario), we observe the variety of phenotypes in the progeny. If the dominant phenotype is present in offspring, the unknown parent must carry at least one dominant allele for that trait.

With the progeny ratios of 22:3:2:23, the unknown's genotype was deduced as "RyYy." This testcross reveals the hidden recessive genes when possible, pinpointing the exact genetic mixture of the parent plant. It is a critical tool for geneticists when mapping inheritance patterns.

Phenotype and Genotype

Phenotypes are the observable characteristics of an organism, like color or height. They result from the interaction of the organism's genotype with the environment. A genotype, on the other hand, is the genetic makeup of an organism, composed of alleles inherited from its parents.

This exercise helps illustrate these concepts through corn kernel color and plant color as phenotypic traits. The genotype includes alleles such as "R" and "r" or "Y" and "y" from each parent plant, forming different combinations like "RyYy."

The observed traits, or phenotypes, result from these genetic combinations. A "colored aleurone, green plant" phenotype stems from the presence of at least one dominant "R" allele and one "Y" allele in the genotype, demonstrating the direct link between genotype and phenotype.

Understanding the distinction and relationship between phenotype and genotype is fundamental in genetics, enabling us to predict the likelihood of traits in offspring and understand genetic variability within populations.