Question

Two different genes, located on two different chromosomes, are responsible for color production in the aleurone layer of com kernels. For color production (either purple or red), the dominant alleles of these two genes \((C \text { and } R\) ) must come together. Furthermore, a third gene, located on a third chromosome, interacts with the \(C\) and \(R\) alleles to determine whether the aleurone will be red or purple. While the dominant allele ( \(P\) ) ensures purple color, the homozygous recessive condition (pp) makes the aleurone red. Determine the \(\mathrm{P}_{1}\) phenotypic ratio of the following crosses: (a) \(C C r r P P \times \operatorname{ccRRp} p\) (b) \(C c R R p p \times C C R r p p\) (c) \(\operatorname{CcRrPp} \times\) CcRrpp.

Short answer

Answer: The phenotypic ratios of the offspring for each cross are as follows: (a) 100% Purple, (b) 100% Purple, and (c) 2:1 Purple:Red.

Step-by-step solution

Cross (a): CCrrPP x ccRRpp

In this cross, we will first determine the gametes produced by each parent and then find the possible genotypic combinations for the offspring. Since C and R are dominant and required for color production, we will need the presence of at least one C and R allele for the color to be expressed: Parent 1: CCrrPP - gametes: CRP and CRp Parent 2: ccRRpp - gametes: cRp Offspring genotypes: CcRRPp Since the offspring has at least one C and R allele, color will be produced. The presence of the dominant P allele ensures a purple color. Therefore, the P1 phenotypic ratio for this cross is: 100% Purple

Cross (b): CcRRpp x CCRRpp

In this cross, we will follow the same procedure as before to determine the possible genotypic combinations: Parent 1: CcRRpp - gametes: CRp and cRp Parent 2: CCRRpp - gametes: CRp Offspring genotypes: CCRRpPp and CcRRPp Both genotypes have at least one C and one R allele, so color will be produced. However, both genotypes are heterozygous for the P gene (Pp), which results in a purple color. The P1 phenotypic ratio for this cross is: 100% Purple

Cross (c): CcRrPp x CcRrpp

This cross involves all heterozygous parents with respect to the three genes. To find the gametes produced by each parent, we need to consider the possible allele combinations for each gene. Parent 1: CcRrPp - gametes: CRP, CRp, cRP, cRp Parent 2: CcRrpp - gametes: CRp, cRp Now, let's find the possible genotypic combinations for the offspring and the corresponding phenotypes by considering the presence of the required alleles: Offspring genotypes and phenotypes: 1. CCRrPp (Purple) 2. CcRRPp (Purple) 3. CCRrpP (Purple) 4. CcRrpP (Purple) 5. CcRRpp (Red) 6. CcRrpp (Red) The P1 phenotypic ratio for this cross is: 4:2 or 2:1 Purple:Red

Key concepts

Dominant and Recessive Alleles

The concept of dominant and recessive alleles is fundamental to understanding genetic inheritance. Alleles are different forms of a gene, and they can be dominant or recessive. A dominant allele is one that expresses its traits even if there is only one copy present. This means that if an organism inherits a dominant allele from one parent, the trait associated with it will appear in the offspring.
In contrast, a recessive allele requires two copies to express its traits. If an organism has one dominant and one recessive allele, the dominant trait will be the one that is manifested.

• Dominant alleles are often represented by uppercase letters (e.g., C, R, P).
• Recessive alleles are represented by lowercase letters (e.g., c, r, p).

In the given exercise, the alleles C and R are dominant and crucial for the production of color. If an organism lacks either one of these dominant alleles, the color trait will not be properly expressed. The P allele influences the shade of color, with P for purple being dominant over p which results in red when present in homozygous form.

Phenotypic Ratio

The phenotypic ratio is used to describe the relative number of offspring displaying particular traits from a genetic cross. It reflects how likely different traits are to show up in the progeny.
After performing a genetic cross, we analyze the possible outcomes to determine phenomena such as color, shape, or size, based on the inherited combinations of alleles.

• A phenotypic ratio gives us a sense of the distribution of phenotypes among the offspring.
• In genetics problems, common ratios include 3:1, 9:3:3:1, etc., depending on the type of cross and number of traits involved.

For instance, in the exercise provided:

• The phenotypic ratio for cross (a) is 100% Purple, as all offspring show the purple trait.
• In cross (b), similarly, the ratio is 100% Purple since all have at least one P allele.
• In cross (c), we saw a mix resulting in a 4:2 ratio, or simplified, a 2:1 Purple to Red phenotype.

Allelism

Allelism refers to the variations or different forms any one gene may have. In genetics, when discussing allelism, we are focusing on how different alleles can cause variations in the trait of an organism, which in turn affects its phenotype.
Each parent contributes one allele for every gene leading to a pair of alleles. These alleles can be dominant or recessive, and they determine the expression of traits.

• Alleles can be homozygous, where both alleles are the same (e.g., CC or pp).
• They can also be heterozygous, where the alleles differ (e.g., Cc or Pp).

In the provided task, we see allelism in action as the variation in the P allele determines whether the aleurone layer is purple or red. Understanding allelism helps us predict the outcome of genetic crosses and the resultant phenotype based on allele combinations from each parent.

Dihybrid Cross

A dihybrid cross is a genetic mix between two individuals who have heterozygous genotypes for two specific traits or alleles. This kind of cross helps demonstrate how different genes independently assort and combine to pass traits from the parents to the offspring.
The classic example of calculating outcomes in a dihybrid cross involves predicting all combinations of two genes, each having two alleles.

• A typical dihybrid cross might involve traits represented as four possible allele combinations, like CcRr.
• The Punnett square method is often used to ascertain the four possible genotype combinations for the offspring: CR, Cr, cR, cr.

In our context with corn kernels' alleurone color, examining crosses such as *Cross (c): CcRrPp x CcRrpp*, demonstrates a dihybrid cross where alleles assort independently. This results in varied phenotype ratios and requires evaluating each gene combination separately to identify which dominant and recessive traits will manifest in the offspring. The analysis gives insight into the possibility of different phenotypic outcomes, such as the mentioned Purple or Red colors, based on the genetic makeup.