A dark-red strain and a white strain of wheat are crossed and produce an intermediate, medium-red F₁. When the F₁ plants are interbred, an F₂ generation is produced in a ratio of 1 dark-red: 4 medium-dark-red: 6 medium-red: 4 light-red: 1 white. Further crosses reveal that the dark-red and white F₂ plants are true breeding
Assign symbols to these alleles, and list possible genotypes that give rise to the medium-red and light-red phenotypes.

Verified step by step guidance

1. Step 1: Recognize that the inheritance pattern described is an example of incomplete dominance, where the heterozygote shows an intermediate phenotype between the two homozygotes. The parental strains are dark-red and white, and the F₁ is medium-red, an intermediate color.
2. Step 2: Assign allele symbols to the two alleles involved. For example, let \( R_d \) represent the dark-red allele and \( R_w \) represent the white allele. Since the F₁ is intermediate, the heterozygote \( R_d R_w \) shows medium-red color.
3. Step 3: Analyze the F₂ phenotypic ratio: 1 dark-red : 4 medium-dark-red : 6 medium-red : 4 light-red : 1 white. This suggests multiple genotypic classes with varying dosages of the alleles, indicating that the trait is controlled by more than one allele or by gene dosage effects.
4. Step 4: Propose genotypes for each phenotype based on allele dosage. The homozygotes \( R_d R_d \) and \( R_w R_w \) correspond to dark-red and white, respectively (true breeding). The intermediate phenotypes (medium-dark-red, medium-red, light-red) likely correspond to heterozygotes with different combinations or dosages of the alleles, such as \( R_d R_w \), \( R_d R_d R_w \), or \( R_d R_w R_w \) if multiple alleles or gene copies are involved.
5. Step 5: Specifically list possible genotypes for medium-red and light-red phenotypes. For example, medium-red could be the simple heterozygote \( R_d R_w \), while light-red could be a genotype with more white allele dosage, such as \( R_d R_w R_w \) or a similar combination depending on the genetic model.

Key Concepts

Incomplete Dominance

Incomplete dominance occurs when heterozygous individuals display a phenotype intermediate between the two homozygous parents. In this wheat example, the F₁ medium-red color results from blending of dark-red and white alleles, rather than one being completely dominant over the other.

Genotype-Phenotype Relationship and Allele Symbol Assignment

Assigning symbols to alleles helps track inheritance patterns. Typically, the darkest phenotype is assigned the dominant allele (e.g., R), the lightest the recessive (e.g., r), and intermediate phenotypes arise from heterozygous or multiple allele combinations. Understanding which genotypes correspond to each phenotype is key.

F₂ Phenotypic Ratios and True Breeding

The F₂ generation's phenotypic ratio reflects the segregation and combination of alleles. True breeding plants (dark-red and white) are homozygous, confirming allele assignments. The complex 1:4:6:4:1 ratio suggests multiple allele interactions or codominance, requiring analysis of genotype combinations for intermediate phenotypes.