Question

Three gene pairs located on separate autosomes determine flower color and shape as well as plant height. The first pair exhibits incomplete dominance, where the color can be red, pink (the heterozygote), or white. The second pair leads to personate (dominant) or peloric (recessive) flower shape, while the third gene pair produces either the dominant tall trait or the recessive dwarf trait. Homozygous plants that are red, personate, and tall are crossed to those that are white, peloric, and dwarf. Determine the F₁ genotype(s) and phenotype(s). If the F₁ plants are interbred, what proportion of the offspring will exhibit the same phenotype as the F₁ plants?

Accepted Answer

Identify the genotypes of the parental plants based on the given traits and inheritance patterns. For the first gene pair with incomplete dominance (flower color), assign RR for red (homozygous dominant) and WW for white (homozygous recessive). For the second gene pair (flower shape), use PP for personate (dominant homozygous) and pp for peloric (recessive homozygous). For the third gene pair (plant height), use TT for tall (dominant homozygous) and tt for dwarf (recessive homozygous). Thus, the red, personate, tall parent is RR PP TT, and the white, peloric, dwarf parent is WW pp tt. Determine the F₁ genotype by crossing the two homozygous parents. Since each parent is homozygous for each gene, all F₁ offspring will be heterozygous for each gene pair: RW for color, Pp for shape, and Tt for height. Write the F₁ genotype as RW Pp Tt. Describe the F₁ phenotype based on the genotypes and inheritance patterns. For incomplete dominance in color, RW results in pink flowers. For flower shape, Pp results in personate flowers (dominant trait). For height, Tt results in tall plants (dominant trait). So, the F₁ phenotype is pink, personate, and tall. To find the proportion of F₂ offspring (from F₁ interbreeding) that exhibit the same phenotype as the F₁ plants, analyze each gene separately. For incomplete dominance (color), the heterozygote RW shows pink flowers. For flower shape and height, the dominant phenotype appears in both homozygous dominant and heterozygous genotypes. Calculate the probability for each trait to show the F₁ phenotype in the F₂ generation: For color, the heterozygous RW genotype frequency is 1/2 (from RW x RW cross). For flower shape, the dominant phenotype frequency (PP or Pp) is 3/4. For height, the dominant phenotype frequency (TT or Tt) is also 3/4. Multiply these probabilities to get the overall proportion of F₂ offspring with the same phenotype as the F₁ plants.

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Key Concepts

Incomplete Dominance

Mendelian Inheritance of Independent Genes

Genotypic and Phenotypic Ratios in Dihybrid and Trihybrid Crosses