Incomplete Dominance & Snapdragon — NEET Biology
✅ Asked in NEET 2024When experiments on peas were repeated using other traits in other plants, it was found that sometimes the F₁ had a phenotype that did not resemble either of the two parents and was in between the two. The inheritance of flower colour in the dog flower (snapdragon or Antirrhinum sp.) is a good example to understand incomplete dominance. In a cross between true-breeding red-flowered (RR) and true-breeding white-flowered plants (rr), the F₁ (Rr) was pink. When the F₁ was self-pollinated the F₂ resulted in the following ratio 1 (RR) Red: 2 (Rr) Pink: 1 (rr) White. Here the genotype ratios were exactly as we would expect in any mendelian monohybrid cross, but the phenotype ratios had changed from the 3:1 dominant: recessive ratio. What happened was that R was not completely dominant over r and this made it possible to distinguish Rr as pink from RR (red) and rr (white).
Incomplete dominance occurs when the heterozygous phenotype is intermediate between the two homozygous phenotypes, not a blend of both traits but a distinct middle form. In snapdragons, RR = red, Rr = pink, rr = white. The common mistake is confusing incomplete dominance with codominance or assuming the F₁ shows one parent's trait completely. Remember: the F₂ ratio remains 1:2:1 genotypically, but the phenotype ratio also becomes 1:2:1 (not 3:1) because the heterozygote is visibly different. NTA repeatedly tests this distinction through snapdragon crosses, expecting students to identify that the intermediate phenotype proves the dominant allele is not completely dominant over the recessive one.
This paragraph was tested 3 times in NEET.
A pink flowered Snapdragon plant was crossed with a red flowered Snapdragon plant. What type of phenotype/s is/are expected in the progeny? (NEET 2024)
A certain plant homozygous for yellow seeds and red flowers was crossed with a plant homozygous for green seeds and white flowers. The F₁ plants had yellow seeds and pink flowers. The F₁ plants were selfed to get F₂ progeny. Assuming independent assortment of the two characters, how many phenotypic categories are expected for these characters in the F₂ generation? (NEET 2022)
In Antirrhinum (Snapdragon), a red flower was crossed with a white flower and in F₁ generation all pink flowers were obtained. When pink flowers were selfed, the F₂ generation showed white, red and pink flowers. Choose the incorrect statement: (NEET 2019)
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