Snapdragon Genetics Calculators

0 calculators tagged with “Snapdragon Genetics”

Snapdragons (Antirrhinum majus) are a classic genetics model organism that demonstrates incomplete dominance — a inheritance pattern where heterozygous offspring show an intermediate phenotype rather than one of the two parental phenotypes. When a red-flowered snapdragon (RR) is crossed with a white-flowered one (WW), all F1 offspring are pink (RW), not red or white. This blending of phenotypes challenges the simple dominance-recessiveness framework and reveals that gene dose can affect pigment production. The F2 generation from pink × pink produces a 1:2:1 ratio (red:pink:white) rather than the 3:1 ratio seen in simple Mendelian inheritance.

All Calculators

No calculators found for this topic.

Incomplete Dominance in Snapdragons

In snapdragon flower color, the R allele codes for a functional enzyme that produces red pigment (chalcone synthase pathway). The W allele is nonfunctional. In RR homozygotes, sufficient enzyme is made for full red pigment. In WW homozygotes, no pigment is produced (white flowers). In RW heterozygotes, only half the normal enzyme level is produced, resulting in pink flowers — an intermediate phenotype.

F1 and F2 Cross Results

  • P: RR × WW → F1: all RW (pink)
  • F1: RW × RW → F2: 1 RR (red) : 2 RW (pink) : 1 WW (white)

The 1:2:1 phenotype ratio is characteristic of incomplete dominance because heterozygotes are distinguishable from both homozygotes.

Incomplete Dominance vs. Codominance

Incomplete dominance produces a blended intermediate phenotype (pink from red + white). Codominance is different — both alleles are fully expressed simultaneously. In codominance, a red × white cross would produce offspring with distinct red and white patches, not a blended pink. The ABO blood type system is a classic codominance example (AB blood type expresses both A and B antigens).

Molecular Basis

Incomplete dominance at the molecular level typically reflects haploinsufficiency — a single functional copy of a gene is insufficient to produce the full wild-type phenotype. This is common for enzymes in biosynthetic pathways where product concentration scales with enzyme dose.

Glossary

Incomplete Dominance
An inheritance pattern where heterozygotes show a phenotype intermediate between the two homozygous phenotypes; caused by gene dosage effects on product concentration.
Haploinsufficiency
A condition where a single functional copy of a gene is insufficient to produce the full wild-type phenotype; the molecular basis of many incomplete dominance examples.
Codominance
An inheritance pattern where both alleles are fully expressed simultaneously in heterozygotes, producing a combined phenotype rather than a blended intermediate.

Frequently Asked Questions

Snapdragon flower color is the textbook example of incomplete dominance. When red-flowered (RR) and white-flowered (WW) plants are crossed, all F1 offspring are pink (RW). Pink is an intermediate phenotype — neither allele is fully dominant. In the F2 generation from pink × pink, a 1:2:1 ratio of red:pink:white appears, showing that heterozygotes produce only half the normal amount of red pigment enzyme.

Incomplete dominance is an inheritance pattern in which heterozygotes show an intermediate phenotype between the two homozygous phenotypes. Neither allele is fully dominant — the heterozygote RW is distinguishable from both RR and WW. This contrasts with complete dominance, where heterozygotes look identical to the dominant homozygote. Incomplete dominance often results from gene dosage effects on enzyme concentration.

In incomplete dominance, heterozygotes show a blended intermediate phenotype (e.g., pink from red and white). In codominance, both alleles are fully and simultaneously expressed in heterozygotes — for example, AB blood type expresses both A and B surface antigens, not a blend. The key distinction is blending (incomplete dominance) vs. simultaneous full expression of both alleles (codominance).

Snapdragons provide a clear, visible demonstration of incomplete dominance that students can observe directly. The three distinct flower colors — red, pink, and white — are easy to score, the 1:2:1 F2 ratio is diagnostic, and the plants are easy to grow and cross. They also show that Mendel's simple dominance-recessiveness model is a special case — many genes show intermediate or additive effects, making snapdragons a valuable pedagogical example.