Why is red color dominant in 4o'clock plant?

[PhyStudent20]

Mirabilis Jalapa exhibits incomplete dominance.
So, if we cross Red and White colored plants (Parental Gen), we get, pink flowers (Filial I Gen).
When these are selfed, we get red, pink, white in ratio 1:2:1
So, how do you determine that Red is dominant, because, we are dealing with incomplete dominance here?

 

[Simon Bridge]

The dominant one is usually which of the two alleles present in the heterozygote is expressed, such that the organism is phenotypically identical to one of the two homozygotes ... but in this case, none of the heterozygotes are identical to the homozygote. That your problem?

Probably the red allel is the one that makes the color while the "not red" allel doesn't... which means you need to look more closely.
The red is then the result of twice as much pigment getting made - since the action of the R defeats the default of r it is dominant.
I don't know if that is the case for the specific plant you cite. Not a botanist.
Look up "haplosufficiency"... there are other ways.

 

[Suraj M]

how do you determine that Red is dominant

Well you can't,
nothing can be said, red is not the dominant gene, simply because of incomplete dominance. so there is no dominant gene in this case :)(similarly in snapdragon also)