Why are some genes recessive and dominant and some mix; tall and short flowers, and red and white combine pink

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Why are some genes recessive and dominant and some mix; tall and short flowers, and red and white combine pink
Some alleles are dominant because the gene product (the protein) "overwhelms" any recessive alleles that are also expressed.



For example, if you make a functional enzyme, and also produce a non-functional enzyme, but you can get on fine with only 50% of your enzyme functioning, then the functioning enzyme will be dominant. Also - the feedback mechanisms to control gene expression may well stay "on" if not enough functioning protein is made, so you'll end up with 200% of the normal amount of protein, with only half of it functional, but the end result is the same as if it was 100% made with it all functional. So if you cross red and white flowers, and the red allele makes pigment, but the white allele doesn't, you'll still end up with a red flower in the progeny.



Other times, the non wild-type allele can produce a protein that actually *interferes* with the function of the normal, wild-type protein. For example, maybe it binds irreversably to the substrate, preventing normal binding of the wild-type protein. In this case, the mutant allele acts like an inhibitor of the normal protein, and will be dominant.

So if you cross white and red flowers, and the "white" allele interferes with the metabolic pathway for making the red pigment, the progeny will all be white.



"Mixing" can happen when the different alleles make different end-proteins that are both expressed, but don't interfere with each other. So if a red and white flower are crossed, the progeny might make both red and white pigment, and will be pink; this is "Codominance" (and is what is seen in human ABO blood types)

Alternatively, the plant may just make half the normal amount of pigment, and will therefore be pink (Incomplete Dominance).