On the surface, we can see two things are happening with the melanin pigments in recessive red birds. First, the feathers have mostly red melanin, with very little black and brown. Second, the melanin is distributed evenly in all feathers, disrupting any underlying pattern or spread genotype.
Inside a bird’s cells, the molecular mechanism of recessive red is fairly complex and not completely understood. But the parts we do understand are interesting and worth the effort to learn.
The color and the recessive red genes both affect the synthesis of pigments called melanins, which are made in structures called melanosomes in cells called melanocytes. But the two genes code for proteins that work on different steps of melanin synthesis. The color gene is Tyrp1, and it codes for the TYRP1 protein. (For more details, visit color.) The recessive red gene is called Sox10, and it codes for the SOX10 protein.
The SOX10 protein’s normal function is to switch other genes ‘on,’ including Tyrp1. When genes are switched on, proteins are made from them, and the proteins are available to do their jobs.
The ‘not recessive red’ allele is a “normal” version of the Sox10 gene. The gene is normally turned on in melanocytes. The SOX10 protein that’s made from it switches the Tyrp1 gene on, and TYRP1 protein is made (or in the case of the brown allele, not made). TYRP1 then goes off to do its job in the melanin synthesis pathway.
The ‘recessive red’ allele is a “broken” version of Sox10. The allele is missing a large chunk of DNA that allows it to be switched on in melanocytes. While it can still be switched on in other cells, its activity is very low in melanocytes. When a bird has just one broken Sox10 allele, the other allele still makes enough SOX10 protein to switch on color/Tyrp1 (and other genes). But when both alleles are broken, little SOX10 protein is made, and the genes it regulates stay switched off. This mechanism explains the recessive inheritance pattern of recessive red.
The reason recessive red is epistatic to color is because recessive red works “upstream” of color. If you think of melanin production as an assembly line, the ‘recessive red’ allele shuts down the assembly line at an earlier step, prior to the step that TYRP1 protein carries out.
We can deduce that SOX10 works on additional genes beyond Tyrp1, because removing just TYRP1 activity causes the brown phenotype. Brown differs from recessive red in both pigment color and distribution across the body.
While we don’t understand molecularly why recessive red birds also have a spread-like phenotype, a reasonable guess is that SOX10 protein operates upstream and in the same molecular pathway as the spread gene.
Changes to DNA sequence in a gene's regulatory switches can affect when, where, and how much protein is made from that gene. Changes to switches are important drivers of evolutionary change. For more examples, visit
Old Genes, New Tricks.