Epistasis: Why should a recessive allele be a hypostatic gene?

Question

Let us take the example of Recessive epistatsis, an epistasis in which a double recessive gene mask the phenotypic expression of alleles of another locus. _{(adapted from: An Introduction to Genetic Analysis)}

An example from the same book:

In plant blue-eyed Mary (Collinsia parviflora), the biochemical pathway is as follows:

The w and m genes are not linked. If homozygous white and magenta plants are crossed, the F1 and F2 are as follows:

Complemention results in a wild-type F1. However, in the F2, a 9:3:4 phenotypic ratio is produced.

This kind of interaction is called epistasis, which literally means “standing on”; in other words, an allele of one gene masks the expression of the alleles of another gene. In this example, the w allele is epistatic on m+ and m$^1$. Conversely, m+ and m can be expressed only in the presence of w+.

Interpretation & Question:

The gene product of m$^+$ is needed to form blue pigment from magenta. On the other hand, in the absence of w$^+$ allele (i.e. w w genotype), no magenta (precursor of blue) is formed so the phenotypic expression of m$^+$ allele(s) is masked. Now, what has m got to do here? $^1$ It is not encoding anything.

Note:

On referring other books I found they do not suggest a hypostatic gene to be recessive (as $m$ has been considered here).

My idea of recessive allele is that one that due to loss-of-function mutation either produces a non-functional variant of the protein or the same protein at a lower quantity.

Has recessive allele been considered an hypostatic allele here considering that it might be able to code normal functional protein at a lower rate?

Scope of answer:

Which concept should be followed? Why (ideally with citation)?

Answer 1

In this example, the $w$ locus is epistatic on the $m$ locus or, in other words, the phenotypic expression of the $m$ locus is dependent on the $w$ locus. It is called recessive epistasis because the homozygous recessive genotype $w/w$ prevents the phenotypic expression of the $m$ locus. A $w/w$ organism will be white regardless of the alleles at the $m$ locus.

Now, to get to your question, consider the situation where the organism is $w^+;m^+$:

$$\ce{white \stackrel{w+}{\rightarrow} magenta \stackrel{m+}{\rightarrow} blue}$$

You can see that the phenotypic expression of the $m^+$ allele is a blue colour. Next consider a $w^+;m/m$ organism:

$$\ce{white \stackrel{w+}{\rightarrow} magenta \stackrel{m/m}{\nrightarrow} blue}$$

You're right that, at a molecular level, $m$ likely codes for some defective enzyme. But, in the case of epistasis, it is important to consider its expression at the level of phenotype. The phenotypic expression of $m/m$ is a magenta colour. Finally, look at a $w/w;m/m$ organism:

$$\ce{white \stackrel{w/w}{\nrightarrow} magenta \stackrel{m/m}{\nrightarrow} blue}$$

Despite having the $m/m$ genotype, which should give a magenta colour, the organism will be white because of epistasis. In other words, the expression of the $m/m$ phenotype is dependent on the $w$ locus.

I hope that answers your question. Let me know if I still misunderstand or something needs to be explained better.

Answer 2

In the context of this textbook, m is simply an allele that codes for a non-functional protein product. The protein product of m is incapable of converting magenta pigment into blue pigment.