I know that some adjectives are written after the hyphen "-" like sentence one below. Actually I don't know the grammatical rule behind that, but could I replace sentence one with sentence two?
They are genes-related issues.
They are genes-relevant issues.
Grammatically, I do not see why not. What is happening here is that adjectival clauses usually (always?) follow the noun being modified.
They are issues that are related to genes
They are issues that are relevant to genes
It is a very common ellipsis to drop the introductory "that are" (or similar construction) and to turn that adjectival clause into an adjectival phrase that likewise follows the noun being modified..
They are issues related to genes
They are issues relevant to genes
Some people like to create a compound adjective and to place it in the normal place for a regular adjective, namely in front of the noun being modified.
They are genes-related issues.
That is too common a practice to label it ungrammatical or unidiomatic.
But as to style, which admittedly is personal, I would never use such compounded adjectives in any kind of formal writing or even in anything but the most casual speech. To my ear, it sounds pretentious. Moreover, and more important, it is vague. Any information on how or why the issues are relevant or related to genes is simply lost by cramming what should be a clause or a sentence into a word. Now presumably you intend to supply that information later so you are merely providing an introduction. But you could find a more informative introduction while being less verbose.
These are issues that are relevant to genetic research in the following two ways.
The sentence above at least specifies a topic. It is of course unnecessarily verbose.
These issues are relevant to genetic research in the following two ways.
For another example:
These issues relate to the etiology of three types of genetic defect.
So grammatically, you are free to go. Stylistically, I'd advise taking a different approach.
You're talking about using a hyphen to create a compound adjective - when two or more hyphenated words come before a noun they modify and act as a single idea.
They are genes-related issues.
This is essentially the same as saying "These issues are related to genes". By creating a compound adjective you have reduced the sentence from 6 words to 5 making it more succinct.
The difference between your two numbered examples is really the same difference between the words "related" and "relevant". Something related to genes just involves genes. "Relevant" carries some importance - it suggests that the issues are important to genes.
Personally, I've not heard "relevant" used in compound adjectives, but that doesn't mean it is wrong. There are some well-accepted compound adjectives such as "time-sensitive" which, if you over-analyse it doesn't make much sense. It doesn't mean something is sensitive to time, it just means that there is a deadline or it is urgent.
