The details of how the gene that causes ‘mutants’ works and it’s mode of inheritance has been inconsistent and vague throughout the 50+ year history of the X-Men franchise. It has been explained as a sex-linked trait that males pass on to their offspring (Bryan Singer’s “X2”), a product of incomplete penetrance and variable expressivity (New X-Men #141), or only expressed when the X-gene is activated by environmental stressors (starvation, fear, etc.) or mutagens (radiation, chemicals, etc.). However, instead of attempting to rectify all of these diverse (and usually scientifically implausible) genetic theories together, I thought we could examine three pedigrees from some of the most important families in the X-Men universe and attempt to explain how the ‘X-gene’ is transmitted.
The first family is the Xavier family, where Charles and his twin sister (who was stillborn, but still present in the cosmic universe, don’t worry its complicated) both inherited telepathic powers and the ‘mutant gene’. Xavier then has a son with a human (non-mutant) named Gabrielle and their son is a mutant with psionic powers. Interestingly, Xaviers step-brother, the anti-hero Juggernaut, also has a son with a human woman and also produces a son with identical powers (super human strength and durability).
The second family is the Eisenhardt/Maximoff family, where Magneto was the first person to manifest mutant powers during internment at Auschwitz in WW2. He then has a child (Anya) with a human woman named Magda whose power status is unknown, as she died at a young age. The couple also has twins: Wanda and Pietro who both manifest mutant powers which, like their father, revolve around controlling things (in Wanda’s case: magic, and Pietro’s: speed/time). Pietro had a mutant daughter with another mutant woman (Crystal, who can control the four elements), and their daughter, Luna, has the power to control other people’s emotions. Wanda had twins with the Vision, though they were constructed from magic (since the Vision is an android and incapable of reproduction). The twins present powers, but it unclear if this is due to a mutation.
You though the second family tree was weird? Just wait, I saved the weirdest for last: the Summers/Grey pedigree. Jean Grey was the first mutant in the Grey family, however, her sister did not seem to have any powers. Scott, Alex and Gabriel Summers were all children of Corsair and Katherine Summers and all presented mutant powers, all relating to energy beams, and varying in where the energy presented physiologically (eyes/hands/or both). Scott and Jean (both mutants) had three children (all appearing in alternative realities) and all of them had powers similar to Jean Grey’s. Rachel Summers is fairly more powerful than Jean Grey (the non-Phoenix version of course) as she has telepathy, psychometry, telekinesis, and limited time manipulation. Nate Grey was artificially created by the supervillian Sinister from the genetic material of Jean and Scott, he had telekinetic and telepathic powers and aged quickly. Lastly, Nathan Summers (yes, they’re both named Nate), more commonly known as Cable, is the son of Scott Summers and Jean Grey’s clone (yes, a clone). He is also telekinetic and telepathic like his siblings and mother, and has a son with identical powers. Scott Summers also has a daughter with Emma Frost, Ruby Summers, who inherits both powers from her parents with slight modifications (ruby form instead of diamond form, for example.)
When we are looking at these pedigrees it is obvious that there must have been some sort of mutagen, carcinogen or cosmic intervention which caused some children to be born as mutants. It is difficult to say the exact decade (as the birthdates of characters vary with universes), however, it is safe to say that the “founding mutants” were born in around the WWII era (could nuclear weapons be the source of these extreme mutations?). However, after these initial mutations took place it becomes obvious that the mutant gene must be dominant as having one mutant parent guarantees one’s mutant status. It is also evident that the type of powers one is inherited, and the child usually gains the more potent parental power (yay alliteration!). This means that there is less variation in the mutant phenotypes (types of powers) as generations go on, leading to a more consistent (and powerful) phenotype in the mutant species. As time goes on mutants become less and less like humans (and mate more with each other), eventually this will cause two distinct species to be formed.