Even though alternate realities can be a mixed bag of things, there still are some unspoken rules than govern these differences.
For example, despite having no powers and not becoming the Flash, Barry Allen is still a forensic scientist in the CCPD and shares a relationship with Iris West.
Lineage is another thing that seems to be generally preserved with Iris and Joe continuing to be daughter and father. In fact, Iris’s career as a police officer and Joe displeasure of her career choice mirrors Earth-1 Iris’ desire to join the CCPD but is disallowed from doing so by Earth-1’s Joe.
And the same goes for Henry Allen/Jay Garrick. We know from Henry that his mother’s (Barry’s grandmother) maiden name was Garrick. Therefore, it can be assumed that the father of Jay Garrick was MIA. Nevertheless, they are essentially the same person who has simply lived through different incidents.
Which would mean the 1990 Flash would have to adhere to this rule as well… but Barry’s grandmother’s maiden name is never mentioned, making it impossible to confirm. Or is it?
Let’s, for a moment, assume that the name Jason (for which ‘Jay’ is short) is of significance to Barry’s grandmother. Maybe Jason was the name of her father, or a sibling, or, very likely, the first name of Henry/Jay’s biological father. In realities which Henry’s father–this Jason Allen–is nowhere to be seen, she names her son after his missing father, and the child obviously grows up to be Jay Garrick. In realities where good Mr Allen sticks around, the child is given the name Henry instead.
In both Earth-1 and the 1990 show, we know that the case is that Henry is named with his father’s surname. And with his mother’s maiden name never mentioned this whole theory would be stuck in a rut.
Enter Jay Allen.
That’s right. In the 1990 series, Barry was not an only child. He had an older brother named Jay who was a police officer with the CCPD. It is his death, at the hands of a gang leader that puts Barry on his path towards becoming a hero.
So if we were to assume that Jason, or Jay, was indeed the name of Henry’s father, it is entirely possible that in at least one reality, he’d have named his elder son after his father–therefore ensuring that the lineages of the characters in all presented realities are somewhat similar.
This even extends to the Smallville universe where we meet a speedster who goes by Jay, Barry, Wally and Bart–identities he claims to have stolen. While it is later confirmed that he is indeed Bart Allen, we are also introduced to a Jay Garrick in the comics (though first seen in a cameo in the show).
In this reality, it is established that all speedsters share a telepathic link via the Speed Force, and the names Jay, Barry, and Wally may have been latently planted in his sub-conscious.
While the Jay and Barry connection is never explored in the Smallville universe, Jay does share a mentor-like relationship with Bart (similar to the comics) and, for a while, is almost a grandfather-like figure to the young speedster… who is the actual grandson of Barry Allen.
So what do all these mean?