Frank and Monica are generally portrayed as terrible parents for the Gallagher children to have, due to their neglectful tendencies, extreme selfishness, and not being above deserting the family home and children when a better offer comes their way. But when you start to look at it from another angle, Lip has a high IQ, and Fiona, Debbie and Ian are extremely giving and proactive. Carl can be seen as the black sheep, because he's fairly dumb and a trouble maker more than not. Even Liam is proving himself to be a prodigy. You can't help but wonder if they would have had the motivation to push themselves on like that, had their family home been 'too welcoming and homely'. Frank and Monica are terrible parents, but it was their terrible parenting that ironically led to their children all being far more independent and proactive than they otherwise could be, simply because they had no choice but to fend for themselves.
Lampshaded by Lip and Fiona as they begin to acknowledge in Season 1's finale that leaving their younger siblings with Frank and Monica might not be so bad after all.
In Season 2, when Jimmy reassures Fiona he won’t leave her, she counters that “everybody leaves”. She ends up being heartbreakingly right when Jimmy does leave her, as well as the various other love interests she has later on.
And then finally she leaves herself, at the end of Season 9.
In the Season 4 finale “Lazarus” Lip says to Fiona that alcoholism isn’t genetic, but the nurture part of nature and nurture. In Season 6, Lip becomes much like father Frank in that he drinks a lot, and, by Season 7, is an alcoholic.
Ian’s bipolar disorder, which he got as a result of mother Monica, who also has the disorder, is hinted at strongly in earlier seasons, particularly this exchange with Fiona way back in the Pilot:
Fiona: It’s just...you look like mom more than any of the rest of us. You probably just scare [Frank].
Ian: He ain’t seen nothing yet.
Another time Lip’s addiction and Ian’s bipolar disorder are foreshadowed is when the two have this conversation in Season 1:
Ian: Like if we got the alcoholic gene from Frank.
Lip: Yeah, or the dummy gene from Monica.
Carl asks Fiona what jail was like in the Season 4 episode “A Jailbird, Invalid, Martyr, Cutter, Retard, And Parasitic Twin”, to which Fiona responds, “Don’t end up there.” Towards the end of Season 5, Carl is put in juvie after attempting to smuggle drugs and gets caught up in a violent gang.
Carl behaves like a poser in season 6, including himself in the black community, telling people he is black, and calling everybody else white. Many fans think this is funny, but remember - Liam is black, and is confirmed via DNA test to be Carl's full sibling. Carl really thinks he is part black, only learning he isn't after taking a DNA test.
