Apparently it matters. A Patchwork Mind discusses how some of the genes we get from each parent are imprinted to work differently. It's not as simple as "half from mom, half from dad, and the recessive gene only works if there's no dominant gene." Maternal genes "turn off" for some characteristics, and paternal ones for others:
it seems that maternal genes play a more important role in the formation of some brain areas, such as those for language and complex thought, and paternal genes have more influence in regions involved in growing, eating and mating.
When the turning off, or imprinting, goes wrong, this fault in how the different genes express causes some rare illnesses:
Among the rare disorders that result from imprinting errors is Angelman syndrome, which affects one out of 12,000 to 20,000 children in the world. Children with the syndrome are hyperactive and often smiling and laughing. In addition, studies suggest that more than 40 percent of affected kids suffer from autism spectrum disorders as well—experiencing great difficulty with language and social skills. The syndrome is marked by a reduction of maternally expressed proteins in a small section of chromosome 15, which is also usually paternally imprinted. In other words, genes from Dad are silenced as usual, but Mom’s genes are also imprinted by mistake—they are not as active as they should be to balance Dad’s imprinting effects. The brains of these children develop abnormally: their cerebral cortex is slightly smaller than usual, and a 2008 study in mice showed that cells in the cerebellum are also atypical.
Scientists are exploring whether gene expression issues may also be the cause of more common problems:
Badcock and evolutionary biologist Bernard Crespi of Simon Fraser University in British Columbia have since developed this theory, having most recently published an essay in Nature on the potential role that genomic imprinting plays in autism and psychotic disorders. “These disorders are opposites to one another, and imprinting is one of the mechanisms that can mediate that opposing feature,” Crespi posits. Although imprinting usually builds a balanced brain, if one parent’s contribution outweighs the other’s, then autism spectrum disorders (the result of too much net paternal influence, they argue) or psychosis (the result of too much net maternal influence) may instead develop, they say.
Alzheimer's and Tourette's also seem to be affected by maternal or paternal genes expressing differently from expected. There's no evidence from studies yet, but the concept is interesting. If we can prove that gene expression causes these problems, we may already have some tools to treat them:
If imprinting is solidly linked to the development of common mental disorders, then it may one day be appropriate to treat patients with drugs that manipulate gene expression. One method could be dialing down the activity of targeted genes, using a therapy called RNA interference—because it interferes with gene expression.