mutation to grandchildren. Epigenetic Several epigenetic models of autism have been proposed. These are suggested by the occurrence of autism in individuals with fragile X syndrome, which arises from epigenetic mutations, and with Rett syndrome, which involves epigenetic regulation factors. An epigenetic model would help explain why standard genetic screening strategies have so much difficulty with autism. Genomic imprinting Genomic imprinting models have been proposed; one of their strengths is explaining the high male-to-female ratio in ASD. One hypothesis is that autism is in some sense diametrically opposite to schizophrenia and other psychotic-spectrum conditions, that alterations of genomic imprinting help to mediate the development of these two sets of conditions, and that ASD involves increased effects of paternally expressed genes, which regulate overgrowth in the brain, whereas schizophrenia involves maternally expressed genes and undergrowth. Environmental interactions Though autism's genetic factors explain most of autism risk, they do not explain all of it. A common hypothesis is that autism is caused by the interaction of a genetic predisposition and an early environmental insult. Several theories based on environmental factors have been proposed to address the remaining risk. Some of these theories focus on prenatal environmental factors, such as agents that cause birth defects; others focus on the environment after birth, such as children's diets. All known teratogens (agents that cause birth defects) related to the risk of autism appear to act during the first eight weeks from conception, strong evidence that autism arises very early in development. Although evidence for other environmental causes is anecdotal and has not been confirmed by reliable studies, extensive searches are underway. Sex bias Autism spectrum disorder affects all races, ethnicities, and socioeconomic groups. Still, more males than females are affected across all cultures, the ratios of males-to-females is appropriately 3 to 1. A study analyzed the Autism Genetics Resource Exchange (AGRE database), which holds resources, research, and records of autism spectrum disorder diagnosis. In this study, it was concluded that when a spontaneous mutation causes autism spectrum disorder (ASD), there is high penetrance in males and low penetrance in females. A study published in 2020 explored the reason behind this idea further. It is commonly known that the main difference between males and females is the fact that males have one X and one Y sex chromosome whereas female have two X chromosomes. This leads to the idea that there is a gene on the