2013 study of identical twins showed identical twins are more likely to be like where both individuals are either trans or cis than their fraternal counterparts,suggesting a genitical component. It could be entirely possible that, in case of fraternal twins, one twin might be reluctant to come out as trans due to social pressures.
https://doi.org/10.1080/15532739.2013.750222
A cytogenic and molecular analysis of 442 trans women and 473 trans men suggests a association between being trans and particular variants of some genes.
https://onlinelibrary.wiley.com/doi/abs/10.1111/jsm.12673
Another analysis of 380 trans women and 344 men showed the occurrence of a high frequency of particular DNA variants of 4 genes that have the capacity to alter sex hormones signaling in women while they had been developing in utero. This suggests one's sense of gender identity itself, as opposed to biological sex, is determined in the womb for humans.
https://doi.org/10.1210/jc.2018-01105
study by Hare reported that trans women have a longer androgen receptor gene than cis men, which is less effective at binding testosterone, potentially preventing complete masculinization of the brain (prenatal androgen exposure or sensitivity, or lack thereof, is an often cited mechanism to explain observed brain-structure differences).
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3402034/
Miscellaneous
https://www.nature.com/articles/d41586-019-01237-z
http://sitn.hms.harvard.edu/flash/2016/gender-lines-science-transgender-identity/