How does heredity work with Endogenous retrovirus?
For a provirus to be heritable, the infection must occur in the germ line. The initial integration would occur at one site in the genome and thus the locus could be described as hemizygous. Inheritance would indeed be probabilistic, depending on whether an offspring inherits the provirus-containing chromosome or its uninfected homolog. 

Eventually, the endogenous retrovirus (ERV) can become fixed or lost in a population by genetic drift and natural selection. Additionally, retrotransposition and, more importantly, reinfection can greatly increase copy number and allow ERV persistence despite a lack of fixation at specific loci. It should also be noted that more recently acquired ERVs exist that have not reached fixation in humans. 



References and Further Reading:

Boeke JD, Stoye JP. Retrotransposons, Endogenous Retroviruses, and the Evolution of Retroelements. In: Coffin JM, Hughes SH, Varmus HE, editors. Retroviruses. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 1997.

Belshaw R, Pereira V, Katzourakis A, Talbot G, Pačes J, Burt A, Tristem M. 2004. Long-term reinfection of the human genome by endogenous retroviruses. Proc Nat Acad Sci 101(14):4894–4899.