Here is the information about connection between slow feathering gene and avian leucosis I've promised before.

Quote:

Recently, the endogenous viral gene ev21 has been shown to be closely linked to the sex-linked dominant slowfeathering gene (K) that is used to feather-sex a large proportion of commercial white-egg-producing chickens (3,20). ev21 has been shown to code for the complete endogenous virus EV21.

If this subgroup E endogenous virus is congenitally transmitted to fast-feathering female progeny that lack gene ev21, these birds could have a reduced immune response to exogenous virus infection. Such a mechanism could lead to the increased ALV infection rate and poor performance observed in fast-feathering female progeny of slow-feathering dams (15).

Certain live poultry vaccines are highly effective when administered on day 18 or 19 of incubation (19). Contamination of these vaccines with infectious endogenous viruses could limit the immune response of vaccinated chickens to exogenous ALV infection. However, a detrimental effect would be expected only if the embryos were susceptible to subgroup E ALV infection and if infection of embryos on day 18 or 19 of incubation rather than on day 6 indeed limits immune response to exogenous ALV.
Since endogenous-virus-induced limitation of immune response
to ALV is clearly specific for that class of viruses (6; Crittenden, unpublished data), the detrimental effects of endogenous-virus expression would not adversely influence chicken flocks that were free of exogenous ALV infection.


It is from here:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC254012/pdf/jvirol00094-0092.pdf

Other quotes will follow, but this one has been the easiest to comprehend (by me smile )