Roan boar - Possible lethal white gene??

If both parents have the roan allele, there is a 1 in 4 chance that they will produce babies with two copies of the roan allele and therefore be lethal whites. So, the answer to your question is yes, he can father lethal babies.

I wouldn't adopt out any of my piggies to a breeding home (I run what I call a micro-rescue). Apart from the risks associated with pregnancy and birth (especially depending on the ages of the sows), three sows can potentially produce quite a lot of babies. There's no shortage of piggies in the world, and there are no guarantees the resulting babies will find good homes.

I think it might be a possibility for him to father roan babies even if the mother doesn't have the roan gene, but I'm not sure on that.

I also would definitely avoid adopting out to a home that would breed your pig, even if it's not a roan. The sows might be too old to safely give birth, and if your male is kept with the sows all the time, he will mate with them very soon after they give birth, which is incredibly hard on the poor sow. They would get a whole lot of guinea pigs in a relatively short time, especially if they don't separate male babies at 3 weeks. Then you're dealing with a whole lot of inbred guinea pigs, as well as potentially lethal pigs. With the amount of pigs that die in shelters or sit in homes that take poor care of them, I would not contribute to breeding in any way, which is exactly what you're doing by adopting him out to a person who will breed.

I had a roan baby at one time and was told by the lady that he should never be bred. Not that that was my intention, he was to be a cage mate for another male of mine. She was telling me just in case he got out or whatever, but NO don't breed him the chance is there and it is a very strong chance

As said by @stray hares, lethals are produced when both parents carry the roan gene. However in a guinea pig with any patch of solid white it is possible for them to be carrying the roan gene without it being visible.

This occurs because the gene causes dispersed white furs but does not normally express itself all over the body. So it is therefore possible for the gene to be only displaying in the area of the guinea pig that also happens to be white, causing a roan guinea pig with no visible roaning. Obviously the more white a guinea pig is the more likely this is, unless you know the full genetic background of the individual (but really how often is that the case? Never, that I know of).

I'm a geneticist so hopefully I can explain this is a way that everyone understands, rather than just what makes sense in my head. Fun semantic fact for you: when people say "roan gene" they actually mean "roan allele" much like when people refer to the "breast cancer gene" they mean "breast cancer allele". All guinea pigs have the gene responsible for roaning, it's the same gene responsible for non-roaning in fact, but not all guinea pigs have the variation (allele) of that gene that produces roaning. Phew.

But yes, Soecara is right. The roan allele is co-dominant, which kind of means it vies for expression with the allele for non-roaning, resulting in variable expression. Which means a pig can have patches of solid colour and patches of roaning, which as you can see in the pictures of those very cute little bubs is a grizzled appearance, made from hairs lacking pigment and therefore appearing white interspersed with other coloured hairs. You do run into issues with pigs with normal white patches, where roaning can be, I guess I'd say, cryptic. Even if you know the parents of a white piggy, crossing a roan pig with a non-roan pig results in a 1 in 2 chance of babies being roan. So the only way to know if a piggy with white on it, where one of their parents is roan, and there is no visible roaning on the white piggy, is to breed that white piggy with a piggy that definitely isn't roan and see if any of the babies end up roan. Obviously, genetics is a roll of the dice so there are no guarantees you'd get a roan piggy even if one of the parents is roan, so really you might need a couple of litters before you can be more sure. I'm exhausted just typing that, so I can imagine what the pigs must feel like after all those babies.

Lethal piggies result when both parents are roan, having one copy of the roan allele each, and then they both pass on that roan allele, resulting in a baby with two copies of the roan allele. One of the fascinating things I find with lethal white piggies is that they are white because of the double roan allele (called being homozygous for the roan allele), which affects melanocyte expression a great deal. Which means they're only white in appearance, lethal white piggies can be genetically black or chocolate or gold or agouti or any other coat colour that exists in guinea pigs, it's just that double hit of the roan allele prevents that colour from being expressed. I always find that really interesting to think about anyway.

Sorry for the small essay, I just woke up and am feeling uncharacteristically enthusiastic.

Humm, very interesting Stray Hares! Thanks