There are five main types of arthritis. The two which are autoimmune, and so could possibly be hereditory, are rheumatoid and psoriatic. I don't know how well informed you are medically but an autoimmune disease is when your body effectively attacks itself, causing damage, in this case to the cartilage of the joints. Septic arthritis occurs after the onset of a joint infection and gout is due to uric acid building up in the joints and causing inflammation. As your mum's arthritis developed early on, it is unlikely to be the most common cause (that typically experienced by the elderly), osteoarthritis which is caused by "general wear and tear", a joint infection and/or age. However, if your mum has obviously abnormal anatomy this may have caused the onset of osteoarthritis...but it's unlikely. Seeing as your grandma developed it young too, the two types I mentioned first are the most likely. The genetics of autoimmune diseases are pretty complicated (again if you know anything about Biology you'll understand when I say it's because of polygenic inheritance) and it can be very hard to say whether or not you will develop the same disease your family has. However, where autoimmune disease of any kind runs in families it can be a good idea to keep an eye out for other illnesses where the cause is your body turning on itself. The most common (as far as I'm aware) are diabetes and hypothyroidism. Not saying it's particularly likely you will get either of those but it's worth considering that if there is an autoimmune tendency in your family you will have an increased risk of developing an autoimmune disease. In terms of preventative measures for developing arthritis itself you can take a glucosamine and/or chondroitin supplement, eat a diet high in calcium and fatty acids and take regular exercise (preferably non-weight bearing e.g. swimming) to strengthen your ligaments.