Ask Mr. Smarty Plants

From Wikipedia: "The grapefruit (Citrus × paradisi) is a subtropical citrus tree known for its bitter to semi-sweet fruit, an 18th-century hybrid first bred in Barbados."

 Since the Lady Bird Johnson Wildflower Center is committed to the growth, propagation and protection of plants native to North America but also to the area where they are being grown; in your case, Travis County, TX, we are not prepared to answer this question. However, we will take a look online and see if we can find some information to help you.

From Aggie Agriculture, here is an article on Home Fruit Production - Grapefruit. Note this paragraph on Propagation:

"Either T-budding or inverted T-budding onto sour orange seedling rootstocks is the primary means used to propagate grapefruit trees in Texas. Because of the high degree of nucellar embryony (seeds come true-to-type) in most grapefruit varieties, they can be grown from seed. However, seedage has two major drawbacks: 1) the seedling-grown trees will be short-lived because of their susceptibility to Phytophthora disease (both foot rot and root rot) and 2) fruit production will usually be delayed for several years until the seedling trees grow through juvenility and become capable of bearing."

Here is another article on Texas Grapefruit History in which the Ruby Red is discussed as a mutation, sometimes helped by radiation. Whether a seed from a grapefruit you purchased at a market would breed true is unlikely; plus, we really don't have the weather in Austin for growing citrus.