The reality is such in human society that any couple or in fact any single person who wishes can have an offspring. Human society attempts to control this by laying down rules about who can marry whom or dictate the number of children a couple should bear, but in reality the sex urge drives the process, followed closely by the urge to become parents. Individuals who break the rules run the risk of ostracism or physical punishment, but even forbidden racial mixing occurs, surreptitiously. This is not the case during genetic engineering, which only proceeds after a long evaluation period and is a deliberate intellectual decision and right granted by the Council of Worlds. In many cases a genetic engineering project involving the merging of two species is followed closely or even overlapped by additional genetic engineering to bring in selective DNA from a third or fourth species, but in most cases it is simply upgrading one intelligent species with DNA from another with desirable traits.
For the Earth at this time, the Zetas were selected to be the species, as we have explained. Our selection was not simply due to our genetic compatibility, our having an iron based hemoglobin for instance, but was due to our emotional control. Humans are almost ruled by emotion at times, not only by blind rage which can cause them to slaughter their loved ones in a fit of jealousy or frustration, but by fear. Fear can cripple humans, taking control of the mental processes with a flood of images on what might occur, so that a palpitating heart or weak knees and mental ruminating on what might happen engross the human. As we have explained, fight or flight emotions were necessary for survival on Earth, due to the number of large carnivores here, but during 4th Density such defensive emotions are not needed for survival.
No other alien species are participating in the development of a hybrid species, not by contribution of genetic material or as genetic engineers, as the traits desired - high IQ and greater emotional control - are traits we have in abundance.