Organism belonging to different species can mate but the offspring produced will not have re productivity or fertility.
For example -
When a healthy horse and a healthy donkey mate, they produce mule. Mule is also a healthy species but is a hybrid species of the two and thus infertile.
Therefore, two organisms that belong to the same species must mate together in order to produce offspring with high re-productivity, fertility and viability.
He needs to know if the disorder occurs in the members of family (it would suggest genetic cause of a disease) or in housemates/colleagues from work (it would suggest environmental cause if sick people share the environment)
