let me give an example. say the recessive genes of your parents have down syndrome. in a punnet square, one out of the 4 possible children will have both recessive genes and get diagnosed with down syndrome. Parents have Rr and Rr, the punnet square has RR, Rr, Rr, and rr. R being dominant normal gene, without disease, and r being having offspring with a chance of having down syndrome. its the same for any other genetic disease.