The Genetic Combiner allows one to combine the genes from different seeds of the same type to create seeds with combinations of those Traits, or new Traits. When viewing unlocked Traits, the mouseover tip will show the specific combination of genes needed to produce that trait in a seed. There are sixteen slots in a gene, and they can be active (blue), inactive (orange), or when viewing a specific trait they can be irrelevant (black).
To splice the genes of two seeds, place them in the first two boxes on the splicer. The third will hold the spliced seed, when you are done. The genes for each seed will be displayed to the right of the seed, and the Traits (and trait-specific genes) of the combination will be shown at the bottom of the screen. By moving the scissor icon to the left and right, you can control where the gene is spliced, which will alter which Traits are passed on or created. Once you like the combination, press the 'Splice' button.