If you wanted to overhaul genetics to make it more realistic AND fun, you'd:
A] Cut out the randomized "does the gene change" check where you click 15 times trying to get a gene to D+.
B] Remove the notion of gene blocks entirely.
C] Change genetic manipulation from point alteration to cut and pasting.
D] Massively increase the number of genetic symptoms.
Our symptom list is: All of the virology symptoms, slimeness, nearsightedness, seizures, coughing, Tourette's Syndrome, nervousness, TK, Hulk, Cold Resistance, X-Ray, Blindness, Deafness, Clumsiness, Strangeness, thirteen traits for Blackness, Blueness, Greenness and Redness of the Hair, Facial Hair and Eyes several traits for skin colour, a trait for maleness, and ones for whatever new genetic info we add.
Each is randomly assigned a 2 letter hex codon at round start out of the 256 possible 2 letter hex codons.
Each copy of a codon you have gives you a chance to manifest the associated gene. If there are multiple genes for a single trait (such as for hair colour) each that manifests is applied in sequence. For example, each Hair Redness gene you have makes your hair one step more red. Species (Virus, Monkey, Human, Xenomorph, Changeling, etc.) is determined by the length of your genome. Humans have a genome length of 42, Monkeys have 40, Changelings add their absorbed DNA onto the end of their current genome to become bigger and bigger, but have their appearance determined by the first 42 (or 40, if a monkey) and shuffle their genomes around as they transform. Viruses have genomes that can be analyzed by the virologist and inserted into genomes in genetics. If you have a virus codon, you have the effects of that symptom genetically. Slimes and slime people have the slime genome which makes you a Slime Person if you have a genome length of 42 and a Slime if you have some other number.
Instead of an emitter that randomly alters the genes at a point, you get two tools, the Genetic Extractor, which cuts out the gene you've selected and adds it to the end of the string in the Extractor's buffer, and the Genetic Splicer, which inserts the contents of the Extractor's buffer at the selected location.
Genes aren't block based, so the string 243AD codes for codons 24, 43, 3A and AD all at once. With whatever results that brings you.