The theory of evolution has three basic parts: 1) it is possible for an organism’s DNA to change or mutate; 2) the change is harmful, beneficial, or neutral; and 3) after a long period of time, the mutations cause new species to form.[11]
https://genetics.thetech.org/about-genetics/mutations-and-disease
DNA is constantly subject to mutations, accidental changes in its code. Mutations can lead to missing or malformed proteins, and that can lead to disease.
Few mutations are bad for you. In fact, some mutations can be beneficial … the mutations we hear about most often are the ones that cause disease. Some well-known inherited genetic disorders include cystic fibrosis, sickle cell anemia, Tay-Sachs disease, phenylketonuria and color-blindness, among many others. All of these disorders are caused by the mutation of a single gene.
Most inherited genetic diseases are recessive, which means that a person must inherit two copies of the mutated gene to inherit a disorder. This is one reason that marriage between close relatives is discouraged; two genetically similar adults are more likely to give a child two copies of a defective gene.
(My nephew, Michael Levine, a geneticist, is Vice-chairman for Research at Berkley. I'm not really bragging since he's from my wife's gene pool.)
https://vcresearch.berkeley.edu/faculty/michael-levine
Title
Professor of Genetics and Development
When the family gets together, I'm sure he like to discuss things like this:
"There are ~50 Dorsal target genes in the Drosophila genome, and a variety of bioinformatics methods allowed us to isolate enhancers for 15 of these genes. Coordinately regulated enhancers that respond to similar thresholds of the Dorsal gradient contain shared sequence motifs. Dorsal target enhancers are typically 500 bp in length and contain clustered binding sites for transcriptional activators and repressors (including binding sites for Dorsal itself). Simple clustering of binding sites is not sufficient to unravel a "cis-regulatory code", which links primary DNA sequence to differential gene activity. The elucidation of a cis-code depends on "grammar", that is, a fixed organization of cis-regulatory elements, such as helical phasing between two adjacent activator sites that work in a synergistic fashion to stimulate transcription. We are obtaining evidence that a fixed grammar governs the immune response in Drosophila larvae. Many immunity genes contain closely linked binding sites for Rel and GATA transcription factors. The binding sites are organized in the same relative orientation, and inverting the sites causes a severe reduction in the immune response.
The sea squirt, Ciona intestinalis, is a simple chordate with a small genome that has been recently sequenced and assembled. The Ciona genome represents a simplified version of vertebrate genomes. Large gene families in vertebrates are represented by just a few members in Ciona. For example, there are 22 FGF genes in a typical vertebrate, but only 6 in Ciona, and each of these corresponds to a subfamily of 3 or 4 vertebrate genes. The Ciona tadpole is initially composed of just ~1,000 cells and there is complete lineage information"