What is the essence of being a human? Is it our thoughts, our emotions? Are we defined by what we do, our achievements; Our great successes and failures? Or are we merely the result of the interactions of about 25000 genes, segments of DNA that instruct cells how to make proteins, contained in what is collectively known as the human genome?We are all these things but it is undeniable that at the core of our being lies the genome - the "master code" of any species. Many scientists say that technology now allows us to make improvements to this code, to "edit" it and improve upon nature, possibly eliminating genetic diseases such as cystic fibrosis and even improving cognitive function and life expectancy.It is a seductive idea. This week, however, a call for caution by some of the world's leading research funders was made. This is not a fatuous argument against "playing God". Humans have been undoing the work of the Almighty ever since we invented medicine and reduced infant mortality rates.Neither is it an argument against genetic tinkering per se; there is nothing mystical or sacrosanct about DNA, and the calls to ban the genetic modification of, say, wheat plants to enable them to thrive without the use of harmful pesticides are specious and motivated by silly superstition.But the human genome is different. It is, quite literally, who we are. Alter the genetic code present in germ-line cells (sperm and eggs) and the changes are carried into future generations. If this can be used to eliminate diseases, then what is the harm?Writing in the magazine Nature this week, five biomedical funders, including the Wellcome Trust and the Academy of Medical Sciences, call for an "urgent national debate" on the ethics of modifying human embryos and the germ line to prevent serious diseases. This might seem moot; after all, the first draft of the human genome was published 15 years ago and to date the promised "genomic revolution" has failed to emerge.But in recent years there has been significant progress in the field of genomic editing.A technique called Crispr-Cas9, invented in 2013, uses an enzyme to alter the DNA of cells. This could be used to treat a particular disease - cancer, say. Most scientists see little danger in gene therapies such as this, because the effects are limited to the recipient.Using the same technique, however, it is also possible - in theory - to alter, significantly, the genes of germ-line cells and of embryos. These are the changes that would be passed on. There are two potential problems here: one practical, one philosophical.The first concerns unintended consequences. Fixing "faulty" genes might have side effects. Conventional gene therapy is not without risk but gene therapy, the risks of which are apparent only generations down the line, would be a different ball game. A patient can consent to treatment; her great-great grandchildren cannot.The second objection is more profound. Germ-line alterations could be used to eliminate disease; they could be used to make improvements. The "designer baby" argument has so far been applied only in the limited example of embryo screening, in which embryos created during IVF are tested and eliminated if found to be carrying unwanted genetic abnormalities.But genome editing carries this further. It might be possible not only to eliminate the unfit but to inculcate "desirable" traits, such as intelligence, beauty and the "correct" sexuality - all of which have significant genetic components.No one would mourn the elimination of haemophilia, but what about Down's Syndrome, autism or dwarfism? Are Down's people "ill", or on the spectrum of what it is to be human? Anyone with an ounce of humanity would surely say the latter.We are a long way from all this being reality but the scientists are right in saying that we should be having this debate now.A world free of genetic disease, full of the bright, the beautiful and the fit may sound like heaven. But in reality it could resemble another place altogether. ..