For many years molecular biologists have been mystified by the fact
that very little of an organism’s DNA seems to serve any useful
function.

I have solved the mystery.

The reason why only 30% of human DNA performs any useful function is that
the rest of it is comments.

Once we decode a typical human genome, we see that the contents begin as
follows:

=================

/* HUMAN_DNA.H
*
* Human Genome
* Version 2.1
*
* (C) God
*/

/* Revision history:
*
* 0000-00-01 00:00  1.0  Adam.
* 0000-00-02 10:00  1.1  Eve.
* 0000-00-03 02:11  1.2  Added penis code to male version. A bit messy –
*                        will require a rewrite later on to make it
*                        neater.
* 0017-03-12 03:14  1.3  Added extra sex drive to male.h; took code from
*                        elephant-dna.c
* 0145-10-03 16:33  1.4  Removed tail.
* 1115-00-31 17:20  1.5  Shortened forearms, expanded brain case.
* 2091-08-20 13:56  1.6  Opposable thumbs added to hand() routine.
* 2501-04-09 14:04  1.7  Minor cosmetic improvements — skin colour made
*                        darker to match my own image.
* 2909-07-12 02:21  1.8  Dentition inadequate; added extra ‘wisdom’
*                        teeth.
*                        Must remember to make mouth bigger to
*                        compensate.
* 4501-12-31 14:18  1.9  Increase average height.
* 5533-02-12 17:09  2.0  Added gay option, triggered by high population
*                        density, to try and slow the overpopulation
*                        problem.
* 6004-11-04 16:11  2.1  Made forefinger narrower to fit hole in centre
*                        of CD.
*/

/* Standard definitions
*/

#define SEX male
#define HEIGHT 1.84
#define MASS 68
#define RACE caucasian

/* Include inherited traits from parent DNA files.
*
* Files must be pre-processed with MENDEL program to provide proper
* inheritance features.
*/

#include “mother.h”
#include “father.h”

#ifndef FATHER
#warn(”Father unknown — guessing\n”)
#include “bastard.h”
#endif

/* Set up sex-specific functions and variables
*/
#include <sex.h>

/* Kludged code — I’ll re-design this lot and re-write it as a proper
* library sometime soon.
*/
struct genitals
{
#ifdef MALE
Penis *jt;
#endif
/* G_spot *g;   Removed for debugging purposes */
#ifdef FEMALE
Vagina *p;
#endif
}

/* Initialization bootstrap routine — called before DNA duplication.
* Allocates buffers and sets up protein file pointers
*/
DNA *zygote_initialize(Sperm *, Ovum *);

/* MAIN INITIALIZATION CODE
*
* Returns structures containing pre-processed phenotypes for the organism
* to display at birth.
*
* Will be improved later to make output less ugly.
*/
Characteristic *lookup_phenotype(Identifier *i);

===

….and so on.

[ Note that God uses three-space tabs ]

Via Mathew Murphy who released this joke under GPL - http://www.gnu.org/fun/jokes/dna.html