The rediscovery of Mendel's laws of heredity in the opening weeks of the 20th century sparked a scientific quest to understand the nature and content of genetic information that has propelled biology for the last hundred years. The scientific progress made falls naturally into four main phases, corresponding roughly to the four quarters of the century.

1º) The first established the cellular basis of heredity: the chromosomes.

2º) The second defined the molecular basis of heredity: the DNA double helix.

3º) The third unlocked the informational basis of heredity, with the discovery of the biological mechanism by which cells read the information contained in genes and with the invention of the recombinant DNA technologies of cloning and sequencing by which scientists can do the same.

4º) The last quarter of a century has been marked by a relentless drive to decipher first genes and then entire genomes, spawning the field of genomics. The fruits of this work already include the genome sequences of 599 viruses and viroids, 205 naturally occurring plasmids, 185 organelles, 31 eubacteria, seven archaea, one fungus, two animals and one plant.

And now, the human being...

The draft genome sequence was generated from a physical map covering more than 96% of the euchromatic part of the human genome and, together with additional sequence in public databases, covering about 94% of the human genome.

The sequence of the human genome is of interest in several respects. It is the largest genome to be extensively sequenced so far, being 25 times as large as any previously sequenced genome and eight times as large as the sum of all such genomes. It is the first vertebrate genome to be extensively sequenced. And, uniquely, it is the genome of our own species.

New data are continually being added (the total amount of finished sequence is now around 1 Gb.), with improvements being made to the physical map, new clones being sequenced to close gaps and draft clones progressing to full shotgun coverage and finishing. The draft genome sequence will be regularly reassembled and publicly released (

Matt Ridley's Genome: The Autobiography of a Species in 23 Chapters is a good introduction to human genetic structure. For each chromosome, Ridley selects some feature as the starting point for a chapter about human DNA. Though that's a bit of gimmick, it's a clever way to organize a book.

For example, chromosome 4 includes the gene that determines whether you'll get Huntington's disease. Ridley discusses the disease, its history, and the genetic markers that distinguish people who will be stricken with Huntington's from those who won't. He then explores the ethical and psychological question of whether people should be tested to determine whether they'll get Huntington's disease, which has no cure.

Ridley covers a wide range of topics. A majority of them deal with variations in the genetic code between different individuals, and the ways in which those variations change people's traits and health. Ridley also discusses the structure of the genome, genetic similarities and differences across multiple species, genetic engineering, junk DNA, and other things.

One surprise was the extent to which behavior is determined by ones genes, instead of the environment. For example, children of physically abusive parents are more likely to become abusive as adults. Genes, not childhood conditioning, cause that correlation. Abused stepchildren don't tend to become abusive, while adopted children of abusive biological parents do.

Genome covers lots of interesting material that I hadn't seen before. For example, evolutionary competition exists between individual genes, within a single creature, to determine which genes are propagated to future generations. That competition explains, for reasons I won't go into, why the Y sex chromosome is so small.

Overall, I'd strongly recommend this book.

A genome is the complete set of genetic material (DNA) possessed by an organism. Most of the DNA is usually found in the chromosomes of the cell nucleus, but DNA is also present in organelles, such as the mitochondria of animal cells, the chloroplasts of plants, and the plasmids of bacteria). In humans, every cell other than the red blood cell contains a complete genome. (A red blood cell doesn't have a nucleus.) Genome is also used to refer to the RNA of RNA viruses and other parasitic quasi-life forms.

Actually, not all of the DNA consists of active genes (euchromatin). Only 2% of the human genome, for example, is euchromatic.

The smallest genome of free-living organisms is possessed by bacteria (from about half a million base pairs, which constitute about 500 genes). Below bacteria, DNA viruses have genomes as small as a few thousand base pairs. HIV has about 9,000. Humans are close to the top the list of genome sizes, with about 3 billion base pairs, which are organized into 24 chromosomes and a number of genes that is still a matter of guessing, but is likely to be on the order of 30,000. Above us are the fugu (more genes, but less 'junk' DNA), rice (!), the amphibians (up to 30 times as much DNA as humans), and the whisk fern (one of the simplest plants). The simpler organisms that have genomes bigger than ours have large amounts of repetitive DNA that has no genetic importance that we know of.

The Human Genome Project

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