DNA is a recipe, NOT a blueprint

The recent media coverage of the naked mole rat genome sequence prompted an alarming number of declarations such as “Naked mole rat’s genome “blueprint” revealed”, doubtless encouraged by an author of the sequencing study using the word “blueprint” twice during a statement that was widely repeated (e.g. here and here).

But the analogy of the genome as a blueprint is misleading for two reasons:

  1. There is no one-to-one mapping between a part of the genome and a part of the organism
  2. Changing one part of the genome does not change one particular part of the organism; instead, it can have many effects on different parts and processes

Richard Dawkins details a much better analogy in many of his books: the genome as a recipe, or set of instructions for making the organism.  This is much more accurate because:

  1. There is no way to map one crumb of a cake to one part of the recipe (in the same way as one part of the organism cannot be mapped to one part of the genome).
  2. A discrete change in the recipe (or genome) can affect the cake (or organism) as a whole i.e. substituting baking powder for yeast makes the whole cake more bready

Dawkins extends this second point in The Blind Watchmaker, to clarify an apparent paradox inherent in this recipe description: if parts of the genome do not map to discrete parts of the organism, how can we talk of a gene for a trait such as eye colour?  Considering the cake again, we can trace the difference between a fluffy and a bready cake to the replacement of baking powder with yeast.  In this sense, yeast is the instruction “for” breadiness, even though the cake would not be bready without other ingredients like flour etc.  In the same way, genes acquire functions because of the difference they make to the phenotype (trait): the name “blue-eyes gene” is still valid, because all other things being equal, an organism with the “blue-eyes gene” instead of the “brown-eyes gene” will have blue eyes instead of brown.

This analogy is also usefully extended in The Extended Phenotype to provide a logical argument against the inheritance of acquired characteristics.  Consider a man who loses an arm in a threshing accident.  If his genome were a blueprint, which maps to his body in a reversible, one-to-one manner, one would expect part of his genome to be lost as well, just as part of a blueprint is removed when the wing of a house is demolished.  By extension, he should produce only one-armed children.  Clearly, this is not what we see.

One final argument against the blueprint analogy is featured in Dawkins’ The Ancestors Tale: genome size does not increase as organism size increases.  If we considered the blueprints of a semi-detached house and Blenheim Palace, both drawn to the same scale, we would expect the blueprint of Blenheim Palace to be much bigger.  If the genome (which is at the same “scale” in all organisms) were a blueprint, we would expect a human to have a much larger genome than a naked mole rat: however, as the recent study revealed, the genomes are very similar in size (22,389 genes compared to 22,561).  And under the blueprint analogy we would definitely expect the tiny water flea Daphnia pulex to have a smaller genome: but with 30,907 genes, this has the largest genome of any animal.

Describing the genome as a blueprint is a recipe for disaster.

References

Colbourne, J.K. et al (2011) The ecoresponsive genome of Daphnia pulex.  Science 331: 555-561

Dawkins, R. (2006) The Blind Watchmaker pp. 295-297.  Penguin Books

Dawkins, R. (2005) The Ancestor’s Tale p. 190.  Phoenix, London

Dawkins, R. (2008) The Extended Phenotype pp. 174-176.  Oxford University Press

Kim et al (2011) Genome sequencing reveals insights into physiology and longevity of the naked mole rat.  Nature 000:1-5

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s