of the opposing forces, or just their numbers.
The sequences of the eight fly homeotic genes are quite different. Yet each has a region, a sequence of only 180 base-pairs, that encodes, with small variations, the following string of amino acids:
RRRGRQTYTRYQTLELEKEFHTNHYLTRRRRIEMAHALCLTERQIKIWFQNRRMKLKKEI.
This is the homeobox. In the sub-microscopic bulges and folds of a homeotic protein’s three-dimensional topology it is the homeobox sequence, nestling within the grooves of the doublehelix of the DNA, that brings the homeotic proteins to their targets, the hundreds, perhaps thousands, of genes under their control. Subtle differences in the homeobox of each protein allows it to control particular suites of genes.
The discovery of the homeobox in 1984, distinctive as a Hapsburg’s lip, suggested that the homeotic genes were all related to each other, that they were a family. Other animals, it quickly became apparent, had homeobox genes as well. They were found in worms and in snails, in starfish, fish, mice, and they were found in us. Perhaps they were present in the very first animals that crawled out of the Pre-Cambrian ooze a billion years ago. Most excitingly, if homeobox genes formed the circuits of the fly’s calculator of parts, might they not do so for
all
creatures, even for humans? Molecular biologists are not a breed much given to hyperbole, but when they found the homeobox, they spoke of Holy Grails and of Rosetta Stones.
They were right to do so. Another of Vrolik’s specimens, this time a skeleton, shows why. At first glance it seems a rather dull sort of skeleton. It isn’t bent with rickets or bowed with achondroplasia; there is nothing unusual about it (though its skull, limbs and pelvis have evidently long gone astray). It is only an undulating vertebral column with brownish ribs on a rusted metal stand – an altogether abject thing. It is not even on display in the public galleries, but lives in a basement where it is shelved with dozens of other skeletons accumulated over a century but now largely surplus to requirements. And yet this skeleton enjoys a quiet renown. Each spring it sees the light of day as it is displayed to anew batch of the Rijkuniversiteit’s medical students who are invited to identify its anomaly. This is surprisingly hard to spot, though obvious once pointed out – it is an extra pair of ribs.
Extra ribs have always caused trouble. In his
Pseudodoxia epidemica
Sir Thomas Browne relates how once, when the anatomist Renaldus Columbus dissected a woman at Pisa who happened to have thirteen ribs on one side, ‘there arose a party that cried him down, and even unto oaths affirmed, this was the rib wherein a woman exceeded’. ‘Were this true,’ Browne continues, ‘this would oracularly silence that dispute out of which side
Eve
was framed.’ The influence of Genesis II: 21–22 on popular anatomy has been a baleful one. I recently asked a class of thirty biology undergraduates (among them Britain’s best and brightest) whether men and women had the same number of ribs: about half a dozen of them thought not. ‘But,’ as Sir Thomas says with customary vigour, ‘this will not consist with reason or inspection. For if we survey the Sceleton of both sexes, and therein the compage of bones, we shall readily discover that men and women have four and twenty ribs, that is, twelve on each side.’ Just so. And yet extra ribs are surprisingly common: one in every ten or so adults has them (but they are no more or less frequent in women than men).
Most of us have thirty-three vertebrae. Starting at the head, there are seven neck vertebrae, then twelve rib-bearing vertebrae, then five vertebrae in the lower back, and another nine fused together to make the sacrum and coccyx or tail bones. In most people with extra ribs, this pattern is disrupted. A vertebra that normally does not bear ribs has become transformed into one that does. Sometimes this means the loss of a neck
The sequences of the eight fly homeotic genes are quite different. Yet each has a region, a sequence of only 180 base-pairs, that encodes, with small variations, the following string of amino acids:
RRRGRQTYTRYQTLELEKEFHTNHYLTRRRRIEMAHALCLTERQIKIWFQNRRMKLKKEI.
This is the homeobox. In the sub-microscopic bulges and folds of a homeotic protein’s three-dimensional topology it is the homeobox sequence, nestling within the grooves of the doublehelix of the DNA, that brings the homeotic proteins to their targets, the hundreds, perhaps thousands, of genes under their control. Subtle differences in the homeobox of each protein allows it to control particular suites of genes.
The discovery of the homeobox in 1984, distinctive as a Hapsburg’s lip, suggested that the homeotic genes were all related to each other, that they were a family. Other animals, it quickly became apparent, had homeobox genes as well. They were found in worms and in snails, in starfish, fish, mice, and they were found in us. Perhaps they were present in the very first animals that crawled out of the Pre-Cambrian ooze a billion years ago. Most excitingly, if homeobox genes formed the circuits of the fly’s calculator of parts, might they not do so for
all
creatures, even for humans? Molecular biologists are not a breed much given to hyperbole, but when they found the homeobox, they spoke of Holy Grails and of Rosetta Stones.
They were right to do so. Another of Vrolik’s specimens, this time a skeleton, shows why. At first glance it seems a rather dull sort of skeleton. It isn’t bent with rickets or bowed with achondroplasia; there is nothing unusual about it (though its skull, limbs and pelvis have evidently long gone astray). It is only an undulating vertebral column with brownish ribs on a rusted metal stand – an altogether abject thing. It is not even on display in the public galleries, but lives in a basement where it is shelved with dozens of other skeletons accumulated over a century but now largely surplus to requirements. And yet this skeleton enjoys a quiet renown. Each spring it sees the light of day as it is displayed to anew batch of the Rijkuniversiteit’s medical students who are invited to identify its anomaly. This is surprisingly hard to spot, though obvious once pointed out – it is an extra pair of ribs.
Extra ribs have always caused trouble. In his
Pseudodoxia epidemica
Sir Thomas Browne relates how once, when the anatomist Renaldus Columbus dissected a woman at Pisa who happened to have thirteen ribs on one side, ‘there arose a party that cried him down, and even unto oaths affirmed, this was the rib wherein a woman exceeded’. ‘Were this true,’ Browne continues, ‘this would oracularly silence that dispute out of which side
Eve
was framed.’ The influence of Genesis II: 21–22 on popular anatomy has been a baleful one. I recently asked a class of thirty biology undergraduates (among them Britain’s best and brightest) whether men and women had the same number of ribs: about half a dozen of them thought not. ‘But,’ as Sir Thomas says with customary vigour, ‘this will not consist with reason or inspection. For if we survey the Sceleton of both sexes, and therein the compage of bones, we shall readily discover that men and women have four and twenty ribs, that is, twelve on each side.’ Just so. And yet extra ribs are surprisingly common: one in every ten or so adults has them (but they are no more or less frequent in women than men).
Most of us have thirty-three vertebrae. Starting at the head, there are seven neck vertebrae, then twelve rib-bearing vertebrae, then five vertebrae in the lower back, and another nine fused together to make the sacrum and coccyx or tail bones. In most people with extra ribs, this pattern is disrupted. A vertebra that normally does not bear ribs has become transformed into one that does. Sometimes this means the loss of a neck
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