Men vs Women - The Genetic Battle
Before you begin to thrash this thread with comments about men vs women comments, please read it out. It might be something that you never expected
In 1998,William Amos and John Harwood wrote in the Philosophical Transactions of the Royal Society : ”The mammalian Y chromosome is thus likely to be engaged in a battle in which it is outgunned by its opponent. A logical consequence is that the Y should run away and hide, shedding any transcribed sequences that are not essential to its function.’ This is not something you would find in one of the most highly reputed journals. “outgunned”, “battle”, “run away and hide”, dont seem something that applies to genes, but rather to people. What is going on here?
This is the story of human evolution. And the chapter this comes from is the most famous of them all. Sexual antagonism… of a different kind.
Most of you would know that humans have 23 pairs of chromosomes. One of each pair comes from the father, and one of each from the mother. On each chromosomes, we have long “strings of text” called as genes. The genes are responsible for everything you are. Right from that long nose you might have, or that bald patch on your head. Genetic determinism is as frightning as unit tests in programming
Now, except from the other (read boring) 22 chromosome, the only interesting ones are the sex chromosomes. Before any of you get any ideas what they are for, the sex chromosomes determine the gender of the person. When the pair is XX, it determines a female child, and when it is an XY a male child. Barring exceptions, this is followed in almost all mammalian species. The reason they are denoted as X and Y, are because unlike other chromosome, where the inviduals in the pair are almost identical to each other, in sex chromosomes, X and Y are totally different. As you can see below. The X chromosome appears complete, while the Y chromosome appears to be a rather small and non-existent blob of DNA matter.
Now that you have all the basics, lets jump to the interesting part. Why of all things is the Y chromosome, such a small and insignificant (atleast looks wise) blob. The story begins from the very beginning when both X and Y were normal and healthy looking chromosomes. This story predates the mammalian - avian divide, and happened somewhere when the reptiles were roaming the land. Due to evolutionary pressure, and random chance, genes developed on these two chromosomes that functioned towards gender determinism. Y chromosomes developed SRY, X chromosome developed DAX. Since Y chromosomes was found only in men, there was heavy evolutionary pressure for the Y chromosome to develop genes that were suitable only to males. Thus DAX on the X present gives the body female characteristics. But when a Y is present with an SRY, the SRY “defeats” the DAX and the body develops into a male. Thus Y chromosome keeps evolving genes that defeat the purpose of the genes on the X chromosome.
Here comes the interesting part. Since the X chromosome spends nearly75% 66% of its existence in a female body, as compared to a male, the X chromosome has twice more of a chance to develop genes that can take potshots at the Y chromosome, than the Y has to take at X. Thus everytime the Y chromosome develops an attacking gene against some gene on the X, X comes back strongly with a counter attack on Y. And the counter attack is so strong, that the gene on Y is completely eliminated !!!
Lets look at an example. From a population of 5000 individuals, one individual, by random chance develops gene KillBill. KillBill’s objective is to develop a protein, which disables the operation of protein PrBill, which is developed by a gene Bill on chromosome X. This will lead to dwindling numbers of X chromosomes that produce Bill. Now assume that protein Bill is actually important for the development of females. Since X spends more time in a female body, there is higher probability that this X develops a mutation such that Bill is mutated to gene Elle. Elle can no longer be attacked by KillBill. Thus in subsequent generations KillBill eliminates all X chromosomes with Bill, and at the same time, since Elle is immune to it, within a few generations, all X chromosomes now have the Elle gene. Now if, Elle has a gene that attacks KillBill, since Y never spends its life in a body with only Y chromosomes, Y never really gets a fighting chance to develop a gene that fights Elle, and soon, all Y chromosomes with KillBill are eliminated and Y is left smaller than before. Imagine this happening over a million years, and you can safely assume that Y must have lost hundreds of gene’s to attacks from X. Thus Y chromosome has almost shed all of its genes, keeping only those most necessary for the development of the male characteristics, such that there is very limited possibility for X to develop any genes to attack it.
Suppose, for instance, that a gene appeared on the X chromosome that specified the recipe for a lethal poison that killed only sperm carrying Y chromosomes. A man with such a gene would have no fewer children than another man. But he would have all daughters and no sons. All of those daughters would carry the new gene, whereas if he had had sons as well, none of them would have carried it. Therefore, the gene is twice as common in the next generation as it would otherwise be. It would spread very rapidly. Such a gene would only cease to spread when it had exterminated so many males that the very survival of the species was in jeopardy and males were at a high premium. Far-fetched? Not at all. In the butterfly Acrea encedon, that is exactly what has happened. The sex ratio is ninety-seven per cent female as a result. This is just one of many cases known of this form of evolutionary conflict, known as sex-chromosome drive. Most known instances are confined to insects, but only because scientists have looked more closely at insects. The strange language of conflict used in the remarks I quoted above now begins to make more sense.
You might be wondering, that natural evolution was supposed to be for the good of humanity, society etc etc… What is going wrong here. The answer is nothing. Evolution isn’t SUPPOSED to help organisms. That is only a by-product of evolution. The main aim of evolution is the survival of the fittest. Mind you, no one said fittest organism. Thus in this case it is the survival of the fittest gene. In this eternal battleground of genomes, we, are only but their chariots, one which they ride, blazing to glory…
References :
Genome: The Autobiography of a Species in 23 Chapters - Matt Ridley
The Selfish Gene: 30th Anniversary edition - Richard Dawkins
In 1998,William Amos and John Harwood wrote in the Philosophical Transactions of the Royal Society : ”The mammalian Y chromosome is thus likely to be engaged in a battle in which it is outgunned by its opponent. A logical consequence is that the Y should run away and hide, shedding any transcribed sequences that are not essential to its function.’ This is not something you would find in one of the most highly reputed journals. “outgunned”, “battle”, “run away and hide”, dont seem something that applies to genes, but rather to people. What is going on here?
This is the story of human evolution. And the chapter this comes from is the most famous of them all. Sexual antagonism… of a different kind.
Most of you would know that humans have 23 pairs of chromosomes. One of each pair comes from the father, and one of each from the mother. On each chromosomes, we have long “strings of text” called as genes. The genes are responsible for everything you are. Right from that long nose you might have, or that bald patch on your head. Genetic determinism is as frightning as unit tests in programming
Now, except from the other (read boring) 22 chromosome, the only interesting ones are the sex chromosomes. Before any of you get any ideas what they are for, the sex chromosomes determine the gender of the person. When the pair is XX, it determines a female child, and when it is an XY a male child. Barring exceptions, this is followed in almost all mammalian species. The reason they are denoted as X and Y, are because unlike other chromosome, where the inviduals in the pair are almost identical to each other, in sex chromosomes, X and Y are totally different. As you can see below. The X chromosome appears complete, while the Y chromosome appears to be a rather small and non-existent blob of DNA matter.
Now that you have all the basics, lets jump to the interesting part. Why of all things is the Y chromosome, such a small and insignificant (atleast looks wise) blob. The story begins from the very beginning when both X and Y were normal and healthy looking chromosomes. This story predates the mammalian - avian divide, and happened somewhere when the reptiles were roaming the land. Due to evolutionary pressure, and random chance, genes developed on these two chromosomes that functioned towards gender determinism. Y chromosomes developed SRY, X chromosome developed DAX. Since Y chromosomes was found only in men, there was heavy evolutionary pressure for the Y chromosome to develop genes that were suitable only to males. Thus DAX on the X present gives the body female characteristics. But when a Y is present with an SRY, the SRY “defeats” the DAX and the body develops into a male. Thus Y chromosome keeps evolving genes that defeat the purpose of the genes on the X chromosome.
Here comes the interesting part. Since the X chromosome spends nearly
Lets look at an example. From a population of 5000 individuals, one individual, by random chance develops gene KillBill. KillBill’s objective is to develop a protein, which disables the operation of protein PrBill, which is developed by a gene Bill on chromosome X. This will lead to dwindling numbers of X chromosomes that produce Bill. Now assume that protein Bill is actually important for the development of females. Since X spends more time in a female body, there is higher probability that this X develops a mutation such that Bill is mutated to gene Elle. Elle can no longer be attacked by KillBill. Thus in subsequent generations KillBill eliminates all X chromosomes with Bill, and at the same time, since Elle is immune to it, within a few generations, all X chromosomes now have the Elle gene. Now if, Elle has a gene that attacks KillBill, since Y never spends its life in a body with only Y chromosomes, Y never really gets a fighting chance to develop a gene that fights Elle, and soon, all Y chromosomes with KillBill are eliminated and Y is left smaller than before. Imagine this happening over a million years, and you can safely assume that Y must have lost hundreds of gene’s to attacks from X. Thus Y chromosome has almost shed all of its genes, keeping only those most necessary for the development of the male characteristics, such that there is very limited possibility for X to develop any genes to attack it.
Suppose, for instance, that a gene appeared on the X chromosome that specified the recipe for a lethal poison that killed only sperm carrying Y chromosomes. A man with such a gene would have no fewer children than another man. But he would have all daughters and no sons. All of those daughters would carry the new gene, whereas if he had had sons as well, none of them would have carried it. Therefore, the gene is twice as common in the next generation as it would otherwise be. It would spread very rapidly. Such a gene would only cease to spread when it had exterminated so many males that the very survival of the species was in jeopardy and males were at a high premium. Far-fetched? Not at all. In the butterfly Acrea encedon, that is exactly what has happened. The sex ratio is ninety-seven per cent female as a result. This is just one of many cases known of this form of evolutionary conflict, known as sex-chromosome drive. Most known instances are confined to insects, but only because scientists have looked more closely at insects. The strange language of conflict used in the remarks I quoted above now begins to make more sense.
You might be wondering, that natural evolution was supposed to be for the good of humanity, society etc etc… What is going wrong here. The answer is nothing. Evolution isn’t SUPPOSED to help organisms. That is only a by-product of evolution. The main aim of evolution is the survival of the fittest. Mind you, no one said fittest organism. Thus in this case it is the survival of the fittest gene. In this eternal battleground of genomes, we, are only but their chariots, one which they ride, blazing to glory…
References :
Genome: The Autobiography of a Species in 23 Chapters - Matt Ridley
The Selfish Gene: 30th Anniversary edition - Richard Dawkins
Labels: dax, genetics, sexual antagonism, sry, xy chromosome
