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Authors: Marcus Chown
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different versions
of those genes, due to random mutations in each of their familylines. And these variants, known as alleles, can make all the difference. For instance, there is a gene that determines hair colour. The copy from your mother might, for instance, be a variant that makes you a redhead or it might be a variant that makes you a brunette. Which version of the two genes is expressed in you depends on which gene is dominant and which recessive.
There could many reasons why a copy, or allele, of a gene is dominant or recessive. It all depends on the particular gene. Each allele – one from your mother and one from your father – will make a slightly different protein. But some proteins win out over their fellows. In the simplest situation, one allele makes a
broken protein
. Since the broken protein does nothing, the working protein is dominant. A good example of a recessive allele is red hair. There is a protein called MC1R whose usual job is to get rid of red pigment. When it is not working, therefore, there is a build-up of red pigment and a person ends up with red hair.
By inheriting versions of each gene either from your mother or from your father, you inherit some characteristics from your mother and some from your father. The precise mix is random. This is how sex maximises the novelty in offspring.
Actually, it is not quite true that you have two identical sets of 23 chromosomes. In fact, you have two identical sets of
only
22
. The chromosomes in the 23rd pair differ between males and females. It works likes this. Chromosomes tend to have a characteristic ‘X’ shape. However, the 23rd might have a ‘Y’ shape. Two copies of the X chromosome make a female while an X plus a Y makes a male. 15
All human embryos develop in exactly the same way in the beginning. However, after forty days, a gene on the male’s Y chromosome called the Sex-Determining Region of the Y chromosome,or SRY, becomes active. It contains the instructions for making testosterone, which converts the gonad cells of an embryo into testes, which in turn trigger the development of male sexual organs. If the expression of SRY is blocked, however, the embryo’s gonad cells become ovaries, which trigger the development of female sexual organs. Differences in hormones between the sexes cause as many as one in six mammalian genes to express their proteins preferentially in one sex rather than the other.
Males are the product of testosterone. They are females with an extra gene. And every male on Earth – even the most macho – was in touch with his feminine side for the first forty days of existence.
The big bang of sex
Since most simple organisms are asexual and the first organisms on Earth were single cells, most biologists believe that the earliest life forms were asexual. This is a hugely simpler means of proliferating than sexual reproduction. So how in the world did sex ever arise? 16
Nature tends to adapt to new tasks things it evolved for entirely different purposes. Glutamate, for instance, one of the most important neurotransmitters in the human brain, was used by the very first bacteria for signalling almost 4 billion years ago. 17 Well, sex is no different. The basic components – the fusion of two cells, the mixing of their genes and the separation of those cells – arose for other purposes and then were co-opted for the purpose of sexual reproduction.
A fundamental process was the swallowing of one simple cell by another to create a complex cell, or eukaryote about 1.8 billionyears ago. 18 This involved a multitude of changes inside a cell. For instance, the swallowee’s membrane was replaced by a different type of membrane to permit it to become a cellular organelle. The exact details are not important. The point is that such adaptions made it possible for
one cell to merge with another
.
At some stage in the mists of time – and all that can be done is to speculate plausibly about this – two similar
different versions
of those genes, due to random mutations in each of their familylines. And these variants, known as alleles, can make all the difference. For instance, there is a gene that determines hair colour. The copy from your mother might, for instance, be a variant that makes you a redhead or it might be a variant that makes you a brunette. Which version of the two genes is expressed in you depends on which gene is dominant and which recessive.
There could many reasons why a copy, or allele, of a gene is dominant or recessive. It all depends on the particular gene. Each allele – one from your mother and one from your father – will make a slightly different protein. But some proteins win out over their fellows. In the simplest situation, one allele makes a
broken protein
. Since the broken protein does nothing, the working protein is dominant. A good example of a recessive allele is red hair. There is a protein called MC1R whose usual job is to get rid of red pigment. When it is not working, therefore, there is a build-up of red pigment and a person ends up with red hair.
By inheriting versions of each gene either from your mother or from your father, you inherit some characteristics from your mother and some from your father. The precise mix is random. This is how sex maximises the novelty in offspring.
Actually, it is not quite true that you have two identical sets of 23 chromosomes. In fact, you have two identical sets of
only
22
. The chromosomes in the 23rd pair differ between males and females. It works likes this. Chromosomes tend to have a characteristic ‘X’ shape. However, the 23rd might have a ‘Y’ shape. Two copies of the X chromosome make a female while an X plus a Y makes a male. 15
All human embryos develop in exactly the same way in the beginning. However, after forty days, a gene on the male’s Y chromosome called the Sex-Determining Region of the Y chromosome,or SRY, becomes active. It contains the instructions for making testosterone, which converts the gonad cells of an embryo into testes, which in turn trigger the development of male sexual organs. If the expression of SRY is blocked, however, the embryo’s gonad cells become ovaries, which trigger the development of female sexual organs. Differences in hormones between the sexes cause as many as one in six mammalian genes to express their proteins preferentially in one sex rather than the other.
Males are the product of testosterone. They are females with an extra gene. And every male on Earth – even the most macho – was in touch with his feminine side for the first forty days of existence.
The big bang of sex
Since most simple organisms are asexual and the first organisms on Earth were single cells, most biologists believe that the earliest life forms were asexual. This is a hugely simpler means of proliferating than sexual reproduction. So how in the world did sex ever arise? 16
Nature tends to adapt to new tasks things it evolved for entirely different purposes. Glutamate, for instance, one of the most important neurotransmitters in the human brain, was used by the very first bacteria for signalling almost 4 billion years ago. 17 Well, sex is no different. The basic components – the fusion of two cells, the mixing of their genes and the separation of those cells – arose for other purposes and then were co-opted for the purpose of sexual reproduction.
A fundamental process was the swallowing of one simple cell by another to create a complex cell, or eukaryote about 1.8 billionyears ago. 18 This involved a multitude of changes inside a cell. For instance, the swallowee’s membrane was replaced by a different type of membrane to permit it to become a cellular organelle. The exact details are not important. The point is that such adaptions made it possible for
one cell to merge with another
.
At some stage in the mists of time – and all that can be done is to speculate plausibly about this – two similar
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