Free The Journey of Man: A Genetic Odyssey by Spencer Wells Page A

non-recombining portion of the Y, is pretty much completely unrelated to the X. Thus it has no paired chromosome withwhich it can recombine, and so it doesn’t. It is passed unshuffled from one generation to the next, for ever – exactly like the mitochondrial genome.
    The Y turns out to provide population geneticists with the most useful tool available for studying human diversity. Part of the reason for this is that, unlike mtDNA, a molecule roughly 16,000 nucleotide units long, the Y is huge – around 50 million nucleotides. It therefore has many, many sites at which mutations may have occurred in the past. As we saw in the last chapter, more polymorphic sites give us better resolution – if we only had Landsteiner’s blood types to work with, everyone would be sorted into four categories: A, B, AB and O. To put it another way, the landscape of possible polymorphisms is simply much larger for the Y. And critically, because of its lack of recombination, we are able to infer the order in which the mutations occurred on the Y – just like mtDNA. Without this feature, we can’t use Zuckerkandl and Pauling’s methods to define lineages, and Ock the Knife can’t help us with the ancestors.
    How does the Y manage to exist without recombination – doesn’t this contradict the idea that we need to create diversity in case it’s necessary to react to a changing environment? The short answer is that there almost certainly are negative evolutionary consequences to the lack of recombination – part of the reason for the low number of functional genes found on the Y. The number of active genes varies greatly among different parts of the genome. In the mitochondrion, for instance, there are thirty-seven. The total number of genes in the nuclear genome is around 30,000 – approximately 1,500 per chromosome, on average. Most of the thousands of genes that would have been found in the bacterial ancestor of the mitochondria have been lost over the past few hundred million years as mitochondria have become more parasitic, giving up autonomy for a cosseted life inside another cell. Some have actually been inserted into the nuclear DNA, leaving us in the odd situation of having small pieces of our genome that are bacterial in origin. So in the case of mitochondrial DNA, it does look like there was pressure for it to lose its genes, transferring the critical ones to the nucleus where recombination can keep them in shape for the evolutionary race.
    We see the same pattern of gene loss for the Y-chromosome.Although the average human chromosome has roughly 1,500 active genes, only twenty-one have been identified on the Y. Some of these are present in multiple, tandem copies – as though the copying machine stuttered as it was duplicating that gene at some point in the past; these are counted as a single gene in our tally. Interestingly, all of the twenty-one genes on the Y are involved in some way in the creation of ‘maleness’ – particularly the gene known as
SRY
, for ‘
S
ex-determining
R
egion of the
Y
, which is the master switch for creating a male out of an undifferentiated embryo. The rest have secondary functions involved in making men look (and act) like men. For the most part, though, the DNA that makes up the Y is devoid of any discernible function. It is so-called ‘junk DNA’, which means that it is transmitted from one generation to the next without conferring any utility. But while it may be biological junk, it is like gold dust to population geneticists.
    As we have seen, we can only study human diversity by looking at differences – the language of population genetics is written in the polymorphisms that we all carry around with us. These differences define all of us as unique individuals – unless we have a twin, no other person in the world has an identical pattern of genetic polymorphisms. This is the insight behind a DNA ‘fingerprint’, used to identify criminals. Applied to the Y-chromosome, it