Women in Science: Cracking the code in the battle of the sexes
I’m an evolutionary geneticist, and I’m interested in how all the genes in the genome work together to make different animal forms.
In my lab, we use the differences between males and females in animals to untangle how these different forms are encoded by a set of genes that is nearly identical. For example, in humans, the genetic difference between females and males is confined to the Y chromosome which is passed from father to son. This means that men have one X and one Y chromosome, and women have two X chromosomes.
The human Y chromosome carries just 27 distinct genes, although many of these are present in multiple copies, and these genes seem to play a key role in male fertility. This is less than 1 per cent of the genes in the genome, and the other 99 per cent of genes are present in both sexes. It is therefore difficult to understand how the profound differences we see between the women and men in morphology, behaviour, disease incidence and many other traits all comes down to less than 1 per cent of the genome. This is where my lab comes in.
In addition to studying the sex chromosomes, we examine the way that females and males use the remainder of the genome, the vast majority that is shared between them, in different ways to produce the differences we observe between the sexes. We aren’t focused so much on the genetics of initial sex determination, which happens very early on in development and acts as the initial switch toward either male or female forms, but rather what genes are involved in later stages of development and at maturity.
There are a lot of genes, on and off the Y chromosome, that are involved in male and female differences. More than half of the genes in the shared portion genome are expressed differently between females and males. Gene expression is the way in which genes are turned into protein, and this is the primary route by which the same gene can create multiple forms, in this case, male and female forms.
We work on many different animals, though at the moment we’re focused a great deal on birds. Bird sex chromosomes are converse to the human system, and instead of a Y chromosome, they have a W chromosome that is confined to females and passed is from mother to daughter. Females therefore have one W and Z chromosome, and males have two Z chromosomes. Female heterogametic sex chromosome systems like this aren’t at all uncommon, and are known in many other animals, including butterflies, moths, and some eels and snakes.
We aren’t entirely sure how many genes are on the bird W chromosome, but it is likely less than 50 genes in total, and this is again a very small proportion of the genome. The genes on the W chromosome seem to all play a major role in female fertility, and yet this small piece of the genome cannot explain the extreme differences we see between the sexes in some bird species, such as the peahen and peacock.
Birds are quite unusual in the fact that females and males in some species, like peafowl, have profound differences, and others such as many species of swans and geese, are very difficult to tell apart. We’re currently comparing the genomes and gene expression patterns (the way that genes are turned into proteins) in several species of birds to unravel how profound sex differences evolve, the degree to which the sex chromosomes are responsible for these differences, and how the shared portion of the genome is used differently in females and males.
At this point, you may be wondering why this all matters, and there are a lot of reasons that it might. The W chromosome in chickens, key to female fertility, may be an important component in agricultural productivity for egg producers. More broadly, understanding how males and females in animals in general differ at the genetic level helps us understand human sex differences as well.
More information about my lab and our work is available at http://www.ucl.ac.uk/mank-group/index.htm.
Professor Judith Mank is spoke at ZSL and L’Oréal-UNESCO’s Soapbox Science on Southbank, 16 July 2012 www.zsl.org/soapboxscienceTagged in: chromosome, DNA, gender, genes, genetics, science, sex, women
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