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Pregnancy Timeline by SemestersFemale Reproductive SystemFertilizationThe Appearance of SomitesFirst TrimesterSecond TrimesterThird TrimesterFetal liver is producing blood cellsHead may position into pelvisBrain convolutions beginFull TermWhite fat begins to be madeWhite fat begins to be madeHead may position into pelvisImmune system beginningImmune system beginningPeriod of rapid brain growthBrain convolutions beginLungs begin to produce surfactantSensory brain waves begin to activateSensory brain waves begin to activateInner Ear Bones HardenBone marrow starts making blood cellsBone marrow starts making blood cellsBrown fat surrounds lymphatic systemFetal sexual organs visibleFinger and toe prints appearFinger and toe prints appearHeartbeat can be detectedHeartbeat can be detectedBasic Brain Structure in PlaceThe Appearance of SomitesFirst Detectable Brain WavesA Four Chambered HeartBeginning Cerebral HemispheresEnd of Embryonic PeriodEnd of Embryonic PeriodFirst Thin Layer of Skin AppearsThird TrimesterDevelopmental Timeline
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May 10, 2012--------News Archive Return to: News Alerts


The female Leghorn chicken has a "W" sex chromosome which, like the
human male "Y" chromosome, does not recombine with reproduction.
Recombination of chromosomes helps get rid of faulty mutations

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Male Sex Chromosomes Are Here to Stay

The results of a new study confirm that although male sex chromosomes have shrunk over millions of years and lost many of their original genes, those that remain are extremely important in predicting fertility and are, therefore, unlikely to become extinct

The study, published in the journal Proceedings of the National Academy of Sciences (PNAS) looked at how genes on sex-linked chromosomes are passed down generations and linked to fertility, using the specific example of the W chromosome in female chickens.

Professor Judith Mank, from the UCL Department of Genetics, Evolution and Environment and senior author said: "Y chromosomes are here to stay, and are not the genetic wasteland that they were once thought to be."

W chromosomes in female chickens are entirely analogous to Y chromosomes in men in that they are sex-limited and do not re-combine when males and females reproduce, as the other regions of the genome do. Recombination allows chromosomes to break up linked genes, which makes selection more effective and helps get rid of faulty mutations. Some scientists think that Y and W chromosomes are doomed because of this lack of an ability to recombine.

The study, which involved researchers at UCL, Oxford and the Swedish Agricultural University, compared DNA regions on the W chromosome in different breeds of chickens, whose fertility rates are very easy to measure simply by counting eggs.

Genetic information from two breeds, the Minorca and Leghorn, which lay more than 250 eggs per year, were compared with two breeds selected for male traits (fighting and plumage) called Yokohama and Old English Game. The researchers also looked at Red Jungle Fowl, a wild ancestor of chickens.

The researchers measured gene expression levels from the W-linked genes in all the breeds, and showed that selection for laying lots of eggs has led to elevated gene expression for almost all the W-linked genes in the layer breeds. At the same time, relaxed female selection in the fighting and plumage breeds has led to a loss of W gene expression.

This means that female-specific selection related to fertility acts to shape the W chromosome, and that the chromosome is able to respond to that selection despite all the problems with the lack of recombination.

Professor Mank said: "We have shown that Y and W chromosomes are very important in fertility – the Y in males and the W in females. It is the ability of the W-linked genes to evolve that is the key to their survival, and which suggests that both the Y and the W chromosomes are with us for the long haul."

The research was funded by the Biotechnology and Biological Research Council (BBSRC) and the European Research Council.

For more information or to interview Professor Judith Mank, please contact Clare Ryan in the UCL Media Relations Office on tel: +44 (0)20 3108 3846, mobile: +44 07747 565 056, out of hours +44 (0)7917 271 364, e-mail:clare.ryan@ucl.ac.uk.

'W chromosome expression responds to female-specific selection" is published online yesterday in the Proceedings of the National Academy of Sciences (PNAS). Journalists can obtain copies of the paper by contacting UCL Media Relations.

About UCL (University College London)
Founded in 1826, UCL was the first English university established after Oxford and Cambridge, the first to admit students regardless of race, class, religion or gender, and the first to provide systematic teaching of law, architecture and medicine. We are among the world's top universities, as reflected by performance in a range of international rankings and tables. UCL currently has 24,000 students from almost 140 countries, and more than 9,500 employees.

Original article: http://www.eurekalert.org/pub_releases/2012-05/ucl-mcr050812.php