Human handedness – inherited or developed?

This post is a little off the topic of what I normally write about, but its something I’ve been wondering about lately as the little munchkin grows up. She’s 6 months old now and is starting to use her hands a lot, going through that grabby stage where anything within arms length is grabbed and shoved in the mouth.  I’ve noticed that more often than not she’s using her left hand for this exploring.  It’s her left hand that grabs her left foot, and her left hand that grabs the spoon during my attempts to introduce her to the wonderful world of solid food.

Does this mean she’s going to be left-handed?  Coming from an extended family of determinedly right-handed people this comes as a bit of a surprise, and makes me wonder if there is an inherited component to handedness, or if there is some trigger during development that makes babies favour one hand over the other.

So now you know…

This from Stuff.co.nz:

British scientists believe they have cracked the answer to the age-old question of which came first, the chicken or the egg?

Researchers have found that a protein called ovocleidin (OC-17) is crucial in the formulation of eggshells, and it is produced in the pregnant hen’s ovaries, the Daily Express reports.

Therefore, the answer to the conundrum must be that the chicken came first.

Using a high-tech computer to look at the molecular structure of a shell, the team of scientists from the Universities of Sheffield and Warwick found that OC-17 acts as a catalyst, kick-starting the conversion of calcium carbonate in the chicken’s body into calcite crystals.

They make up the hard shell that houses the yolk and its protective fluids while the chick develops.

“It had long been suspected that the egg came first but now we have the scientific proof that shows that in fact the chicken came first,” said Dr Colin Freeman, from Sheffield University.

“The protein had been identified before and it was linked to egg formation but by examining it closely we have been able to see how it controls the process.”

But the researchers have not yet got an answer to how the protein-producing chicken existed in the first place.

I may have more to say about this later…  I don’t quite see how this is “proof” that the chicken came first.

Update:  Here’s PZ Myer’s take on this.  In a nutshell:

You simply can’t make the conclusion the reporter was making here. The species ancestral to Gallus gallus laid eggs, the last common ancestor of all birds laid eggs, the reptiles that preceded the birds laid eggs…the appearance of egg laying was not coincident with the evolution of ovocleidin. The first chicken that acquired the protein we call ovocleidin now by mutation of a prior protein also hatched from an egg.

Bird sex gene identified

 In mammals, sex is determined by genes contained on sex chromosomes – males have an X and a Y chromosome, and females have two X chromosomes.  In birds things are quite different, as it is the male that has two of the same type of sex chromosome.  Male birds have two Z chromosomes and female birds have a Z and a W chromosome.  In mammals, the Y chromosome contains a gene called SRY, which “switches on” the male sex determining pathway.  So if you have the SRY gene you develop testes, and if you don’t you develop ovaries. 

Until now, the identity of the master sex-determining gene in birds has been a mystery.  The Z and W chromosomes of birds are not related to the X and Y chromosomes of mammals, and birds do not have an SRY gene.  Research published in Nature yesterday appears to have solved this mystery, with evidence that the DMRT1 gene, located on the Z chromosome, is the bird sex determining gene. 

Sex determination – even more complicated than you thought

Reptiles do all sorts of crazy things when it comes to sex determination.  Some use good old fashioned sex chromosomes like mammals do, but in other species the temperature that the egg is incubated at determines the sex of the offspring.  For instance in the tuatara, eggs that are incubated at 23 degrees are uniformly male, and eggs incubated at 18 degrees always turn out female.   Some reptiles seem to use a combination of both - they have sex chromosomes, but these are overridden when the eggs are incubated at extremely high or low temperatures.  Now it seems it could be even more complicated for some species, as a recent study out of Rick Shine’s lab at the University of Sydney has found that egg size is also a determining factor.  In their study of the Eastern three-lined skink (Bassiana duperreyi), they found that sex chromosomes, temperature and egg size interact to determine the sex of the offspring, with larger eggs incubated at low temperatures producing females, regardless of their sex chromosomes.   

Ed Yong from Not Exactly Rocket Science has a nice write-up about this paper - go check it out!

The paper citation is: Radder, Pike, Quinn and Shine (2009) Offspring Sex in a Lizard Depends on Egg Size. Current Biology in press doi:10.1016/j.cub.2009.05.027

Hox genes rap & reptiles

I found this rap video on Hox genes (yes really) on a couple of other blogs over the past week.  Its so good I thought I’d post it here.

 

And now that you’re all familiar with the basics of Hox genes…   a recent study in Genome Research has found some rather unusual features of this gene family in lizards.  Hox genes play an important role in specifying the anterior-posterior body plan in the developing embryo, and are found in ordered clusters in the genome.  Vertebrates have 4 clusters (A, B, C, and D), with each cluster containing multiple Hox genes.  

This figure shows the 4 Hox clusters in mammals, colour-coded according to where they are expressed in the developing embryo

Hox clusters are unusual in that there is very little repetitive DNA  between the genes (repetitive elements being a large fraction of most eukaryotic genomes).  Thus, in most vertebrates Hox genes are located close together, in a tightly ordered sequence.