MadSci Network: Genetics

Re: Are hair whorls an inherited trait?

Date: Tue Jun 20 19:59:27 2000
Posted By: Todd Wylie, Staff, Bioinformatics: Human Genome Project, Genome Sequencing Center: Washington University Medical School
Area of science: Genetics
ID: 959953651.Ge


	Congratulations on having twin brothers in your family. You must be very
proud and excited. In answer to your question - hair whorls between
identical twins are not necessarily identical. In fact - they can grow in
opposite directions altogether! 
	Identical twins have, in the past, been considered by most to be carbon
copies of one and other. However, scientists in the last few years have
modified their definitions of the "identical" part of identical twins. It
has been found that though identical twins are for all appearances exactly
alike they still have differences in such characteristics as fingerprints,
hair whorls, moles, and other visible traits. Some identical twins known as
"mirror image twins" even have their internal organs arranged opposite one
and other.
	As you probably know - identical, or "monozygotic," twins actually are
formed from the same zygote in their mother. A zygote is the mother's egg
that has been joined by the father's sperm. A zygote is the very first
stage of a growing baby. Usually the cells of a zygote just keep on
dividing but the zygote stays a single mass of cells. Sometimes the zygote
separates into two masses of cells that develop into two babies. These two
babies are referred to as identical twins and should have identical genes.
So how do they get slight differences in appearance if they have the same
genes - right? Well, it gets a little complicated here.
	 There is a debate in the scientific community called "nature versus
nurture". Basically, it is a debate over how much does the environment and
our genetic composition affect who we are and how we look and act. Twins
may have started out as the same zygote - but once divided they are
subjected to different environmental conditions at different times (even in
the womb). Scientists now believe that such environmental conditions as how
much blood supply, oxygen, and nutrition a baby receives in the womb (as
well as polarity) can play a part in how a twin developed. This may be the
cause of such visible variations as differing hair whorls. Also, even
though twins may have exactly the same number and composition of genes -
the genes probably aren't all turned on at the same time or producing exact
amounts of product in both twins - as these things can be influenced by
environment. So one twin may have a gene that is producing a product that
is not present in the other at that time - or possibly producing much more
product than the other twin is producing.  Though twins theoretically have
"identical" DNA - if a scientist were to examine every molecule between
them they would find tiny differences (called mutations). We're talking
VERY slight differences here between identical twins - but technically
differences just the same. This may not seem like a big deal to say that a
set of identical twins are something like 99.9999% genetically similar, but
let's consider that chimpanzees and humans are thought to be (at current
estimate) 99% genetically similar in coding and non-coding regions to each
other! We certainly don't look or act very much like chimpanzees even
though we're 99% similar genetically. So it is that very small changes in
genetic code can have a huge outcome on what an organism is like and how it
acts - the extent of which we just aren't sure of yet.
	One thing is apparent concerning genetic research at this point - the
pathways and mechanisms are extremely complex and scientists are just
beginning to scratch the surface of what is yet to be known. Hopefully, in
the not too distant future, we will have a better understanding of how
these things work, Victoria. Thanks for the question and take care.


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