[Jacob-list] Color patterns in clones

[Anderson, Gary B.]

An earlier message raised the question of identical features in clones and
the observation that the cat cloned by Texas A&M was not identical in
appearance to the cat that has been cloned (i.e., the cat that donated the
genetic material to produce the clone).  This situation is unusual in that
the donor cat had a color pattern called tortoise-shell (as I read, but
perhaps calico by appearance in the photos), which is a mixture of yellow
and black patches throughout the body.  In addition, the donor cat carried
white spotting genes.  The gene for tortoise-shell is located on the X
chromosomes.  In tortoise-shell cats, which are only females (XX), one X
chromosome carries the gene for yellow pigment and the other the gene for
black pigment.  Only one of the two X chromosomes is active (i.e., one is
'inactive' or turned off) in each cell of a patch of hair, and a patch of
hair produces either yellow or black pigment but not both (hence, yellow and
black patches throughout the body).  Which of the two X chromosomes is
'inactivated' in a given patch is not genetically determined; thus, the
clone could have a yellow patch where the donor cat has a black patch,
despite that the two are genetically identical.  If this concept is extended
across the entire body of the two cats, two genetically identical cats need
not look alike.  DNA analysis (conducted at UC Davis) confirmed the genetic
identity of the two cats, however. 

That that two cats would not look alike could easily have been predicted.
It's a nice example of how clones, which are genetic copies of one another
(by definition), might well differ in numerous characteristics (not the
least of which is learned behavior).

Embryo-splitting procedures, which have been available commercially to
produce identical twins in livestock since the mid-1980s, have demonstrated
that, as stated in the earlier messages, genetically identical twin
Holsteins are not exact copies of one another.  In this instance, genetics
determines the number of pigment cells produced during development but not
precisely where the pigment cells migrate during fetal development.  One
would not expect to have a mostly white calf with a mostly black twin; the
two calves would have approximately the same degree of black versus white,
but the color patterns would be expected to differ as two 'patches' of
pigment cells might migrate together to create a single larger spot on one
calf while the other calf maintains two separate spots.

The same would be expected in Jacob sheep. No one to my knowledge has
conducted an experiment to know for sure, but some breeders are wondering
about it.  Last fall Fred Horak contacted me to ask what features identical
Jacob twins might share.  For example, would knee patches be found on both
lambs of an identical twin set?  What about a slipped eye patch?  What about
horn placement?  My laboratory has used for research purposes embryo
splitting and other embryo manipulation procedures in various livestock
species, and I agreed to consider an experiment aimed at producing identical
sets of Jacob lambs (admonishing Fred for his great idea but lousy timing,
as no self-respecting sheep in California is nonpregnant by mid-fall).
Using Jacob ewe lambs (which in my experience are crummy embryo donors but
the only nonpregnant ewes that I had) and ewe lambs from the University
flock that did not become pregnant from natural breeding (not the ideal
animal to use as an embryo recipient), and with lots of volunteer help from
graduate students and staff in my lab, in late October we attempted an
embryo-splitting experiment aimed at producing sets of identical twin Jacob
lambs.  To make a much longer story short, one ewe to which was transferred
two half-embryos was pregnant (by ultrasonography) with twin embryos/fetuses
at 30 and 60 days gestation, but by 90 days she was resorbing the pregnancy.
Her due date is in 2 weeks, but last week she started to pass fluid and
debris from the failed pregnancy.  

The experience has stimulated me to try again next fall, this time using
adult ewes as embryo donors and adult recipient ewes that are withheld from
breeding.  Survival of each half-embryo is about 50%, and in only about 1/4
of twin half-embryo transfers would one expect survival of both (i.e.,
identical twins).  With no available large funding source, next fall's
experiment will have to be small-scale, but it should be possible with
proper planning to produce several sets of identical twin Jacob lambs.  THEN
when a breeder asks about the degree to which spotting patterns and other
traits are genetically-based, a science-based answer might be available.