This article was reproduced here
with the direct permission of the author and was first published in "OUR
DOGS"on FEB 17th, 2005
Merle Chihuahuas- time to call a halt
by Dr. Malcolm B. Willis
Coat colour in the Chihuahua is quite complex
because a range of colours is acceptable in both varieties. One colour that
does not exist naturally in the breed is Merle.
Merle is a gene that causes patches of lighter colour (usually greyish blue)
to appear in the coat. There are two alleles which are termed M (merle) and
m (non merle) with merle being dominant to non merle. All breeds carry the
merle gene but most have the non merle m in duplicate and are thus homozygous
for non merle (mm). The dominant merle gene is found in Shetland sheepdogs,
Rough Collies, Border Collies, Australian Shepherds, Dachshunds (where it
is called dapple) and a few other breeds. A variant exists in Great Danes
and Australian Shepherds carry Tweed which is a variant of merle. The Chihuahua
does not have M and all Chihuahuas should be mm.
Recently a number of Chihuahuas that carry merle have appeared in USA and
are gaining some credence as 'fashion accessories' and the like. Most reputable
breeders are against the gene and it would be fair to say that it must have
come in through a crossing... probably with Dachshunds. Since the AKC would
not register crossbreeds as Chihuahuas one has to conclude that somewhere
Dachshunds have appeared in pedigrees as Chihuahuas... with false names? Crossbreeding
for a specific purpose, such as Dr Cattanach's work seeking to bring a docked
tail into the Boxer, needs to be done under KC approval and supervision so
that pedigrees remain correct even if more than one breed is involved. I have
no evidence of that in the Chihuahua merle situation.
In case some of you feel that another colour would be fine it is crucial to
point out that merle is a dangerous gene. The homozygous merle MM is usually
white and has very often impaired hearing and sight. For this reason some
Kennel Clubs do not recognise merles and many KC's and breeders do not mate
merle to merle. Merle to merle would be Mm to Mm which would give rise (in
large numbers) 25% MM, 50% Mm and 25% mm and clearly there are serious problems
with the 25% homozygous merles.
The
popular belief is that heterozygous merles Mm are quite normal'
However some thirty years ago Hannover workers showed eye problems in merle
Dachshunds (Wegner and Reetz, 1975 Dausch et al, 1977) sperm imperfections
(Treu et al,1976) and impaired hearing (Reetz etal, 1977). These problems
were found in homozygous merles and also heterozygous merles. Hearing faults
ranging from complete deafness to slight hardness of hearing occurred in 54.6%
of homozygous merles and 36.8% of heterozygous merles, Based on this work
and their own work on eye failings Klinckmann et al (1986) suggested restricting
the breeding of merles on welfare grounds.
This means that the long held belief that MM dogs had problems but not Mm
is not true and thus merle is a gene that would be best eliminated as a defect
and certainly not introduced to new breeds where it does not really exist
such as the Chihuahua.
The KC should ban merle as a colour in Chihuahuas and not register any merles
on the grounds that it is a serious defect but also because any merle Chihuahuas
may have false pedigrees later back if the crossing with Dachshunds is how
the gene was introduced. It may be impossible to prove this but DNA testing
might be helpful. On the other hand non merle dogs mm regardless of ancestry
are safe as regards merle because they cannot have it. If a dog has all four
grandparents merle but is non merle then it cannot carry the merle gene. On
the other hand if it carries Dachshund "blood" it will run the risk
of carrying Dachshund problems that may not exist in Chihuahuas at present.
Aside from fashion accessory (God save us!) there is no gain and some loss
to be made from the merle in the Chihuahua and thus it needs a combined effort
by all kennel clubs to eradicate the gene from the breed.
Let me state that I have a vested interest in that my wife has Chihuahuas!
* Since writing the above article I have come into possession of a report
issued by the Board of Directors of the Chihuahua Club of America Inc to members
of that club (dated1st May 2004). This has also been circulated to members
of the British Chihuahua CIub.
The report highlights the fact that historically the breed has permitted any
colour. I am in favour of such policies which contrasts with some breeds where
specific colours are frowned upon or disqualified for no logical reason beyond
historical legacy or personal dislike. Thus the Newfoundland standard accepts
black-and-white and brown but not brown-and-white, or Tibetan terriers should
not be liver. There
is no logic in such rules but merle is biologically dangerous and not
like any other colour.
A colour associated with eye and hearing defects should be selected against,
however attractive some people may consider the merle variant. Moreover, the
argument that it has been around about 10 years plus does not explain whence
it came. There are suggestions that merle can lie hidden (camouflage merles)
but merle is a dominant and thus any merle must have at least one merle parent,
however pale the pattern is expressed. There is a need to DNA test merles
and compare these with DNA tests on Dachshunds whence it appears the colour
infiltrated the Chihuahua breed. Are we to breed Chihuahuas or crossbreeds?
REFERENCES
Dausch, D. et al (1977) Dtsch. Tierarztl. Wschr. 84: 469-75
Klinckmann, G. et al (1986) l . Vet.Med. A. 33: 674-88
Reetz L et al (1977) Dtsch.Tierarztl.Wschr. 84: 273-7
Treu, H. et al (1976) Zuchthyg. 11: 49-61
Wegner, W. & Reetz,l. {1975) Tierarztl prax, 3: 455-9.
© 2005 Dr Malcolm B.Willis all
rights reserved.
No image or script may be reproduced without expressed written permission.
Thinking of buying
a merle Chihuahua? They are the result of cross breeding. Every geneticist
that has been contacted has given the same comment, "It is highly unlikely
that a mutation has occurred, merle Chihuahuas are most likely the result
of cross breeding." That means not only are they not purebred dogs but
you may possibly be getting the genetic problems of several different breeds.
Of course the breeders of these dogs will claim something different because
their main motivation is to sell you what they claim are *rare* dogs. The
truth is available here.
Dr Malcolm B. Willis,
genetics professor and author of "The Genetics Of The Dog",
has published an article in "Our Dogs" and has kindly given me permission
to reprint it in its entirety.
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