[Jacob-list] Genetics: Horns / Eyes
Jacobflock at aol.com
Jacobflock at aol.com
Mon Mar 14 16:55:40 EST 2005
In a message dated 3/5/2005 8:30:07 PM Central Standard Time,
labradorridgejacobs at yahoo.com writes:
> Which would prove dominate in the genetics 2-horns, or 4? Or is there a
> dominate/recessive marker with Jacobs horns. What typical combination thereby
> leads to 6-horned animals?
>
Some Jacob flocks have a mix of two horn and four horn Jacobs; very few aree
six. Questions often asked include:
q Can I breed two horn to four horn?
q Why does my four horn ram produce two horn offspring? And vice versa.
q Why does a four horn ram over my four horn ewe produce a two horn
offspring?
Chromosomes: The Jacob has 27 chromosomes; 26 plus an XX chromosome for the
ewe, 26 plus an XY chromosome for the ram. The 26 chromosomes are autosomal;
i.e., they are similar and line up in pairs, or homologous, for rams and ewes.
The X chromosome and Y chromosome are "sex" chromosomes.
The horn chromosome of interest to the Jacob breeder is Chromosome 10. This
is the chromosome that carries the horn gene. The fact that the horn gene is
on chromosome 10 is rather unique. Cattle and goats have their horn gene on
Chromosome 1. Further, the horn gene is autosomal; it is not on a "sex"
chromosome. However, the "sex" chromosome may influence the development of the
horn.
Horn genes and alleles: For an inherited characteristic, such as horns, the
Jacob has at least two (perhaps four or more) genes on the chromosome which are
inherited, one half from each parent. Alleles are alternative forms of genes.
When the sperm and egg unite at fertilization, both contribute one half of
each gene to form one "paired" gene condition. Just as each parent contributes
one half of their genes to the offspring, one half is lost.
When the Jacob progeny has a pair of genes or alleles that are identical for
a characteristic, such as horns, the genes are described as homozygous for a
trait.
When the Jacob progeny has a pair of genes or alleles that are different,
(e.g., one is dominant, the other recessive) they are said to be heterozygous.
When the two genes of a pair are different, one is expressed (you see it), the
other is hidden (it's still there but you don't see it). The gene you see is
called the dominant gene; the gene that is still there but you don't see is
called the recessive gene. Major genes generally produce major effects. For
many genes, you don't see an effect.
Horn gene and its three alleles: The presence of horns (two horns, four
horns is another issue), is probably controlled by a single autosomal locus called
Horns, or, in genetic shorthand Ho. The Ho locus can have three alleles or
forms: (1) Ho+ which produces horns in rams and ewes and would be the allele of
the Jacob, (2) Ho*hl allele which is the hornless allele (horns in rams and
hornless or aberrabt/scurred ewes, and (3) the Ho*P allele which produces
polled sheep.
The existence of the autosomal Horn gene on Chromosome 10 and the allele for
the Horn gene can be concluded beyond what is called a "reasonable doubt".
But, once the observations move beyond the two horn condition, the existence of
alleles becomes probable and the cause of the effect of the gene may have
other possible explanations.
Multiple horns and the scur locus: The presence of multiple horns, that is,
more than two horns, is apparently subject to the presence of a locus called
HNM and is considered an incomplete dominant. The HNM locus is also postulated
as the cause of the congenital defect called Split Upper Eyelid Defect
(SUED), the upper eyelid flesh and the bone of the occipital orbit being
'extensions' of the splitting of the horn core bud.
The hornless allele, Ho*hl is sex limited and perhaps interacts with the Ho
allele producing three observations: aberrant horns, long scurs and short
scurs. (Montgomery and Dolling). Horns and scurs, in cattle are on different
chromosomes; 1 and 17 or 19?. It is possible that the Hohl gene for the Jacob HMN
wanabes is, like cattle, located on a diferent chromosome.
Beginning by counting horns: The Horn locus produces two horns but Jacobs,
and other breeds, are known to produce four or, in some cases, five or six. A
Jacob with more than two horns is said to be multi-horned or polycerate. A
confusion in counting can arise in those cases where it appears a Jacob is a two
horn. Upon close examination a seam representing the complete fusion of the
horns can be seen. This seam may be almost invisible in the yearling but often
becomes more evident over time, say by age two to four. While it seems simple
to distinguish between two and four horns, or four horn and five or six,
mistakes do occur.
Multiple horn allele: The genetic cause of the multiple horn Jacob demands a
gene and or allele. It is assumed that the allele for two horns might be HN+
and the allele for the multiple horn would be HNM.
G. L. Alderson (1980) described a horn number locus which increases the
number of horns to more than the nominal two horns. Alderson observed the results
of breeding two and four horned sheep and concluded that there were two loci
and neither two nor four horn allele was dominant.
Possible inheritance: One possibility that can be used to explain a two or
four horn condition is a two gene theory; two "horn genes" that determine
whether a Jacob will be a two horn or four horn. How does one come to such a
theory? First one must review enough progeny "expressions" to come to a probable
conclusion about the number of genes or alleles that have been inherited from
two and four horn sires and dams. The progeny tells the observer something
about the parents' genes.
What are possible expressions if we assume there is a four horn gene and a
two horn gene? "Basic Horn Genetics in Jacob Sheep, H. Smith, JSS The
explanation postulates a rather straight forward Mendelian pattern which would have to
be confirmed by many observations including all progeny.
Let H represent the four horned gene.
Let h represent the two horned gene.
Thus, HH would be a homozygous four horn Jacob;
hh would be a homozygous two horn;
Hh would be a heterozygous Jacob.
Further, let us assume the four horn trait H is dominant to the two horn h.
Thus a heterozygous genotype Hh is expressed as the phenotype four horn
Jacob.
The two gene theory: If one accepts these definitions and assumptions; the
following breeding possibilities follow using Mendelian logic:
q A 4 horn HH ram and 4 horn HH ewe
HH x HH
HH HH HH HH
(All offspring are four horned)
q A 4 horn HH ram and 4 horn Hh ewe
HH x Hh
HH HH Hh Hh
(All offspring appear four horned but half
are homozygous, half are heterozygous.)
q A 4 horn HH ram and 2 horn hh ewe
HH x hh
Hh Hh Hh Hh
(All offspring appear four horned but
all are heterozygous.)
q A 4 horn Hh ram and 2 horn hh ewe
Hh x hh
Hh Hh hh hh
(Half of the offspring appear four horned but
are heterozygous; half the offspring are homozygous two horn.)
q A 4 horn Hh ram and 4 horn Hh ewe
Hh x Hh
HH Hh Hh hh
(One out four of the offspring is a four horned HH homozygous; one is a two
horn hh homozygous, two more appear as four horn and are heterozygous four
horn.)
q A 2 horn hh ram and 2 horn hh ewe
hh x hh
hh hh hh hh
(All of the offspring are two horned
hh homozygous)
Multiple horns and the "splitting" gene theory: Another possibility that is
widely considered is the "splitting gene" theory. In this theory, the
multiple horn is an incomplete dominant and the same horn gene that splits and this
splitting also is the cause of the congenital Split Upper Eyelid Defect
(SUED). In order to arrive at this theory, I seem to recall that there is a "line
of potential expression" extending from the horn bud and the skull to the upper
eyelid at either end of which an embryonic cleavage can occur. The cause of
the "splitting" is not explained but the congenital SUED appears to be
associated with multiple horns.
What is a horn: What seems a simple observation and counting is open to
error, as indicated above. But is the "horn" we think we see, always a horn? One
must be sure it is a horn. A horn has a core that runs from near the end of
the horn and is affixed firmly to the skull. Identifying a horn on a ram is
relatively easy compared to correctly identifying a horn on a ewe. Horns grow
unless removed with the core; scurs and aberrant horns appear to lack growth
activity associated with horm growth.
The Hornless gene (Ho*hl) is expressed in three ways: (1) the aberrant horn
which has a keratinous sheath but has a minimal core and is weakly attached to
the skull, (2) the long scur and (3) the short scur. Correct identification
of the hornless gene in the ewe can be very difficult prior to four to six
months. What is considered as the more feminine, dainty, horn on the ewe may
actually be an expression of the hornless gene; thois is particularly true of ewe
lambs.
The aberrant horn or scur appears: The hornless gene (Ho*hl) is apparently
recessive. The hornless gene in the ram is not seen; the ram appears to carry
normal horns but the ram can be a carrier. The Ho*hl carrier ram when mated
to a Ho*hl ewe produces aberrant horn and scurred ewes. Like the Ho*hl ram, an
apparently horned ewe can be a carrier of the hornless gene.
An Ho*hl ram carrier (has horns) and a horned ewe that is an Ho*hl carrier:
Ho Ho*hl x Ho Ho*hl
HoHo Ho HoHl Ho Hohl Hohl Hohl
(One out four of the offspring is horned homozygous; one is hornless Ho*hl
and will be observed as an aberrant horn or scurred ewe, two more appear as horn
and are heterozygous hornless.)
If one wanted to test for the hornless gene, one should concentrate on
breeding a 'test ram' to a known heterozygous aberrant horned or scurred ewe. If
the ram produces an aberrant horn or scurred ewe, he is a carrier if even one
out of six ewes is aberrant horned or scurred. Ram lambs will always appear
with normal horns and cannot be included in the six lamb progeny test.
The extreme case would be a ram that is homozygous for the hornless gene. In
this case the ram will appear to have horns and all the ram's male offspring
will appear to have horns. However, ewe offspring will have aberrant horns or
scurs.
Ho*hl Ho*hl x Ho Ho*hl
Hohl Ho Ho Hohl Hohl Hohl Hohl Hohl
(Two out of four of the offspring are homozygous for the hornless trait; two
are heterozygous hornless. Ewes will be aberrant horned or scurred and rams
will be horned)
The appearance of the hornless gene may involve several generations depending
on the genotype of the sire and dam. There are two solutions: one is to
progeny test rams, the second is to cull ewes that exhibit the trait. If one
agrees that the Jacob is horned in both sexes, elimination of the 'hornless'
trait should be important to the integrity of the breed.
Testing theories: In order to have a basis for a two gene or single
"splitting" gene theory, many observations related to two and four horn breedings must
be reviewed. It is after noting each and every progeny's horn expression and
the horn expression of each parent that the theory can be posited and tested.
The theory is offered based on observations of all the progeny and proved by
utilizing and testing its predictive value.
Where does one find a record of many progeny and parents, certainly far more
breedings than one would see in a small flock? The most logical place to look
is the flock book or similar databases. The flock book and related data base
lists the progeny or offspring of all known parents and the number of horns
is listed for progeny and parents, and in many cases grandparents and great
grandparents. But here is the limitation: most flock books and related data sets
contains only those Jacobs submitted for certification or registration.
Why does this limitation exist? Because the registration process "requires"
certain phenotype expressions and observation in lambs is not always accurate.
Further, not all the progeny of every breeding are reviewed and recorded.
This process skews breeding results, genotyping and identification of the genes
and alleles associated with certain traits. It may come to pass that one
erroneously concludes that since registered sheep (albeit hornless) produce
"purbred" sheep (hornless), the Jacob is hornless. There are a number of "horned
ram"/"hornless ewe" brreeds that may be in the gene puddle.
Designing a test: The number and effect of a gene is determined by observing
ALL the offspring of ALL test breedings. The progeny of various breedings
are measured or tested for the phenotype characteristic, in this case number of
horns, and their parents' are similarly measured or tested. Tests must go for
generations.
Every progeny and all matings must be noted. After many progeny measurements
or tests, say a hundred, the progeny are grouped by the parent types (four and
two horn rams over two and four horned ewes). The ratio of two and four
horned progeny suggest the number of genes, whether there is any sex influence
whether dominant, recessive, etc. If any progeny are excluded, the data is
virtually useless and represents no more than a starting point for further
speculation. The breeder's choice to submit only selected lambs to a registry or a
registry's process which discourages all births, clouds the data present in most
registries.
If a sufficient number of all progeny and parents through three or four
generations were able to be documented and categorized it would suggest the genetic
basis for horn inheritance, the final test will be to prove the hypothesis.
If you would like some starting references to explore these theories further
:
G. Montgomery, "Mapping the horns locus in sheep", Journal of Heredity,
Sept/Oct, 1996, pp.358-363.
CHS. Dolling, Breeding Merinos, Rigby Press, Adelaide, 1970.
CHS Dolling, 1961, Hornedness and polledness in sheep: Triple alleles
affecting horn growth in sheep, Australian Journal of Agriculture Research, Vol. 12,
pp253-261.
H. Smith, "Basic Horn Genetics in Jacob Sheep", Jacob Journal, Spring 2000,
pp. 50-52.
>
>
> Along the same line of questioning, I would presume brown to be the dominate
> eye color, with the occassional blue being recessive? I've not actually seen
> (to my knowledge) a blue eyed Jacob, but would imagine them to be
> impressive.
>
>
Eye color observed by the shepherd depends on (1) the "part" of the eye being
observed and (2) actual color. The eye has several parts of course: the iris
which is yellow or gold (looks brown but its not) and black (melanin again),
the sclera (the "baggie" that holds the eye together) in sheep may have
pigmentation which varies in color by breed and individuals, and the choroid (a
"baggie in the baggie") which is a "screen" sometimes specifically called the
tapetum lucidum which is (I believe) white in about 95%, blue in about 5%. Colors
from the sclera often get "colored" by choroid.
The function of the sheep's eye and the colors of the eye tunics are sort of
related to its survival in adverse environments. The shape of the iris (like
a TV screen), amount of melanin and the choroid reflectivity and all the other
cells and muscles spot environemntal threats. Those that are breeding for
blue eyes probably know more about why this is desirable.
> Thanks,
> Chris
>
Fred Horak
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