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Original
Article About Dexters
"The
Significance of an Achondroplasia-Like Condition Met With in Cattle"
Part VII
by F.A.E. Crew
in Proceedings of the Royal Society of London,
volume 95, 1923, pages 228-255.
Page 246 (Cont'd)
VII - The Significance of the Monstrous Calf
The following points have to be considered:
The Dexter had its origin in the mating of two distinct races of cattle and
is peculiar for its bodily conformation characters, being in all probability
a low-grade achondroplast. The mating of Dexter and Dexter results in the
production of four classes of calves in such proportions as to suggest that
the Dexter is a Mendelian di-hybrid in respect of its coat-colour and bodily
conformation characters. A certain proportion of the calves produced by
Dexter and Dexter matings are still-born and exhibit characters which
constitute the condition of high-grade achondroplasia, the characters of
their parents greatly exaggerated. Associated with these foetal characters
there are usually present anasarca and hydramnios and in some cases
hydrocephalus. The cause of death is either the cause of the hydramnios or
the cause of the peculiarities in the skeleton which render normal delivery
impossible. The cause, in all probability, is endocrinal in nature and is
possibly a malfunctioning of the pituitary. The incidence of the condition
is such as to suggest that the condition is genetical in its origin. There
is but one way of producing a monstrous calf of this sort and that is to
mate Dexter with Dexter. (Report has it that the same condition is found in
the small Breton breed of cattle.)
These facts can be accommodated by the following scheme. The old-fashioned
Kerry and the old-fashioned Devon furnished those factors which in
combination yielded the original Dexter, a big-headed, stout-bodied,
short-limbed individual. Or it may have been that the first Dexter was a
mutation. It suffices to suppose that the original Dexter had the factorial
constitution symbolised by the formula Bb (S l1 12) (s
l1 12). (The significance of the symbols l1
and l2 is indicated below.)
Dexter x Dexter
56.25 per cent 18.75 per
cent 18.75 per cent
6.25 per cent.
Black Dexter-type Red Dexter-type
Black Off-type Red Off-type.
Or 2 “achondroplasts” of the same grade as their parents and 1 non-achondroplast
in every four on the average, while the remaining one would be a somewhat
more pronounced case of “achondroplasia” than either of the parents. So far,
the story is not one of pathology, for, considering what is known of the
human subject, it is seen that a low-grade “achondroplast” is
physiologically
Page 247
efficient. In order to interpret the “bull-dog” calf in terms genetical, the
following hypothesis is suggested.
During the formative period of the breed two independent mutations occurred.
Each of these resulted in the appearance of a factor L1 and L2
respectively, the action of which intensified the action of the factors
which resulted in the production of the low grade of the achondroplasia-like
condition. L1 and L2 are modifying amplifying factors
and their action is additive. Either alone produces a greater degree of the
“achondroplasia” characterisation, and together they yield the highest grade
which is seen in the non-viable “bull-dog” calf. The “lethal” constitution
is SS+L1+L2. These mutations occurred in the early
Dexter and not in the parental breeds, and would seem to be linked with the
factor S.
Before the appearance of these mutant factors the gametes provided by the
Dexter were B(S l1 12) , B(s l1 12)
, b(S l1 12) and b(s l1 12);
after the factors Ll and L2 had appeared
independently, and in all probability in different individuals, the series
of gametes would be as follows:
A = B(S l1 12)
E = b(S l1 12)
B = B(s l1 12)
F = b(s l1 12)
C = B(S L1 12)
G = b(S L1 12)
D = B(S l1 L2)
H = b(S l1 L2)
Random matings in a population containing individuals of the following
factorial constitutions
AA AB AC AD AE AF AG AH
BB BC BD BE BF BG BH
CC CD CE CF CG CH
DD DE DF DG DH
EE EF EG EH
FF FG FH
GG GH
HH
would yield a percentage of monstrous calves which would differ according to
the relative numbers of the genotypes present in that population. The
variety of the genotypes resulting is decided in great part by phenotypic
selection. If it is assumed that red coat colour is not desired then the
numbers of the BE EF BG EH FF FG FH GG GH and HH genotypes will be reduced
very quickly. If it is assumed that the makers of the Dexter breed were
aiming at as short-legged a beast as possible, that is, if it is assumed
that
Page 248
actually they were seeking the biologically unattainable, a high-grade
“achondroplast” with the constitution represented by the formula SS+L1+L2,
then the genotype BB would be discarded, for the breeder would choose his
“best” individuals for further mating. The genotypes CD CH DG GH are the
“bull-dog” calves and being still-born would not be available. So there
would be left the following:
AA (S l1 12) (S l1 12)
) Lowest grade of “achondroplasia”
AB (S l1 12) (s l1 12)
) in the Dexter
AC (S l1 12) (S L1 12)
)
AD (S l1 12) (S l1 L2)
) Intermediate
BC (S L1 12) (s l1 12)
) grade
BD (S l1 L2) (s ll1 12)
)
CC (S L1 12) (S L1 12)
) Exhibition Dexter, a higher grade
DD (S l1 L2) (S l1 L2)
) but viable
In random mating the incidence of the monstrous calf would be 12.5 per cent
of the total births. But if, as suggested above, the breeder persisted in
selecting his animals for breeding from the SS + L1 and the SS +
L2 individuals it is probable that only the genotypes AC AD CC
and DD would be used, and in these circumstances the “bull-dog” foetus would
appear much more commonly. The proportion of “bull-dog” calves varies with
the relative proportions of the different genotypes in the Dexter herd and
with the amount of deliberate selection of individuals with certain
characters which are held in esteem.
On to Part VIII
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