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ABSTRACTS
Inbreeding involves crossing two animals related to the extent of 50 per cent or more.
Examples of such matings are : ' full brother and sister, half brother and sister, sire and
daughter, and son to dam. ' Line-breeding involves two animals of between 25 and 50
per cent identity of inheritance. Some workers suppose that line-breeding permits con-
centration of genes from a larger number of individuals without the dangers of the high
concentration and its accompanying weaknesses possible with inbreeding. Variations
between full brothers and/or sisters in all classes of animals confirm the contention that
genes cannot be mixed with uniform results as can two chemical reagents. Some autho-
rities recommend such mixing, however, if under expert guidance. Authorities like
Castleman, Marshall and East refer to the value of inbreeding. Castleman also says,
' We use it but feel that in so doing we are playing with fire. '
East reminds us that very rigid laws have prevented the importation of any cattle,
except for slaughter, to the Jersey or Guernsey islands since .1763. Each of
these islands, therefore, has a breed of cattle that is very highly inbred. He
adds that this fact will be more fully appreciated when it is realized that the
larger island, Jersey, is only eleven miles long by six miles wide. The famous Jersey bull,
Sybils Gamboge, which sold for $65,000, was the result of half brother and sister matings
for four generations. Sixty generations of inbreeding have made it possible for Dr Helen
King at the University of Pennsylvania to produce rats which are the exact duplicates
of their parents. Records exist of flocks of sheep which have been maintained a hundred
years with no introduction of new animals. Horses have been inbred for size and speed
with good results.
Defects as well as points of merit may be concentrated by inbreeding. The practice
must obviously be carried out by an expert breeder and be accompanied by rigid selec-
tion. Inbreeding, however, without discrimination and rigid selection will yield more
evil results than good.
[J. N. W.].
Inheritance of horns and scurs in sheep. H. I. IBSEN and R. F. COX (1940).
J. Heredity 31, 7, 327.
F OUR pairs of genes for horn or scur formation in sheep are discussed in this paper as
follows : 1. H and P, horned and hornless respectively. ' HH animals of both sexes
are horned, provided other genes, mentioned later, do not interfere. PP animals of both
sexes are hornless. In heterozygotes (HP),H is dominant in males and P in females.' 2.
Ih and ih which inhibit horn formation. Dominance is complete in both sexes. Because
of the interaction of a third pair of genes, however, ihih females are sometimes hornless,
whereas ihih males are always horned. 3. Hm and hm. The latter is the horn modifier
gene. Hm prevents hm from having any effect. If either male or female is homozygous
or heterozygous for Hm or if the male is homozygous for hm, modification does not occur ;
if, however, the female is homozygous for hm, modification occurs. 4. Sc and sc mani-
fest themselves only in hornless animals, producing scurs when Sc is present ; sc is the
recessive gene for this character. The expression of Sc, when heterozygous with sc, is
affected by the presence of Ih in pure bred 'polled ' Rambouillet females but not in the
males. Whether females of this breed, if ScSc, have scurs when Ih is present is not known.
The authors mention that crosses between certain horned and hornless breeds give
F 1 progeny of which all males are horned and all female hornless although they develop
scurs later in life. This they explain by assuming that H is dominant in the male and its
allelomorphic gene P is dominant in the female. It also explains both horned and horn-
less progeny of either sex in the F 2 generation of such a cross. (F 1 = HH × PP = HP ;
and F 2 = HP × HP = HH + 2HP + PP). The Dorset Horn and the Suffolk breeds, res-
pectively, may illustrate the horned and hornless breeds involved. No satisfactory
genetic explanation has yet been given, however, for pure breeds in which the males are
horned and the females are hornless.
Arkell's data support the theory that the Rambouillet breed is homozygous for horned
(H) and also homozygous for a horn modifier gene (hm) which causes the female to be
hornless. Thus, in both sexes the constitution will be HH hmhm while ordinary horned
breeds are HH HmHm. Arkell mentioned occasional hornless males of this breed in
1912 indicating P was present although more recently no such condition has been reported.
From the absence, at present, of occasional hornless males, and therefore of females