TheH-2 complex of mice plays a major part in transplantation immunity. Genetical studies by Snell and his colleagues have revealed nine different combinations. Serological investigations have demonstrated five basic antigens,C, D, E, FandK. The distribution so far as it is known is given in the table. We postulate two alleles ofE, and at least three ofD. No allele ofKhas been definitely established as yet. The extreme polymorphism of the system is largely contributed byDand possibly byKas well. Difficulty in demonstrating allelomorphs may be attributable to the following factors: (1) technical difficulties in the demonstration of some partial antibodies in mice, (2) low concentration of certain antigens on the erythrocytes, (3) the small population so far studied serologically. Mutations have been shown to arise not infrequently in some strains studied in America and involve the simultaneous transformations of three antigens, we have found that two of the antigens known to have been involved are separable by crossing-over.
The number of antigenic components detectable in system s such as
H
-2 in mice depends in part upon technical developments. It is also limited by the number of genetically different individuals available for study and a formula for deducing the number of antigenic components detectable for
N
unrelated systems is given in the text. If parental types and descendants derived from them by crossing over are included the possibilities are decreased. A new antigenic factor(s) has been brought to light by technical improvements and two more have been revealed by crossing over. Three cross-over combinations
H
-2
g
,
H
-2
h
and
H
-2
i
are now available as homozygotes. A preliminary map gives the order
DCVK
for genes within the
H
-2 system. The relative positions of
E
and
K
are not yet known but their genes are closely linked. The frequency of recombination between
D
and
K
was 1⋅03% for both sexes and 1⋅4% for females. Values obtained by other workers are given in the text. It is concluded that crossing over must make a material contribution to the polymorphism of the
H
-2 system. No convincing example of mutation within the system has been published as yet. One new cross-over system
H
-2
i
appears to have certain selective disadvantages whilst statistical evidence strongly indicates that
H
-2
b
,
H
-2
d
and
H
-2
k
are favoured by selection under laboratory conditions.
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