Solved by a verified expert:1. Recall that allelic forms of
Ig constant regions exist. In the mouse, the locus encoding Ig heavy chain
constant regions is named IgH. The inbred mouse strains BALB/c and C57BL/6 have
different IgH alleles: BALB/c mice are IgHa and C57BL/6 mice are IgHb. They
also differ in their humoral response to phosphorylcholine (PC). While both
strains make equivalent amounts of PC-specific serum antibody, the response in
BALB/c mice is dominated by antibodies with a VH region called “T15,”
which can be detected with an antiserum (anti-T15). In contrast, the
PC-specific antibodies made by C57BL/6 mice don’t use the T15 VH.
In the early 1970’s it was
suggested that variable and constant regions were encoded by SEPARATE GENES! To
investigate the genetic relationship between V-region and C-region genes, two
researchers typed the mice listed in the table below for IgH, then immunized
the mice with PC-BSA. They assayed the resulting PC-specific responses for T15
VH by testing dilutions of each mouse’s serum for precipitation with anti-T15.
The results are shown in the table below.
Strains, crosses, and genotypes
assessed:
Maximum
serum dilution capable of yielding a precipitin reaction with anti-T15
antisera1
Strain
#
of mice
IgH
genotype
No
ppt
1:4
1:8
1:16
1:32
1:64
1:128
1:256
BALB/c
17
a
a
–
–
–
–
10
5
2
–
C57BL/6
18
b
b
18
–
–
–
–
–
–
–
(BALB/c
x C57BL/6)F1
21
a
b
–
–
–
2
9
7
3
–
F2
17
31
21
a
a
a
b
b
b
–
–
21
–
–
–
–
–
–
3
6
–
11
18
–
3
4
–
–
3
–
–
–
–
[(BALB/c
x C57BL/6)F1 x C57BL/6] backcross
32
34
a
b
b
b
–
34
–
–
–
–
5
–
16
–
8
–
3
–
–
–
What can you conclude about the gene(s) encoding VH and CH?
Do these data disprove the
hypothesis that V and C regions are encoded for by separate genes? Explain
your reasoning.
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