What can you conclude about the gene(s) encoding VH and CH?

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.