/. Embryol. exp. Morph. Vol. 33, 1, pp. 205-216, 1975
Printed in Great Britain
205
Sex chiniaerism and germ cell distribution in
a series of chimaeric mice
By ANNE McLAREN 1
From the Agricultural Research Council Unit of Animal Genetics,
Institute of Animal Genetics, Edinburgh
SUMMARY
1. Of 30 mice born from aggregation of embryos from a multiple recessive strain with Fx
embryos carrying the contrasting alleles, 4 females and 20 males proved to be overtly chimaeric.
2. Three XX\XX females, five XYjXY males and eight XY\XX males were identified by
chromosome analysis. Thus 50 % of the population analysed were sex chimaeras, and all
of these developed as phenotypic males, though one showed evidence of hermaphroditism.
3. In seven XY\XX chimaeras that bred, the genetic component undergoing spermatogenesis coincided in every case with the component identified by chromosome morphology
asJF.
4. The Fx component predominated in metaphase plates derived from cultured blood
cells. Comparison with direct preparations from bone marrow suggested selection in favour
of Fx cells, either through differential proliferation of stem cells in vivo or differential response
to phytohaemagglutinin in vitro.
5. In XY\XX males, the percentage of XXcells detected varied from 1 % to 98 % in blood,
and from 0 % to 80 % in bone marrow.
6. Of eight 'single-sex' chimaeras progeny-tested (three XXIXX, five XY\XY), only one
showed evidence of a mixed population of germ cells. The proportion of the two types of
progeny varied significantly from litter to litter, but was unrelated to the age of the male.
INTRODUCTION
When pre-implantation mouse embryos are united in pairs to produce
chimaeric animals, 50 % of the pairs on average will consist of male/female
unions. Of the sex chimaeras (denoted XYjXX'm this paper) which result when
both cell populations survive, many develop as phenotypically normal fertile
males, some (the proportion depends on strain combination) may develop as
females, and rather few show any evidence of hermaphroditism. In consequence,
the sex ratio of mouse chimaeras is usually characterized by an excess of males
(McLaren, 1972a). There is evidence that in mice, unlike amphibia, only the
XY germ cell population is capable of differentiating into functional gametes
in XY/XX males (Mystkowska & Tarkowski, 1968; Mintz, 1968), so that all
the progeny of such males are derived from one only of the two chimaera
components.
1
Author's address: M.R.C. Mammalian Development Unit, University College London,
Wolfson House, 4 Stephenson Way, London NW.I 2 HE.
206
A. MCLAREN
Some indirect evidence on the incidence of sex chimaerism can be obtained
from the sex ratio of chimaeras and from their breeding records. Assuming a
primary sex ratio of unity, the incidence of sex chimaerism in a sample of m
male and / female chimaeras is estimated as (m—f)/(m+f), if all the sex
chimaeras develop as males. If not all sex chimaeras develop as males, this
approach will underestimate the true incidence of sex chimaerism. From the
breeding records, the proportion of chimaeras producing' single-type' progenies
in which only one component of the chimaera is represented gives a maximum
estimate of the incidence of sex chimaerism. This will almost certainly be an
overestimate, because although sex chimaeras can only give 'single-type'
progenies, by no means all 'single-type' progenies come from sex chimaeras.
Mintz (1968) has suggested that cell selection during gametogenesis may greatly
affect the proportions of the two types of offspring in some strain combinations.
Direct evidence on the incidence of sex chimaerism comes only from chromosomal studies, and these are few in number. Mystkowska & Tarkowski (1968)
reported that, of six overt chimaeras studied as adults, three were sex chimaeras
(all males). Of nine foetuses derived from embryo aggregation, six (three males,
two females, one male hermaphrodite) proved to be sex chimaeras (Mystkowska
& Tarkowski, 1970). Among eight adult presumed chimaeras, Milet, Mukherjee
& Whitten (1972) found four sex chimaeras (two males, one female, and one
intersex).
The present paper describes chromosomal studies and/or progeny testing of
26 adult animals derived from aggregation of embryos of a single strain combination. All but four of the aggregants were overt chimaeras.
MATERIAL AND METHODS
The chimaeras were obtained by aggregation of embryos, as described by
Bowman & McLaren (1970). Each aggregant contained one embryo from a
'multiple recessive' stock homozygous for eight recessive genes (non-agouti,
brown, dilute, pink-eye, chinchilla, waved-2, short-ear, vestigial-tail) and two
biochemical markers (Id-1 and Gpi-1), and one Fx embryo carrying the contrasting alleles in homozygous or heterozygous form, from crosses of C3H/Bi/
McL males with either C57BL/McL females or CBA/Fa females (X4, 5 and 6
only). Observations on some members of the series are described by McLaren &
Bowman (1969), McLaren (1972a) and Griineberg & McLaren (1972). In
table 2 of the last paper, Jf21 was erroneously listed as a male, and XI6 as
a female.
Of 30 aggregants surviving at least 2 weeks after birth (Table 1), 24 were
overtly chimaeric and 6 non-chimaeric as judged by coat colour and external
morphology, as well as by enzyme determinations on a number of different
organs. Three died before reaching sexual maturity (one recessive non-chimaera
, and two chimaeras X6$ and X\5<$), and one recessive non-chimaera
Sex chimaeras in mice
207
Table 1. Data available on 30 mice derived from, embryo aggregation
Non-chimaeric
Aggregants progeny-tested and
examined cytologically
Aggregants only progeny-tested
Aggregants only examined cytologically
Aggregants neither progeny-tested nor
examined cytologically
Total surviving at least 2 weeks
Overtly
chimaeric
Recessive
Dominant
17
1
1
1
2
1
0
0
4
2
4
1
2
24
0
(XI 7c?) produced no progeny and was not examined cytologically. The remaining
26 animals comprised 22 overt chimaeras and 4 non-chimaeras (two recessive,
two dominant).
Female aggregants were mated with males from the multiple recessive stock,
and each male aggregant was mated with two females from the same stock. If
aggregants failed to produce progeny during a 2- to 3-month period, the consorts
were replaced. All surviving progeny were classified by inspection at 2-3 weeks
for non-agouti, brown, dilute, pink-eye, chinchilla, waved-2, short-ear and
vestigial-tail, and the Id-1 type of many of the young was determined by electrophoresis of liver biopsy specimens.
The aggregants were killed at ages ranging from 9 to 23 months. The reproductive organs were examined; part of each gonad was then fixed in Bouin's
fluid for histological examination, and part was fixed in fresh 3:1 methanol:
glacial acetic acid for examination of air-dried meiotic chromosome spreads
(Kofman-Alfaro & Chandley, 1970). Various tissues were taken for enzyme
determinations, and eyes, skeleton and skin were preserved. Blood was cultured
and chromosome preparations made as previously described (McLaren, 19726).
From some of the animals, direct preparations of chromosomes from bone
marrow were also made.
Three methods were used to identify a Y chromosome in order to sex the
chromosome preparations. (1) On preparations stained with carbol fuchsin,
exceptionally well-spread metaphase plates were sexed by the 'morphological'
method of Stich & Hsu (1960), in which the presence of three 'smallest' chromosomes is taken as indicating an XY karyotype. (2) Using autoradiography, a
late-labelled chromosome which was also one of the eight smallest of the complement was taken to be a r chromosome (McLaren, 1912b). (3) Using a technique
modified from Arrighi & Hsu (1971) and Gagne, Tanguay & Laberge (1971),
Giemsa staining revealed the Y chromosome as a small chromosome lacking
darkly staining pericentric heterochromatin. The bone marrow preparations
were mainly sexed by the autoradiographic method.
208
A. MCLAREN
The Giemsa staining technique enabled metaphase plates of the two chimaera
components also to be distinguished. Chromosome 14 shows a greatly reduced
amount of pericentric heterochromatin (forming a dark blob at the centromeric
end) in C3H and CBA mice and also in the multiple recessive strain, but not
in C57BL mice (McLaren, unpublished results). Plates of the 'recessive' component therefore showed two medium-sized 'blob-free' chromosomes, while
those of the Fx 'dominant' component showed only one such chromosome.
RESULTS
Of the 24 overt chimaeras, 4 were female and 20 were male. This ratio gives
an initial estimate of sex chimaerism of 0-67, and does not differ significantly
from the 1:3 ratio of females to males that would be expected if 50% of all
chimaeras were XYjXX, and all these developed as phenotypic males.
Of the 21 chimaeras which bred, all but one produced 'single-type' progeny.
This gives a maximum estimate of sex chimaerism of 0-95. Thirteen of the
progenies were from the recessive component of the chimaera, and seven from
the dominant component. The remaining animal (X2>2>S) sired 32 young from
the recessive and 9 from the dominant component. There was no relation
between the genotype of the progeny (recessive or dominant) and the number
of litters produced or the total number of young born (McLaren, unpublished
results). All young produced were either of the recessive or of the dominant
phenotype at all loci examined, and no evidence of mosaicism was seen (McLaren,
unpublished results). One male (XI) produced no progeny, and no spermatozoa
were found in the uteri of females with which he mated.
The three methods of sexing metaphase plates from the cultured blood cells
are compared in Table 2. Although none of the methods is error-free, they show
a reasonable degree of concordance. The autoradiographic method is the least
accurate, but is free of bias. The Giemsa method tends slightly to overestimate
the proportion of XY cells in a mixed population, since it is easier to spot a Y
chromosome if it is there than to be certain of its absence. The morphological
identification is in every instance based on a smaller number of plates than the
other methods, as only exceptionally high quality plates can be used.
The diagnosis of sex chromosome constitution was made primarily on the
basis of the blood culture chromosome analyses, using other information where
appropriate. A component of opposite chromosomal sex to the phenotypic sex
of the individual was regarded as evidence of sex chimaerism if it amounted to
5 % or more, taking into account the results of all three scoring methods. XI
comes close to the 5 % limit, and has no supporting evidence as to the strain
origin of the cells; however, the results from the bone-marrow analysis confirm
that it was indeed a sex chimaera. ^30 and J^22 appeared on morphological
grounds to be non-chimaeric; chromosome analysis supported this view. As
expected from the sex ratio, the three chimaeric females showed no evidence of
Non-chimaeric
Non-chimaeric
*30
*27
X2Z
X29
X3\
X33
xn
XI
X2
X5
XI
X9
X\0
XI2
XI3
Chimaeric
Identity
JT14
Sex
Chimaeric
Phenotype
00 (95)
3 0 (69)
00 (22)
92-9 (64)
89-7 (72)
88-2 (76)
93-4 (78)
10-8 (76)
97-7 (84)
2-6 (80)
981 (69)
24-8 (62)
1000(130)
94-4 (63)
00 (45)
94-7 (65)
21 (49)
93-9 (59)
00 (31)
89-7 (24)
Autoradiography
2-7(184)
1000 (96)
27-6 (87)
99-5 (206)
97-4(115)
1-7(115)
1000(119)
4-2 (96)
1000 (66)
2-7 (37)
1000 (33)
93-3 (75)
99 1 (113)
1000(195)
150(133)
0-0(16)
1-5(274;
Giemsa
00 (7)
00 (40)
10-0(10)
95 0 (20)
96-2 (26)
86-4 (22)
1000 (24)
00 (2)
1000(20)
00 (26)
1000 (3)
23-1 (13)
94-6 (37)
1000 (39)
6-3 (16)
93-7 (32)
00 (8)
1000 (18)
00 (12)
1000 (9)
Morphology
XYcells detected in blood cultures
(no. of plates scored)
200 (10)
92-9 (14)
89-2 (37)
93-3 (15)
0-0(11)
1000 (23)
94-7(19)
390 (41)
88-9 (27)
1000 (9)
1000 (7)
1000 (21)
1000 (12)
5-9 (17)
63-6(11)
%XY cells
detected in
bone marrow
(no. of plates
scored)
Table 2. Chromosomal diagnosis of sex in cells from mice derived by embryo aggregation
XX\XX
XX\XX
XX\XX
XYjXX
XY\XX
XY\XX
XY\XY
XY\XX
XYKXY or XX)
XY\XX
XY\XY
XY\XX
XY\XY
XYKXY or XX)
XY\XX
XY\XY
XY\XX
XYjXY
XX
XY
Presumed
chromosome
constitution
O
Non-chimaeric
Non-chimaeric
Chimaeric
Chimaeric
Chimaeric
Chimaeric
Phenotype
\SJ
9
3
3
X22
X8
xn
X~i\
X2
X\
X30
xn
X2%
X10
X21
X5
X9
X\2
X2Q>
XI9
X2\
X22
X\A
XI
X33
XI3
X26
X29
X4
X\6
o.
9
9
9
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
Identity
Sex
ffMtC
* R = 'recessive' component.
XY\XY
XY\XY
XY\XY
XY\XY
XY/XY
—
—
—
XYKXY or XX)
XYKXY or XX)
XY/XX
XYfXX
XY\XX
XY\XX
XY\XX
XYjXX
XY/XX
XYjXX
XX
—
XY
—
xx/xx
XXjXX
XX\XX
—
Presumed
chromosome
constitution
If
84-6 (13)
—
83-9(137)
820(150)
88-3 (60)
97-6 (84)
76-8 (56)
970(100)
—
—
—
—
1000(87)
2-5 (80)
91-5(94)
991 (108)
69-8 (63)
97-3 (111)
98-7 (80)
87-5 (48)
—
6-3(16)
—
1000 (33)
—
R, D*
R, D
—
R, D
R, D
R, D
R, D
R, D
R, D
—
—
—
—
—
R
R
R
R
R
R
D
—
R
—
D
—
Component
corresponding
to phenotypic
sex
D = 'dominant ' (i.e. Fx) component.
1-9
2-1
—
11
98-3
97-5
99-2
99-2
97-5
—
—
—
981
970
93-8
13-3
2-4
26-2
1-7
3-3
92-2
93-4
1-2
—
96-3
—
Overall %
XY cells
detected in
blood cultures
% D* cells
detected in
blood cultures
(no. of plates
scored)
R(137)
R(48), D(19)
D(31)
D (110)
D (115)
R(131)
R(5l)
R(91)
D ( l 32)
D(90)
R(137)
R(4)
R(85)
R(31)
R(107)
R(85)
D (148)
—
R(35)
RC26)
D(84)
D(121)
R* (107)
R(63)
R(15)
Component
giving rise
to germ cells
(no. of young)
to
tfl
p
o
>
O
i—»
211
Sex chimaeras in mice
Table 4. The distribution of the two possible types ofprogeny among
animals of various sex chromosome constitutions
Overt chimaeras
Nonchimaeras
XYor XX
A.
Sex
3
Progeny
XYjXY or:
XX/XX
Recessive
Dominant
Recessive
Dominant
Mixed
2
1
1
3
1
XY/XX
Unknown
1
0
3
2
0
0
0
6
1
0
2
0
0
2
Table 5. Segregation of progeny in 12 litters sired by X33, born to
6 females (a-f) of the multiple recessive stock
Number of progeny
Number of progeny
A
Date of birth
of litter
21. iv. 72
12. vi. 72
16. x. 72
6. xi. 72
20. xii. 72
25. xii. 72
Female
' Recessive'
'Dominant'
a
a
b
b
c
d
0
4
7
5
5
5
6
0
0
1
1
1
Date of birth
of litter
Female
21. i. 73
8. iii. 73
9. iii. 73
22. iii. 73
29. iv. 73
5. v. 73
e
b
c
d
b
f
' Recessive'
'Dominant'
7
0
6
1
8
0
0
6
0
1
0
3
sex chimaerism. Of the seven males that failed to show evidence of sex chimaerism, five showed both 'dominant' and 'recessive' cell populations in the blood
and so could confidently be diagnosed as XY/XY chimaeras. One (X27) showed
only 'dominant' cells in the blood and another {X\0) was not scored for the
strain origin of the blood cells: for these males the absence of XX cells in the
blood could therefore not be used as evidence that the animal as a whole was
XYjXY rather than XY/XX.
Omitting the two doubtful animals (X10, X21), we are left with three females
and five males that are not sex chimaeras, and eight males that are sex chimaeras,
a proportion of 0-5. Some of the male sex chimaeras had fewer than 5 % XY
cells in the blood {X\2,178) or in both blood and bone marrow (Z31). Table 3
gives the strain composition and breeding behaviour of the animals. In all
seven known sex chimaeras that bred, the component diagnosed as XY on
chromosome morphology (Table 3) corresponded to the component undergoing
spermatogenesis, as judged by the genotype of the progeny.
As expected, the male (A33) that sired a mixed progeny proved not to be a
sex chimaera. Of the 16 siring single progenies, seven were XYjXX chimaeras
from which mixed progenies would not have been expected, four were XY/XY,
14-2
212
A. MCLAREN
Table 6. The distribution of males with testes showing Leydig cell hyperplasia or
Leydig cell tumours, according to sex chromosome constitution and age at death
Not sex
chimaeras
Sex chimaeras
Not known
Total
A.
Age at autopsy
< 18 months
> 18 months
Total
Normal Affected Normal Affected Normal Affected Normal Affected
4
0
4
0
2
2
1
1
2
2
3
5
2
1
3
0
0
0
7
2
9
2
5
7
and five did not have their sex chromosome constitution determined. There was
no significant difference in litter size or total number of young produced between
the males that were sex chimaeras and those that were not. The relationship
between sex chimaerism and genotype of progeny is shown in Table 4. The
proportion of males siring recessive-type progeny was higher in the sex-chimaeric
XYjXX than in the XYjXY group (6/7 versus 1/4), though not significantly so.
The relative proportions of recessive and dominant progeny of Z33^ were
not evenly distributed between litters (Table 5), but showed significant heterogeneity {X\ID = 52-23, P < 0-001). No trend with the age of the male was seen,
nor with the age of the females with which he was mated, nor was there any
heterogeneity among females once the heterogeneity among litters was taken
into account.
At autopsy, some of the males (including the sterile XX) showed small yellowish
testes, but otherwise the reproductive organs appeared normal in all except X9.
This male, who had sired only one litter of four young, at the age of 7 weeks,
was found on dissection at 20 months to be a hermaphrodite with a minute
testis and normal male organs on the right, and an immature uterus and rudimentary ovary on the left side. Histological examination showed granulosa cells
but no oocytes in the ovary, and an atrophic testis. Leydig cell hyperplasia,
amounting sometimes to advanced non-malignant Leydig cell tumours, was
seen in one or both testes of X\, 2, 7, 12, 13, 28 and 31. Normal tubules, with
brisk spermatogenesis, were found in X5, 8,10,16,18,22, 26,27 and 29. Meiotic
chromosome preparations made from the testes of X2, 5, 7, 8, 10, 13, 18, 22,
26, 27, 28, 29 and 31 revealed only XYgerm cells, although at least five of these
males were sex chimaeras.
Comparison of the condition of the testes with the age at which the animal
was killed and with its sex chromosome constitution (Table 6) showed a tendency for Leydig cell hyperplasia to increase with age and to be more common
among sex chimaeras, but neither tendency was statistically significant.
The relative proportion of .Of and XYceMs in the bone marrow did not always
accord with that found in the blood (Table 2). Of the three males showing less
than 5 % XY cells (all of'recessive' phenotype) in the blood, ^28 had 20 % XY
Sex chimaeras in mice
213
cells in bone marrow, XI 2 had nearly 40 % and X31 nearly 90 %. For XI8, the
proportion of XYcells (again 'recessive') was about 25% in blood but 100%
in bone marrow; X5 and XI both had over 90% XY cells in the blood, with
corresponding proportions in bone marrow of 100% (^5) and 64% (XY).
DISCUSSION
The following conclusions emerge clearly from the above findings:
(1) Fifty per cent of the chimaeras (8/16) proved on chromosome analysis to
be sex chimaeras (XYjXX).
(2) All of the sex chimaeras were phenotypically male, even when less than
5 % of the blood cells were XY. All except one of the males was fertile.
(3) Only one of the XYjXX males showed morphological evidence of hermaphroditism.
(4) The genetic component undergoing spermatogenesis, as judged by progeny
testing, coincided in 7/7 XYjXX chimaeras with the component identified by
chromosome morphology as XY.
The problem remains of why so few chimaeras in this series produced mixed
litters. Seven of the 'single progeny' animals were XY/XX chimaeras, and a
further five may have been; but this still leaves three females and five males that
could have produced mixed litters, of which only one did. Mullen & Whitten
(1971) showed that 'unbalanced' strain combinations characteristically produce
a high proportion of 'single' progenies, but the strain combination used for the
present study is well balanced, as judged by the high percentage of overt
chimaeras, the high sex ratio, and the representation of both components in
substantial amounts in the coat and also in a range of internal organs.' Recessive'
and 'dominant' progenies were produced in approximately equal numbers. If
selection of germ cells has played a part, it appears to have favoured 'recessive'
germ cells in the ovary and 'dominant' ones in the testis, yet the single male to
produce a mixed progeny showed no evidence of a relative increase of 'dominant' spermatozoa with time. Mullen & Whitten (1971) reported 24 'mixed
progeny' out of a total of 65 overtly chimaeric animals from balanced strain
combinations. Assuming that 50 % were sex chimaeras, this gives an estimate of
24/32-5 with mixed progenies, which differs significantly both from 100%
(P < 0-01) and from our proportion of 1/8 (P < 0-01). The initial population of
cells destined to give rise to germ cells may be small enough (perhaps only
2-3 cells) to make random sampling a major factor in producing single rather
than mixed progenies; an additional factor would then be the 'patch size' of
the chimaeric components, which is likely to vary from one strain combination
to another. Studies on the pigmented retina of the eye in these recessive <-> Fx
animals (West, personal communication) yield independent evidence that
'patch size' in this combination may be relatively large.
The tendency in the mixed progeny of X33 for young of one genotype or the
214
A. MCLAREN
other to predominate in any particular litter, with the 'prevailing' genotype
changing literally from one day to the next (see Table 5), can best be explained
on the assumption that 'patch size' is also relatively large for germ cells in the
spermatogenic tubules, and that the spermatozoa used for fertilization from a
single ejaculate represent a spatially localized sample of the spermatogenic
process. It could be, for example, that each spermatogenic tubule is colonized
by the descendants of a single primordial germ cell, and that the tubules release
their contents in rotation. The result is in agreement with the observation of
Burgoyne (1973) that the relative proportions of the two types of phenotypically
distinguishable spermatozoa in the vasa deferentia of a C3H <-+ C57BLchimaeric
male bore little relation to the relative numbers of progeny of the two types
sired within a few days of the time of sperm sampling.
The high proportion of males showing Leydig cell hyperplasia cannot be
interpreted in the absence of data from the control strains. Leydig cell tumours
are said to be rare in mice (Slye, Holmes & Wells, 1919); their incidence can be
increased greatly by oestrogen treatment (Gardner, 1958).
The preparations of blood showed a striking preponderance of F± (' dominant')
over recessive cells. Of 15 mice in which the strain origin of the blood cells was
determined (Table 3), 14 had more than 66 % and 6 had more than 95 % Fx
blood cells. This suggests that Fx lymphocytes are at a strong selective advantage
relative to those of the recessive component, an interpretation that is supported
by the direct preparations of bone marrow cells (Table 2). Strain diagnoses were
not made on these, but in five of the XYjXX chimaeras a comparison between
the blood and bone marrow results is possible, using the relative proportions of
XYand XX cells in the two tissues. In each case the proportion of Fx cells was
lower in bone marrow than in blood, usually by a substantial margin (X\2,
Xl&, X2%, X2>\). The selective advantage could consist in differential growth of
Fx and recessive stem cells in vivo; alternatively, since the blood cells were cultured, it is possible that the asymmetry reflects a greater mitotic response to
phytohaemagglutinin on the part of the F1 lymphocytes.
If a relative advantage of Fx blood cells in vivo or in vitro is assumed, the
suggestion in Table 2 that XX cells may be at a selective advantage in XYjXX
chimaeras disappears. With abnormal ^-chromosome complements, the opposite
situation prevails, at least in vitro, so that the more ^chromosomes a cell carries
the slower it completes a cell cycle (Barlow, 1972). However, the suggestion
(Barlow, 1973) that XY cells may prove to be at a slight proliferative advantage
over XX cells in chimaeras is not borne out by any substantial overgrowth by
XY cells in our XY\XX animals.
I am grateful to Dr P. Bowman (Genetics Department) for making and analysing the
chromosome preparations of bone marrow, to Dr M. Buehr (Genetics Department) for help
with the analysis of the autoradiographs, to Dr A. C. Chandley (M.R.C. Clinical and Population Cytogenetics Unit) for making and examining the air-dried meiotic chromosome spreads,
to Dr. N. MacLean (Pathology Department, Western General Hospital) for examining the
gonads histologically, and to the Ford Foundation for financial support.
Sex chimaeras in mice
215
Note added in prooj
A further male chimaera involving the multiple recessive strain (recessive
< >Q strain) has now produced a mixed progeny, again with significant variation
between litters in the proportions of the two types of young (x%) = 103,
P < 0-02). On the other hand Mystkowska (personal communication) reports
two large mixed progenies from CBA-T6<->CBA-p male chimaeras, showing
no hint of variation between litters (x%8) = 27-0; #(226) = 29-0). The contrast
suggests that the heterogeneity is associated either with the multiple recessive
component as such, or with the genetic disparity between the component
strains, since this is minimal in the CBA-T6<->CBA-p combination.
REFERENCES
F. E. & Hsu, T. C. (1971). Localization of heterochromatin in human chromosomes. Cytogenetics 10, 81-86.
BARLOW, P. W. (1972). Differential cell division in human X-chromosome mosaics. Humangenet ik 14, 122-127.
BARLOW, P. W. (1973). The influence of inactive chromosomes on human development:
anomalous sex chromosome complements and the phenotype. Humangenetik 17, 105-136.
BOWMAN, P. & MCLAREN, A. (1970). Viability and growth of mouse embryos after in vitro
culture and fusion. /. Embryol. exp. Morph. 23, 693-704.
BURGOYNE, P. S. (1973). The genetic control of germ cell differentiation in mice. Ph.D. Thesis,
University of Edinburgh.
GAGNE, R., TANGUAY, R. & LABERGE, C. (1971). Differential staining patterns of heterochromatin in Man. Nature New Biol. 232, 29-30.
GARDNER, W. U. (1958). Testicular tumorigenesis. In Hormones in Endocrine Tumours. Ciba
Fndn Coll. Endocr. 12, 239-249.
GRUNEBERG, H. & MCLAREN, A. (1972). The skeletal phenotype of some mouse chimaeras.
Proc. R. Soc. B 182, 9-23.
KOFMAN-ALFARO, S. & CHANDLEY, A. C. (1970). Meiosis in the male mouse. An autoradiographic investigation. Chromosoma 31, 404-420.
MCLAREN, A. (1972a). Germ cell differentiation in artificial chimaeras of mice. Proc. Int.
Symp. "The Genetics of the Spermatozoon'' (ed. R. A. Beatty & S. G. Waelsch), pp. 313324. Edinburgh and New York: R. A. Beatty & S. G. Waelsch.
MCLAREN, A. (19726). Late-labelling as an aid to chromosomal sexing of cultured mouse
blood cells. Cytogenetics 11, 35-45.
MCLAREN, A. & BOWMAN, P. (1969). Mouse chimaeras derived from fusion of embryos
differing by nine genetic factors. Nature, Lond. 22A, 238-240.
MILET, R. G., MUKHERJEE, B. B. & WHITTEN, W. K. (1972). Cellular distribution and
possible mechanism of sex-differentiation in XXIXY chimeric mice. Can. J. Genet. Cytol.
14,933-941.
MINTZ, B. (1968). Hermaphroditism, sex chromosomal mosaicism and germ cell selection
in allophenic mice. /. Anim. Sci. (Suppl. 1), 27, 51-60.
MULLEN, R. J. & WHITTEN, W. K. (1971). Relationship of genotype and degree of chimerism
in coat color to sex ratios and gametogenesis in chimeric mice. /. exp. Zool. 178, 165-176.
MYSTKOWSKA, E. T. & TARKOWSKI, A. K. (1968). Observations on CBA-p/CBA-T6T6 mouse
chimaeras. /. Embryol. exp. Morph. 20, 33-52.
MYSTKOWSKA, E. T. & TARKOWSKI, A. K. (1970). Behaviour of germ cells and sexual
differentiation in late embryonic and early postnatal mouse chimaeras. /. Embryol. exp.
Morph. 23, 395-405.
ARRIGHI,
216
A. MCLAREN
M., HOLMES, H. S. & WELLS, H. G. (1919). Primary spontaneous tumors of the testicle
and seminal vesicle in mice and other animals. XII. Studies in the incidence and inheritability of spontaneous tumors in mice. /. Cancer Res. 4, 207-228.
STICH, H. F. & Hsu, T. C. (1960). Cytological identification of male and female somatic
cells in the mouse. Expl Cell Res. 20, 248-249.
SLYE,
(Received 14 July 1974)