Observations on Serum Cholesterol Levels in the Twin
Population of Evans County, Georgia
By JOHN R. McDONOUGH, M.D., M.P.H., CURTIS G. HAMES, M.D.,
B. G. GREENBERG, PH.D., F.A.P.H.A., Louis H. GRIFFIN, JR.,
AND ANDREW J. EDWARDS, JR.
Downloaded from http://circ.ahajournals.org/ by guest on June 14, 2017
discrete entity or merely the upper end of a
normal distribution.18
Twin studies provide a technic for finding
out if a genetic component is significantly
affecting variations observed in a trait. Monozygotic twins contain identical genetic material, whereas dizygotic twins vary in genetic
material to the same extent as non-twin siblings. Variation between dizygotic co-twins
significantly above that in monozygotic cotwins is prima-facie evidence that at least
part of the variation observed in the trait is
under genetic control.
Osborne, Adlersberg, DeGeorge, and Wang'9
studied serum cholesterol levels in adult twins
living in New York City. Mean intra-pair
differences were not found to vary significantly between monozygotic and dizygotic
twins or between twins residing together or
apart. Sufficiently suggestive trends, however,
prompted these authors to conclude that their
data supported the hypothesis that variations
in cholesterol levels are determined by both
hereditary and environmental factors.
Gedda and Poggi20 recently studied cholesterol levels in twins aged 6 to 19 years living
in Rome, Italy.* Their data show striking
differences between monozygotic and dizygotic
twins, which led to the conclusion that the
biogenesis of cholesterol is dependent on the
IT HAS BEEN firmly established that elevated levels of cholesterol in human serum
are associated with an increased propensity
for the development of coronary heart disease. '4 Despite this, the biologic relationships
between elevated serum cholesterol levels,
lipid metabolic abnormalities, the atheromathrombotic complex within the coronary arterial wall, and clinical coronary events have
not been adequately delineated, and form the
basis for a great deal of continuing work in
cardiovascular research.
Cholesterol levels have been found to vary
by age,5' 6 sex,7 race,8' 9 geographic location 10, 1 diet12' 13 seasonal changes,14- and
occupational stress.15 There is a great deal of
variation among persons at all ages, with a
tendency for values, to be normally distributed
with some skewing tol the right, at least for
middle-aged white American men.'
The degree to which hereditary factors are
responsible for variations in cholesterol levels
has been the subject of a number of studies.
The clinical syndrome familial hypercholesteremic xanthomatosis has, been clearly shown
to be genetically determined and probably
is caused by an autosomal dominant gene, or
by a polygenic type of inheritance.'6' 17 On
the other hand, it has not yet been determined
whether idiopathic hypercholesteremia is a
Fromi the Evans County Health Department, Heart
Research Project, Claxton, Georgia.
Supported by a contract from the Georgia Department of Public Health, Cardiovascular Disease Control Service, and the Public Health Service Heart
Disease Control Program. Partial support was from
a grant (H-3341) from the National Heart Institute,
U. S. Public Health Service.
Presented at the Section on Arteriosclerosis, American Heart Association, October 20, 1961, Miami
*Data obtained on twins and matching sibling
pairs, zygosity differentiation of like-sexed twin
pairs, and an English translation of the paper by
Gedda and Poggin' have been desposited as Document
number 7069 with the ADI Auxiliary Publications
Project, Library of Congress, Washington, 25, D. C.
A copy may be secured by citing the Document
number and by remitting $3.75 for photoprints, or
$2.00 for 35-mm. microfilm. Make checks payable to:
Chief, Photoduplication Service, Library of Congress.
Beach, Florida.
962
Circulation, Volume XXV, June 1962
CHOLESTEROL LEVELS IN TWINS
963
Table 1
Classification of Twin Pairs Located by Census of Evans County, Georgia, Study
Study
Area
Bordering study
area
i
g
!____Iw__
area
__
data
obtained
genotype. They pointed out that environmenDownloaded from http://circ.ahajournals.org/ by guest on June 14, 2017
tal influences were largely controlled, since
all of the twin pairs were residing together in
the homes of their parents.
The purpose of the present study was to
test by means of the twin system the hypothesis that a genetic component is responsible for
a significant part of the variation found to
occur in serum cholesterol levels.
Materials and Methods
As a basis for conducting a study of the prevalence rates of coronary and hypertensive heart
diseases, a household census of Evans County,
Georgia, and contiguous areas was conducted during January and February 1960. The following
question: "Are there any twins living in the household?" was included as part of the census questionnaire to provide a method of identifying all
of the twin pairs residing in the study area. A
comparison with the population counted by the
Census Bureau in April 1960, has been completed,
and will be reported in detail in another publication. The first census under-enumerated that of
the Census Bureau by 5.1 per cent, and the two
were in close agreement when looked at by age
group, race, and sex.
Eighty-three pairs of twins were discovered by
means of the twin question on the household census. While the data on these twins were being
obtained, five additional pairs were located living
adjacent to but outside the study area. These five
pairs were added to raise the basic roster to 88
pairs. Follow-up of these 88 pairs (table 1) revealed 16 pairs in which the co-twin was deceased,
and another 10 pairs in which the co-twin had
moved too far away from the study area to be included. Six twin pairs were in the age group 0 to
4 years, and were judged too young for inclusion
in the study group. This left 56 pairs in which
Circulation, Volume XXV, June 1962
both co-twins were accessible for study. Distribution by race and age between twins and the general population was quite similar. Data were collected from all 56 pairs and form the basis for
the present report. Fifty-six pairs of siblings,
matched as closely as possible with the twins by
age, race, and sex and living together or apart
were drawn at random from the population and
were similarly studied.
Blood for typing and cholesterol was obtained
at a household visit. Zygosity was determined by
sex, physical appearance, haptoglobin type, and
up to 30 blood-group antisera.*
Fifty twin pairs could be differentiated into 23
monozygotic (MZ) pairs and 27 dizygotic (DZ)
pairs. The remaining six pairs were undifferentiated (UD). All 27 DZ pairs were clearly identified by means of differing sex, different blood
types on the basis of the ABO and Rh antisera,
or serum haptoglobin and Ag phenotypes. All the
identical twins had at least the ABO and Rh
blood-type determinations, most had haptoglobin
determinations, and some had up to 30 different
blood-type antisera determinations. The six UD
twin pairs were lacking in blood-type data, so
that they could not be clearly classified (table 2).
It should be noted that all the DZ twins were
clearly determined by differing sex or differing
blood type. Thus, the DZ twins constitute a pure
group (all known to be DZ twins). It should be
further appreciated that classification into the
MZ twin group cannot be positive but only highly
probable. Thus it is possible (although unlikely)
that a few of the MZ twins are aetually DZ twins.
*For this service, the authors are indebted to Dr.
Baruch S. Blumberg, National Institutes of Health,
Bethesda, Maryland, and Dr. Fred H. Allen, Jr.,
Assistant Professor of Pediatries, Harvard University, and Assistant Director, Blood Grouping Laboratory, Boston, Massachusetts.
I
McDONOUGH ET AL.
964
Table 2
Distribution by Race, Sex, and Zygosity of 56 Twin Pairs
Zygosity
S &
White
9 9
MZ twins
DZ twins
UD twins
Total
5
3
10
6
2
10
18
Total
Negro
a 3
9
5
3
5
2
5
5
9 9 8 9
9 9
8 8
8
5
3
8
2
10
5
2
8
15,-
15
9
4
28
8 9
Total
13
23
27
13
56
6
Table 3
Observed Serum Cholesterol Levels
MZ twins
Downloaded from http://circ.ahajournals.org/ by guest on June 14, 2017
DZ twins
UD twins
Siblings
Age
group
No.
persons
5-14
15-29
18
18
30+
10
5-14
15-29
30+
26
12
16
8
5-14
15-29
30+
5-14
15-29
30+
*S.D. not calculated for N < 10.
\/d/2k
142.3
145.1
212.3
166.7
159.3
194.7
145.1
Cholesterol (mg./100 ml.)
Standard deviation
Range
104-187
83-182
160-257
107-233
103-225
122-271
106-164
23.7
22.5
32.4
33.5
33.2
42.5
0
4
49
33
30
To the extent that this might bias the results, the
bias would operate in the direction of masking
true differences that might occur between the two
groups rather than vice versa. Therefore, significant differences should tend to be more significant
than may be indicated by the data.
Serum cholesterol was done in replicate according to the Abell technic,21 in which 0.5 ml. of
serum is digested with potassium hydroxide, extracted with petroleum ether, color development
carried out by means of the Lieberman-Burchard
reagent, and the sample read at 30 minutes on a
Bausch and Lomb Spectronic 20 spectrophotometer adjusted to a wave length of 620 my. Qualitycontrol technics involving within-run, between-day,
and between-laboratory reproducibility of cholesterol measurements have shown a consistently high
level of performance throughout the entire period
the study was in progress. Based upon 448 replicates from the 224 subjects making up the study,
the technical error according to the formula given
3.42.' By use of a correction
by Moore1 was Se
factor6' 22 to remove any posible effect on the
*Se
Mean
where d=difference between replicates of k pairs of measurements.
168.3
152.9
154.5
205.3
148-199
97-225
28.4
30.6
46.9
91-253
106-309
second duplicates of standing for 24 to 48 hours,
the square roots of the error terms in the analysis
of variance were for MZ twins, 3.48; for DZ
twins, 3.50; for UD twins, 3.40; and for siblings,
3.32; and for the total group, s = 3.40. This
would imply that a factor to adjust for standing
of the second duplicates is not necessary. These
values agree very well with those obtained in
other laboratories in good control.'
Results
Cholesterol levels (mg. per 100 ml.) are
presented in table 3. Although a few individuals were found to have cholesterol levels
that might be classified as hypercholesteremic,
they were not excluded from the analysis (as
was the case with the data of Osborne et al.'9
The data on all types of twins and siblings
were examined for the effect of age, sex, race,
and type of residenee (whether they lived together or apart). The intra-pair differences
(that is, the difference between individuals
within each pair, henceforth referred to as
the difference or [D]), were used throughout
the analysis.
Circulation, Volume XXV, June 1962
CH:OLESTEROL LEVELS IN TWINS
965
Table 4
Effect of Age on Intra-Pair Difference
MZ twins
DZ twins
Siblings
Males
Females
Males
Females
Mixed pairs
Males
Females
Mixed pairs
N
Regression coefficient
Standard error
t
8
15
5
9
13
16
27
13
1.435
.355
1.161
.230
- .0144
- .221
.580
.822
.555
.155
.612
.428
.497
.425
.398
.335
2.58*
2.29*
1.89
.54
.03
.52
1.46
2.45*
*Significant at the .05 level.
Table 5
Mean Intra-Pair Difference by Sex
Downloaded from http://circ.ahajournals.org/ by guest on June 14, 2017
MZ twins
DZ twins
Siblings
Males
Females
Males
Females
Mixed pairs
Males
Females
Mixed pairs
Age
Difference
Standard error
16.7
22.8
17.4
26.6
21.1
18.7
23.9
22.3
19.9
11.3
21.9
29.3
40.3
27.6
32.9
37.5
4.9
1.78
11.3
8.4
7.0
6.8
5.2
7.6
The regression of D on age was computed
for the male, female, and mixed pairs in all
cases (table 4). Both male and female MZ
twins had a significant positive regression of
D on age (p < .05). For MZ twins the slopes
of the lines relating D to the age of the males,
and females are not significantly different
from each other t = 1.86, although the difference between the two slopes is large enough
to be suggestive. The variances about the regression lines are also significantly different
(F = 3.94) and one has to be conservative
in interpreting the previous, result. The DZ
twins did not show a eonsistent relationship.
Pooling all of the data for DZ twins in order
to get a better estimate of the slope still did
not produce a significant relationship to age.
The mixed pa.irs of siblings showed a positive
significant regression on age. The male siblings had a negative regression coefficient that
was not significantly different from zero. The
net result. for this latter group was no consistent, relationship with age, and therefore
the data for siblings were not age-adjusted.
Circulation, Volume XXV, June
1962
No significant differences in D by sex were
found, except for MZ twins where the males
showed a greater intra-pair difference despite
the fact that they were younger on the average (table 5). If male and female MZ twins
are adjusted to age 21, using their separate
regression lines, their adjusted values are
10.7 for females and 26.0 for males. Since
the relationship of D with age is assumed t,o
be different for males, and females, the size
of the difference will depend upon the age to
which the cholesterol levels are adjusted.
Therefore, further tests of significance are
rather meaningless in this instance. As one
would expect, inspection of the DZ twins
shows that the mixed pairs. have larger intrapair differences than the male and female
pairs, but the difference is not significant
(t = 1.48). The same is true for the siblings.
The differences by race and type of residence are shown in table 6. No consistent
difference between races was found. In every
case, however, thos.e that were living apart
had larger differences than those living to-
9McDONOUGH ET AL.
966
Table 6
Mean Intra-Pair Difference by Race and Type of Residence
No.
Together
Age
Difference
No.
Apart
Age
Difference
White
9
14.6
15.2
6
36.3
17.2
Negro
6
11.3
8.3
2
29.5
17.2
White
5
9.8
18.3
9
36.3
46.3
Negro
9
9.3
28.5
4
35.2
33.1
White
18
13.2
24.2
15
40.6
38.8
Negro
17
11.1
28.9
6
33.4
51.4
MZ twins
DZ twins
Siblings
Downloaded from http://circ.ahajournals.org/ by guest on June 14, 2017
Table 7
Cholesterol Differences by Residence
No.
MZ twins
DZ twins
Same sex
Differing sex
15
I
7
Living together
Mean Standard
Mean
difference error
age
Living apart
Mean
difference
Mean
Standard
No.
age
error
13.0
12.7
2.9
8
35.1
17.2
5.7
11.0
8.0
19.3
30.4
7.1
6.0
7
6
35.6
36.5
34.1
51.8
6.5
15.6
12.7
10.5
26.7
4.5
8.1
16
5
37.7
38.0
25.6
41.2
56.5
8.4
4.3
Siblings
Same sex
Differing sex
27
8
gether. Unfortunately, there is so little overlap in the present data between age and residenee that the two are confoiunded.
In order to evaluate the effect of zygosity
within residence groups7 racial groups were
pooled and the analysis was carried out separately for pairs of the same sex and of differing sex (table 7). No significant differeiices
were found between DZ twins and siblings,
either living together or living apart; therefore DZ twins and siblings were combined
for the comparison with MZ twins. The t-test
for the contrast MZ twins versus DZ twins
and siblings of the same sex within the together group is t- 2.02, which is of borderline significance. Within the apart group for
the same comparison t- 2.42, which is signifieant at the 0.05 level. When together and
apart groups are combined, the contrast between DZ twins and siblings of the same sex
and MZ twins becomes t - 3.14, which is sig-
nificant at the 0.01 level. Scatter-diagrams
of D values for MZ and DZ twins. by a.ge and
residence appear in figure 1.
An analysis of covariance was performed
on the data in the three groups (MZ, DZi, siblings) so that the values of D were adjusted
for associated variables. It was recognized
that several of the assumptions underlying
such anaulysis were not fulfilled but an examination of the results, nevertheless, would not
be harmful if these limitations were kept in
mind. The analysis of covariance confirmed
other analyses in that the average D did not
differ between siblings and DZ twins when
adjusted for age, race, sex, and whether living
together or apart. On the other hand, the MZ
twins had a significantly lower average value
of D (7 versus 39) than the DZ twins and
siblings when adjusted for these same factors.
Another way of looking at the data is to
compare the cholesterol differences observed
Circulation, Volume XXV, June 1962
967
CHOLESTEROL LEVELS IN TWINS
Age
65
610
55
0
6
50
MZ Twins
DZ Twins
45
.~~~4I
0
35
.1
o'
0
S
*
25
20to
~~000
0
0
Downloaded from http://circ.ahajournals.org/ by guest on June 14, 2017
0
0
.
30
30
*
12&
10
0
001 0
0
0
0
0
00
oo 5 00
0
100 90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 100
Cholesterol Difference (mg./ 100 ml.)
Figure 1
Intra-pcair differences in serum cholesterol. Living apart, 0; living together 0.
in MZ twins, DZ twins, and UD twins with
their matched sibling pairs (table 8). This
has the advantage of removing any effect of
age, race, sex, or residence in a way similar
to the analysis of covariance but without violating the latter 's ass.umptions. In a comparison of the twins and siblings, only three
pairs turned out to be not completely matched
for residence. This would not appreciably
affect the results. To make this test, the differenee between the twin pair and its matching pair (second row, table 8), is compared
to its standard error. This provides a t-test
of the hypothesis of equality of the difference
of the twins and their matching pairs. It is
obvious that the differences for DZ twins and
UD twins are not significantly different from
the differences of their matching pairs of siblings. However, for the MZ twins t = 21.1/6.8
= 3.10, which is highly significant (p < .01).
It is therefore concluded that the MZ twins
are different from their matched pairs in cholesterol variation and that this difference is
due to the genotype.
Circulation, Volume XXV, June 1962
Discussion
These data support the findings, of Gedda
and Poggi20 that variations in cholesterol
level are to a significant, extent dependent
upon the genotype. Differences for MZ twins
were significantly affected by age for both
males and females. With advancing age,
cholesterol differences for MZ twins increased
at an average of 1.435 mg. per 100 ml. per
year of age for male and 0.355 mg. per 100
ml. per year of age for female pairs (table 4).
Thus, at the earliest age group studied, 5 to
14 years, the difference for MZ twins is very
small, whereas variation in cholesterol level
is seen to be quite large (table 3). This indicates that differing genotypes are responsible
for practically the entire range of variation
seen at these young ages, and, since the variation can be quite large, the genotype must
therefore exert a powerful influence upon
cholesterol variation.
This raises, interesting questions as to what
might be the genetic mechanism responsible
for the control of cholesterol level. Unfortu-
McDONOUGH ET AL.
968
Table 8
Comparison of Cholesterol Mean Intra-Pair Differences Between Twins and Matching
Sibling Pairs
MZ twins
pairs
DZ twins
Matching
pairs
UD twins
14.3
35.4
33.2
34.2
13.8
Matching
Mean intra-pair
difference
Matching pair
minus twin pair
Standard error of
the difference
Significance of the
difference from zero
21.1
6.8
< 0.01
Downloaded from http://circ.ahajournals.org/ by guest on June 14, 2017
nately, twin studies do not provide answers
to genetic mechanisms, gene frequencies, mutation rates, or linkage relations.23 Cholesterol
is a quantitative variable that resembles a
bell-shaped distribution (possibly with some
right-tailed skewing) over a wide range of
values.' It would seem that single factor inheritance, even with multiple alleles, would
provide too few phenotypes to explain the
genetic regulation of cholesterol level. Two
or more genes are quite possibly involved,
perhaps with multiple alleles at some loci. It
would thus seem likely that the genetic regulation of cholesterol is multifactorial.
TLhe data also clearly show that environmnental agents significantly affect the cholesterol level. Without exception, pairs living
apart had larger differences than pairs living
together. Unfortunately, place of residence
could not be analyzed separately from age,
because of insufficient overlap in the ages of
the two groups living together and apart.
The nature of the environmental agents influencing the cholesterol level was not looked
for in this study.
Clinicians have been impressed by the frequency with which cases of heart disease are
found to occur in the same family.24 A retrospective study of the families of medical students has shown coronary heart disease to
occur four times oftener in siblings of affected
parents.25 Coronary heart disease has also
been found to have a higher frequency in
hypercholesteremic families.1'-'
The associations of elevated serum choles-
1.00
6.9
Not significant
Matching
pairs
12.8
-1.00
2.8
Not significant
terol with coronary heart disease, and genotype with cholesterol level would suggest that
the familial aggregation seen to occur in coronary heart disease may be due to the occurrence of certain genotypes within affected
families which maintain the cholesterol level
at the upper end of its distribution.
Summary
Important factors affecting the variation in
cholesterol were found to be the genotype of
the individual, age, and type of residence.
The effect of genotype is clearly shown in
comparing MZ twins with their matched pairs.
The difference between MZ twins and their
matched pairs was large and statistically
significant, while for DZ and UD twins the
difference was so small it could easily have
been due to chance. In interpreting the significance of the effeet of type of residence,
we are also comparing two age groups that
are almost nonoverlapping, so that adjustment
for age by covariance might be questionable,
since it involves extrapolating beyond the
limits of the data.
Thus, it seems that two important sources
of variation of cholesterol have been isolated:
type of residence or age which is confounded
or entangled, and genotype.
Acknowledgment
The authors wish to acknowledge the assista:nce
of the following: Dr. J. Gordon Barrow, Dr. A.
David Bernanke, Dr. Baruch S. Blumberg, Fae
Bowen, James Bowen, Ann Brewton, Dr. Charles T.
Brown, Melrose Callaway, Dr. Alan J. Cooper, Mattie
Circulation, Volume XXV,
June 1962
CHOLESTEROL LEVELS IN TWINS
Lou Daniel, Miriam Funderburk, Dr. Glen E. Garrison, Otis Garvin, Edytha Godbee, Dr. Louis H.
Griffin, Sr., Dr. James E. Grizzle, Dr. Robert F.
Hansen, Lillian Lewis, Leona Lewis, Hubert Manning, Jane MeDonough, Dr. Arthur E. Rikli, Harold
Smith, and Sarah Stulb.
969
11.
12.
References
Downloaded from http://circ.ahajournals.org/ by guest on June 14, 2017
1. REPORT, OF A COOPERATIVE STUDY OF LIPornoTEINS AND ATHEROSCLEROSIS: Evaluation of
serum lipoprotein and cholesterol measurements as predictors of clinical complications
of atherosclerosis. Circulation (part 2) 14:
691, 1956.
2. DAWBER, T. R., MOORE, F. E., AND MANN, G. V.:
Coronary heart disease in the Framingham
study. Am. J. Pub. Health (part 2) 47: 4,
1957.
3. DOYLE, J. T., HESLIN, A. S., HILLEBOE, H. E.,
4.
5.
6.
7.
8.
9.
10.
FORMEL, P. F., AND KORNS, R. F.: A prospective study of degenerative cardiovascular disease in Albany: Report of three years' experience. I. Isehemic heart disease. Am. J. Pub.
Health (part 2) 47: 25, 1957.
KANNELL, W. B., DAWBER, T. R., KAGAN, A.,
REVOTSKIE, N., AND STOKES, J.: Factors of
risk in the development of coronary heart
disease. Six-year follow-up experience. Ann.
Int. Med. 55: 33, 1961.
THOMAS, C. B., AND EISENBERG, F. F.: Observations on the variability of total serum cholesterol in Johns Hopkins medical students.
J. Chron. Dis. 6: 1, 1957.
HAMES, C. G., AND GREENBERG, B. G.: A comparative study of serum cholesterol levels in
school children and their possible relation to
atherogenesis. Am. J. Pub. Health 51: 374,
1961.
LowRy, E., MANN, G. V., PETERSON, A., WYSOCKI,
A. P., O 'CONNELL, R., AND STARE, F. J.: Cholesterol and beta lipoproteins in the serums
of Americans: Well persons and those with
coronary heart disease. Am. J. Med. 22: 605,
1957.
BRONTE-STEWART, B., KEYS, A., AND BROCK,
J. F.: Serum cholesterol, diet, and coronary
heart disease. An inter-racial survey in the
Cape peninsula. Lancet 269: 1103, 1955.
PAGE, 1. H., LEWIS, L. A., AND GILBERT, J.:
Plasma lipids and proteins and their relationship to coronary disease among Navajo
indians. Circulation 13: 675, 1956.
KEYS, A., KIMURA, N., KUSUKAWA, A., BRONTESTEWART, B., LARSEN, N., AND KEYS, M.:
lIessons from serum cholesterol studies in
Circulation, Volume XXV, June 1962
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
Japan, Hawaii, and Los Angeles. Ann. Int.
Med. 48: 83, 1958.
KEYS, A., KARYONEN, J. J., AND FIDANZA, F.:
Serum cholesterol studies in Finland. Lancet
275: 175, 1958.
KINSELL, L. W., PARTRIDGE, J., BOLING, L.,
MARGEN, S., AND MICHAELS, G.: Dietary modification of serum cholesterol and phospholipid
levels. J. Clin. Endocrinol. 12: 909, 1952.
PAGE, I. H., STARE, F. J., CORCORAN, A. C.,
POLLACK, H., AND WILKINSON, C. F.: Atherosclerosis and the fat content of the diet.
Circulation 16: 163, 1957.
THOMAS, C. B., HOLLIES, H. W. D., AND EISENBERG, F. F.: Observations on seasonal variations in total serum cholesterol level among
healthy young prisoners. Ann. Int. Med. 54:
413, 1961.
FRIEDMAN, M., ROSENMAN, R. H., AND CARROLL,
V.: Changes in the serum cholesterol and blood
clotting time in man subjected to cyclic
variation of occupational stress. Circulation
17: 852, 1958.
WHEELER, E. O.: The genetic aspects of atherosclerosis. Am. J. Med. 23: 653, 1957.
EPSTEIN, F. H., BLOCK, W. D., HAND, E. A.,
AND FRANCIS, T.: Familial hypercholesterolemia, xanthomatosis and coronary heart disease.
Am. J. Med. 26: 39, 1959.
McKUSICK, V. A.: Genetic factors in cardiovascular diseases: I. The four major types of
cardiovascular disease. Mod. Concepts Cardiovase. Dis. 28: 535, 1959.
OSBORNE, R. H., ADLERSBERG, D., DEGEORGE,
F. V., AND WANG, C.: Serum lipids, heredity
and environment. Am. J. Med. 26: 54, 1959.
GEDDA, L., AND POGGI, D.: Sulla regolazione
genetica del colesterolo ematico (uno studio su
50 coppie gemellar: MZ e 50 coppie DZ).
Acta genet. med. et gemel. 9: 135, 1960.
ABELL, L. L., LEVY, B. B., BRODIE, B. B., AND
KENDALL, F. E.: A simplified method for
the estimation of total cholesterol in serum.
J. Biol. Chem. 1: 357, 1952.
GREENBERG, B. G.: Letter to the editor. Am. J.
Pub. Health 51: 913, 1961.
STEINBERG, A. G.: Methodology in human genetics. J. M. Edue. 34: 315, 1959.
WHITE, P. D.: The importance of heredity in
coronary heart disease. Symposium on Coronary Heart Disease. Circulation 22: 296, 1960.
THOMAS, C. B., AND COHEN, B. H.: The familial
occurrence of hypertension and coronary artery
disease, with observations concerning obesity
and diabetes. Ann. Int. Med. 42: 90, 1955.
Observations on Serum Cholesterol Levels in the Twin Population of Evans
County, Georgia
JOHN R. MCDONOUGH, CURTIS G. HAMES, B. G. GREENBERG, LOUIS H.
GRIFFIN, JR. and ANDREW J. EDWARDS, JR.
Downloaded from http://circ.ahajournals.org/ by guest on June 14, 2017
Circulation. 1962;25:962-969
doi: 10.1161/01.CIR.25.6.962
Circulation is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
Copyright © 1962 American Heart Association, Inc. All rights reserved.
Print ISSN: 0009-7322. Online ISSN: 1524-4539
The online version of this article, along with updated information and services, is
located on the World Wide Web at:
http://circ.ahajournals.org/content/25/6/962
Permissions: Requests for permissions to reproduce figures, tables, or portions of articles
originally published in Circulation can be obtained via RightsLink, a service of the Copyright
Clearance Center, not the Editorial Office. Once the online version of the published article for
which permission is being requested is located, click Request Permissions in the middle column of
the Web page under Services. Further information about this process is available in the Permissions
and Rights Question and Answer document.
Reprints: Information about reprints can be found online at:
http://www.lww.com/reprints
Subscriptions: Information about subscribing to Circulation is online at:
http://circ.ahajournals.org//subscriptions/