AMERICAN JOURNAL OF HUMAN BIOLOGY 31315-324 (1991)

Comparative Genetic Variance and Heritability of Head and Facial Traits in Northwest Indian and Belgian Twins

KRISHAN SHARMA AND CHARLES SUSANNE Department o Anthropology, Panjab Uniuersit , Chandigarh 260024,

Brussels, Pleinlaan 2, 1050 Brussels, Belgium IC S i India ( K S), L aboratory of Human Genetics, d e e University of

ABSTRACT Genetic variance analysis of 13 head and facial traits is consid- ered in samples of Northwest Indian and Belgian twins. Modified t tests, based on the nested structure of twin data, indicate differences between monozygotic (MZ) and dizygotic (DZ) mean values of bigonial diameter in Indian females and in Belgian males, i.e., for only two of 52 instances. Heterogeneity of variance is observed in about 30% of the craniofacial traits in both samples, invalidating conventional within-pair genetic variance estimates for these traits. Patterns of environmental bias on zygosities differ between sexes within the same population and also between the two samples. Revised average genetic variance ratios are higher in Indian than in Belgian twins. Male twins manifest, on average, higher genetic variance ratios than female twins in both the samples. There is stronger evidence of environmental covariance in MZ than in DZ twins for both Indian and Belgian twins. The pattern of intraclass correlations, based on average values, is as follows: dr, > 0 rMZ > drD, z OrDz in the Indian twins and drMz > Or,, > 0 rDZ > 6 rDZ in the Belgian twins.

Craniofacial traits are often used in an- thropological studies of variability and mi- croevolution and in definin racial tax-

that head and facial morphology is largely genetical1 determined, although alterna-

this h othesis, several stu ies of familial resem IY ances for craniofacial traits have been conducted (e.g., Susanne, 1975; Weaver and Christian, 1980; Sharma et al., 1984; Sharma and Sharma, 1984; Poosha et al., 1984; Sharma, 1987). Results of these stud- ies conform to the model of sim le polygenic

However, some evidence of directional domi- nance has also been re orted (Sharma,

been recorded thus far. Twin studies have also been commonly

used to study the genetic basis of craniofacial traits (Lundstrom, 1954; Horowitz et al., 1960; Watnick, 1972; Nakata et al., 1974). These studies, which anal ze intrapair dif-

assum tions such as equality of environ-

twin types. Later refinements in twin meth-

onomies. This interest stems B rom the view

tive hypot K eses have been su gested. To test %

inheritance of several cranio F acial traits.

1987), but no evidence o P sex linkage has

ferences, are usually base B on questionable

menta f variances and covariances between

odologes take these assumptions into ac- count (Christian et al., 1974; Christian, 1979). The improved methods, when applied to quantitative twin data, have demon- strated significant environmental determi- nation for many dental size and occlusal traits (Sharma et al., 1985; Corruccini et al., 1986; Sharma and Corruccini, 1986). This study further considers these oints in new

west Indian (Punjabi) twins compared with Belgian twins.

MATERIALS AND METHODS A sample of 170 airs of twins at Chandi-

garh have been fo lowed in a lon itudinal growth stud since 1975. The twins elong to an urban Junjabi population of predomi- nantly middle socioeconomic levels and en- joy good nutritional status and medical care according to Indian standards. Of this regis- try, 28 male (14 monozygotic [MZI and 14 diz gotic [DZ]) and 29 female (14 MZ and 15 DZY postadolescent and young adult twin pairs form the sample for the present report.

data on head and facial morpho P ogy of North-

% P

Received August 22,1989; accepted January 18,1991.

01991 Wiley-Liss, Inc.

316 K. SHARMA AND C. SUSANNE

Twin z gosity was determined b concor-

Duffy serologic traits, PTC tasting ability, and ABH secretor factor. The girls were above 14 ears and boys were above 16 years of age. A1 had attained adult secondary sexual characteristic development.

All measurements were taken by one au- thor (K.S.) following standard anthropomet- ric techniques (Martin and Saller, 1957). Measurement error for head and facial traits is lower than that for body traits. Avera e measurement error was ‘ust 0.1 mm for t i? e smaller dimensions an d 1-2 mm for the lar er dimensions, i.e., about 1.25%.

+he Belgian sample consisted of 57 MZ and 39 DZ male twin pairs and 67 M Z and 42 DZ female pairs 18-25 years of age (Susanne et al., 1983). Zygosity was based on 22 blood

oups (Defrise-Gussenhoven et al., 1981). Keasurements were redominantly taken by the same person (8s.) following Martin and Saller (1957). The techniques in both the samples are thus comparable.

The analysis was performed using the methodology of Christian and colleagues (Christian et al., 1974; Christian, 1979). The first step compared means between MZ and DZ twins for each of the 13 variables. This was done by using a modified t test based on the nested structure of twin data in which among-pair mean s uares were used as an error term and the 2 egrees of freedom were approximated (Christian and Norton, 1977). In this mixed method, the analysis of vari- ance involves a hierarchy consisting of zy- gosities (fixed effect), twin pairs within zygosities (random effect), and two members within twin pairs (random effect) in that order. Significant differences between MZ and DZ twins would reflect inherent biologi- cal differences associated with the twinning process (Christian et al., 1977; Christian, 1979).

The second step was one-way analyses of variance treating twin pairs as groups of two to provide amon -pair and within- air

variances between z osities, i.e., TMZ = (AMZ + WMZ) and ‘%Z = (ADZ + WDZ), were contrasted by a two-tailed F’ test. The larger sum of mean squares was retained as the numerator with a probability level dou- ble that in usual F tables as suggested by Christian et al. (1974). An 0.2 probability level was used with the F‘ test to control type I1 error because the test is relatively insensi-

dance f$r Al A2 BO, CcDEe, MN, k ell, and

mean squares for M l and DZ twins. &,a1

tive to common variance in the zygosities. Under the model used, the expected values of mean squares can be represented by the following:

E(Mmz + M m z ) = WADZ + Mwoz) = 2 4 + 2u: + 4uge (1)

A significant difference as revealed by the F‘ test contrasting the sum of mean s uares

environmental component unique to each twin type (uaeMZ = environmental variance component for MZ twins and uBeDZ = envi- ronmental variance component for DZ twins). This inequality of environmental fac- tors may be attributed to competitive or convergent influences that differ for twin types, including both re- and postnatal fac- tors (Kempthorne an a Osborne, 1961).

According to the model used, genetic vari- ance estimates would also be biased by in- equality of environmental covariances. If the environmental covariance was greater for MZ than for DZ twins, the heritability estimates would be biased upward. To test this, Christian et al. (1975) su gested an F test comparing within-pair an f amon - air mean squares of DZ twins (F = ADZ&BZ). If this ratio fails to exceed significantly a value of one, then there is a greater chance of relatively higher environmental covariance for MZ twins.

Within-pair genetic variance was tested by the usual F ratio: F = WDZ/WMZ. If there was evidence of variance heterogeneity, the modified among-component ratio was used

between zygosities may be caused % y an

to estimate unbiased genetic variance as su ested b Christian (1979): Fa, = C d 8 Z + AMd/(WMZ + ADZ).

As per Christian’s model, the fraction of enetic variance (GT) estimated by twin data

[as the following expected value:

GT =1/2~2, + 3 / 4 4 + (1 - flu:. (2)

The two estimates of the genetic variance (GT) are GWT = MwDz - MWZ, with the ex- pected value as

E(G:WT) = GT + CMz - CDz + 2(~, , - U&)

+ .“,z - &z (3)

and

COMPARATIVE GENETIC VARIANCE IN HEAD AND FACIAL TRAITS 31 7

with the expected value as

GwT and GCT are the within-pair and among- pair component estimates of genetic vari- ance. The significance of these estimates can be tested by the F ratios explained above. CMz and CDz are the covariances among environmental effects between members of a twin pair in MZ and DZ twins, respectively; uge is the covariance between genetic and environmental effects in the same individ- ual, and u& is the covariance between ge- netic effects on one member of a twin pair and environmental effects on the other mem- ber of that twin pair. According to Chris- tian’s model, at least two assum t i o g must

good estimate of genetic variance GT) when the F‘ test comparing u2 MZ and he,, is not significant. uowever, w&en uzev 7 u2RuB, the estimate GCT must be preferre- thoug it will have a larger variance than GWT.

Means, variances, and Falconer’s herita- bility coefficients based on intraclass corre- lations (h2 = 2[rMz - rDZI) were also calcu- lated to facilitate comparison with other studies.

RESULTS

be made: uge = $!,, and CMz.= cp DZ. GWT is a

Descri tive statistics for Indian twins are given in pr ables 1 and 2. Corresponding de-

KiVen scriptive data for Belgian twins are elsewhere (Susanne et al., 1983). Resu ts of mean and variance equality between zygosi-

ties in Indian and Belgian twins are con- trasted in Tables 3 and 4. Among the 52 comparisons, the t’ test of MZ-DZ mean difference fails to reject the null hypothesis except for bigonial diameter in Indian fe- males and Belgian males. On average, In- dian MZ twins have smaller dimensions, especially females, than DZ twins, althou h the differences are not statistically signi 9 i- cant. In contrast, Belgian twins do not show such a trend but for minor variations.

The F’ test shows that MZ and DZ twins differ more in variances than in means. Fun- damentally different atterns are observed

tween sexes of the same population. In In- dian males, heterogeneity of variance is observed in two of the 13 traits (head len h and nasal height), and in both traits $Z twins are significantly more variable than DZ twins. In Belgian males, variance hetero- eneity is observed in four of the 13 traits ? head breadth, mouth breadth, ear height,

and ear breadth). In two of the traits (head and mouth breadths), MZ twins are more variable, in the other two (ear height and ear breadth), DZ twins are more variable. The mean TDZPTMZ ratio is 0.98 (SD = 0.38) for Indian and 1.02 (SD = 0.36) for Belgian male twins.

Table 4 shows that variance heterogeneity is more apparent in female Indian twins than in their Belgian counterparts. The null hypothesis among zygosities is rejected for seven of 13 dimensions in Indian females. Conversely, in Belgian females the null hy- pothesis is rejected for only three of 13 traits.

between Indian and i elgian twins and be-

TABLE 1. Means (em) and variances for head and facial traits of Indian twins

Mean Variance Males Females Males Females

Measurement MZ DZ MZ DZ MZ DZ MZ DZ

Head length Head breadth Frontal breadth Bizygomatic breadth Bigonial breadth Psysiognomic facial

Morphological facial

Nasion-stomion length Nasal height Nasal breadth Mouth breadth Ear height Ear breadth

height

height

18.26 14.48 10.41 13.11 9.84

17.41

11.50

7.30 5.27 3.63 4.73 6.15 3.32

18.58 14.75 10.48 13.33 9.81

17.25

11.46

7.26 5.25 3.66 4.92 6.08 3.24

17.46 13.87 10.12 12.52 8.76

16.37

10.51

6.77 4.98 3.22 4.57 5.68 3.05

17.68 0.89 13.91 0.21 10.10 0.13 12.69 0.36 9.21 0.40

16.44 0.66

10.77 0.53

6.81 0.24 4.86 0.25 3.29 0.04 4.65 0.18 5.74 0.18 3.05 0.08

0.46 0.36 0.18 0.30 0.27 1.04

0.50

0.23 0.20 0.03 0.18 0.22 0.06

0.34 0.22 0.05 0.09 0.08 0.67

0.33

0.16 0.18 0.06 0.07 0.09 0.04

0.38 0.13 0.12 0.23 0.17 0.40

0.24

0.14 0.10 0.04 0.09 0.05 0.06 -

31 8 K. SHARMA AND C. SUSANNE

TABLE 2. Mean squares among and within twin pairs among Indian twins

Males Females MZ DZ MZ DZ

Measurement AMZ WMZ ADZ WDZ AMZ WMZ ADZ WDZ

Head length 1.81 0.03 0.68 0.25 0.66 0.03 0.68 0.10 Head breadth 0.37 0.05 0.58 0.17 0.40 0.06 0.18 0.07 Frontal breadth 0.26 0.01 0.31 0.06 0.09 0.02 0.14 0.10 Bizygomatic breadth 0.74 0.01 0.54 0.07 0.17 0.02 0.32 0.14 Bigonial breadth 0.81 0.01 0.36 0.19 0.13 0.03 0.27 0.08 Physiognomic facial 1.22 0.15 1.26 0.84 1.29 0.10 0.57 0.24

height

height Morphological facial 1.04 0.05 0.63 0.38 0.64 0.04 0.39 0.09

Nasion-stomion length 0.49 0.02 0.35 0.12 0.30 0.02 0.22 0.07 Nasal height 0.50 0.10 0.18 0.06 0.36 0.01 0.13 0.06 Nasal breadth 0.07 0.01 0.03 0.04 0.11 0.01 0.05 0.03 Mouth breadth 0.35 0.02 0.29 0.07 0.13 0.01 0.12 0.05 Ear height 0.37 0.01 0.33 0.13 0.18 0.01 0.05 0.05 Ear breadth 0.15 0.01 0.08 0.03 0.08 0.003 0.10 0.02

TABLE 3. Tests for equality of means and uariances among male twin zygosities

t' F' F = ADZ/WDZ Trait Indian Belgian Indian Belgian Indian Belgian

Head length -1.05 -0.02 1.99l 1.27 2.69 2.28 Head breadth -1.47 -0.83 1.74 1.43' 3.44 2.41 Frontal breadth -0.45 0.61 1.39 1.13 5.24 1.74' Bizygomatic breadth -0.99 -1.13 1.22 1.33 7.94 2.17 Bigonial breadth 0.14 -2.66' 1.52 1.33 1.8@ 1.35? Physiognomic facial height 0.55 0.37 1.54 1.29 1.49'' 2.58 Morphological facial height 0.18 0.86 1.08 1.31 1.673 2.87 Nasion-stomion length 0.23 0.49 1.08 1.19 3.05 6.84 Nasal height 0.09 0.15 2.11' 1.37 2.77 5.21 Nasal breadth -0.04 -0.75 1.10 1.27 0.76? 2.94

Ear height 0.46 0.03 1.19 1.452 2.61 3.26 Ear breadth 0.89 0.09 1.36 1.612 2.45 11.68

'MZ significantly more variable than DZ. "7. significantly more variable than MZ.

'P < 0.05

Mouth breadth -1.20 -1.08 1.01 2.191 4.18 0.9o3

3P > 0.10.

ThemeanTDZrnMZratiois 1.16(SD = 0.70) in Indian females and 1.03 (SD = 0.21) in Belgian females. The results thus show that variability in Indian female twins is much higher than that in Indian male twins. No such trend is evident in Belgian twins.

The ADZNDZ ratio also exhibits differ- ences between the two sam les and between

significant for six of the 26 traits (23%) in Indian twins and for four of the 26 traits (15.4%) in Belgian twins. Both samples re- veal hi her environmental covariance of MZ than D !i twins in males than in females, but different traits are affected in the two sexes and samples except for bigonial diameter in male twins. Most estimates of genetic vari-

sexes (Tables 3 and 4). T!l e F test is not

8 ance and heritability for traits showin higher MZ environmental covariance woul be biased upward. These results will tend to confound any inference drawn from tradi- tional twin studies for these traits.

Genetic variance ratios (GVRs) are pre- sented in Table 5 for males and in Table 6 for females. By employing Christian's notation, i.e., substituting among-component in place of within-pair estimates wherever re uired,

male twins (Table 5 ) . The revised average enetic variance ratio is 7.29 (SD = 4.17) in

Belgian male twins (mean = 2.70, SD = 1.21). The average is reduced by a 0.54 frac- tion in Indian male twins by substituting the

GVRs are significant for all traits in I ndian

f ndian males, which is higher than that in

31 9 COMPARATIVE GENETIC VARIANCE IN HEAD AND FACIAL TRAITS

TABLE 4. Tests for equality of means and variances among female twin zygosities

Trait t' F' F = ADZ/WDZ

Indian Belgian Indian Belgian Indian Belgian

Head length -1.11 Head breadth -0.26 Frontal breadth 0.32 Bizygomatic breadth -1.47 Bigonial breadth -4 .1F~~ Physiognomic facial height -0.29 Morphological facial height -1.44 Nasion-stomion length -0.28 Nasal height 0.92 Nasal breadth -1.00 Mouth breadth -0.95 Ear height -0.66 Ear breadth -0.08

'MZ significantly more variable than DZ. "DZ significantly more variable than MZ.

4 P < 0.05 "P > 0.10.

0.51 -0.47 -1.01

0.67 0.39

-0.07 -0.31 -0.35 -0.66 -0.76 -1.18 -0.75 -0.91

1.13 1.79l 2.142 2.4a2 2.172 1.72l 1.40 1.14 1.94l 1.52 1.22 1.78l 1.51

1.00 1.67' 1.18 1.37' 1.08 1.18 1.40' 1.10 1.10 1 .oo 1.02 1.08 1.22

6.79 2.79 2.53 3.02 1.413 2.84 2.31 2.63 3.25 2.70 2.31 2.24 4.56 3.66 3.20 4.79 2.13 3.54 2.10 3.54 2.48 1.773 0.923 2.84 4.65 5.77

TABLE 5. Genetic variance estimates for males

Measurement

Genetic variance ratio (GVR) Indian Belgian

Among- Among component component

Within- (if valid) Within- (if valid) pair F P F P pair F P F P

Head length Head breadth Frontal breadth Bizygomatic breadth Bigonial breadth Physiog. face height Morph. face height Nasion-stomion length Nasal height Nasal breadth Mouth breadth Ear height Ear breadth

8.38 3.19

11.86 9.55

12.95 5.64 7.29 6.61 4.42 5.89 3.34

15.90 6.75

0.000 0.019 0.000 0.000 0.000 0.001 0.000 0.001 0.004 0.001 0.017 0.000 0.000

5.27 2.80 3.10 2.91 4.00 3.64 2.24 2.35 2.20 3.00 1.45 2.81 1.33

0.000 0.000 0.000 0.000 0.000 0.000 0.003 0.002 0.003 0.000 0.092 0.000 0.154

- 0.005 - - - - - - - -

0.000 0.523 0.945

among-com onent estimate wherever ap- ?he average within-pair GVR in

Belgian male twins, the avera e value is

within- air GVR in Belgian male twins is 2.85 (S6 = 1.05). In Belgian males, GVR is not significant for two traits (ear hei ear breadth). Ear height shows the GVR in Indian male twins, len h shows the highest GVR in Belgian

In Indian female twins (Table 6),10 traits reveal significant genetic variance esti- mates. Two traits (frontal breadth and ear hei ht) display insignificant GVRs. For na-

propriate. ndian male twins is 7.82 (SD = 3.83). In

lowered by a 0.15 fraction. T a e average

ma ? e twins.

sal a eight, the GVR almost approaches criti-

cal value (P = 0.056). The revised average GVR is 3.26 (SD = 1.921, which is lower than the within-pair average GVR (4.82, SD = 2.55). On the other hand, in Be1 'an female twins, the revised avera e G k is 2.13

within-pair estimate (2.19, SD = 0.81). In Belgian females, two traits (nasion-stomion length and morphological facial hei ht) ex-

traits, the & is significant. Indian twins demonstrate higher genetic

variance ratios than the Belgian twins. Be- tween sexes, male twins manifest, on aver- age, higher genetic variance estimates than females in both the samples.

The intraclass correlation coefficients, rMZ

(SD = 0.841, which is slight s y lower than the

hibit nonsi ificant GVRs. For a1 ? other

320 K. SHARMA AND C. SUSANNE

TABLE 6. Genetic variance estimates for females

Genetic variance ratio (GVR) Indian Belgian

Among- Among- component component

Within- (if valid) Within- (if valid) Measurement pair F P F P pair F P F P

Head length 3.24 0.012 - - 2.40 0.000 - -

Head breadth 1.15 0.396 2.53 0.026 1.28 0.179 1.82 0.007 Frontal breadth 6.13 0.000 1.41 0.230 2.25 0.001 - -

Bizygomatic breadth 8.75 0.000 2.31 0.039 1.40 0.107 1.57 0.028 Bigonial breadth 2.95 0,019 3.25 0.007 4.30 0.000 - - Physiog. face height 2.46 0.041 2.31 0.039 2.34 0.001 - -

Nasion-stomion length 3.68 0.007 - - - - Nasal height 4.74 0.002 2.13 0.056 1.69 0.028 - - Nasal breadth 4.55 0.002 - - 2.08 0.003 - - Mouth breadth 5.39 0.001 - - - - 2.00 0.006 Ear height 8.94 0.000 0.92 0.567 2.95 0.000 - - Ear breadth 8.22 0.000 - - 2.10 0.003 - -

Morph. face height 2.47 0.040 - - 2.42 0.000 0.96 0.574 1.20 0.246

and rDz, estimated by the difference of amon - and within-pair mean s uares di-

Indian Z twins, these coefficients are insi - nificant in 10 of 26 instances (38.4%). fn Belgian DZ twins, this trend is observed in only three of 26 instances (11.5%). All other coefficients are significant (P < 0.05) in a one-tailed test.

DISCUSSION As a prelude, it would be worthwhile to

discuss the fundamentals underlying quan- titative genetics to enable readers to inter- pret the results in a proper perspective. A

vided%h their sum, are listed in + able 7. In

phenoty e of any quantitative trait is com- posed o P genoty ic and environmental com- ponents, so the First step in genetic analysis is to partition total phenotypic variance into its components. It is now an established fact that quantitative traits are 01 genic, so more than one locus is involvecf &is allows the possibility of nonallelic interaction among loci, epistasis. Moreover, alleles at a locus always act in pairs, resulting in the possibility of dominance plus additive inher- itance. Genetic variance can be further re- solved into three components: additive, dom- inance, and epistasis (uG = (T + u + ui). The additive genetic variance of a pofygenic

TABLE 7. Intraclass correlations

Indian twins Belgian twins Males Females Males Females

Trait MZ DZ MZ DZ MZ DZ MZ DZ

Head length 0.97 0.46 0.91 0.74 0.85 0.39 0.78 0.47 Head breadth 0.75 0.55 0.73 0.43 0.85 0.41 0.77 0.50 Frontal breadth 0.96 0.68 0.71 0.17' 0.79 0.27 0.73 0.48 Bizygomatic breadth 0.98 0.781 0.83 0.40 0.84 0.37 0.71 0.45 Bigonial breadth 0.96 0.31 0.65 0.53 0.79 0.15l 0.86 0.46 Physiog. facial height 0.78 0.201 0.86 0.40 0.88 0.44 0.75 0.38 Morph. facial height 0.90 0.25' 0.90 0.64 0.82 0.51 0.75 0.57 Nasion-stomion length 0.93 0.51 0.89 0.52 0.87 0.75 0.68 0.66 Nasal height 0.94 0.47 0.93 0.36' 0.80 0.12' 0.71 0.56 Nasal breadth 0.82 -0.14l 0.90 0.33' 0.78 0.49 0.72 0.42 Mouth breadth 0.89 0.61 0.87 0.43 0.67 -0.06' 0.63 0.28 Ear height 0.96 0.45' 0.93 0.04' 0.76 0.53 0.81 0.48 Ear breadth 0.94 0.42' 0.94 0.65 0.81 0.84 0.83 0.71 Mean for 13 variables 0.91 0.44 0.85 0.43 0.81 0.40 0.75 0.49 S.D. 0.08 0.21 0.09 0.20 0.06 0.24 0.06 0.11 Number of pairs 14 14 14 15 57 39 67 42

'Intraclass correlations not significant a t or below 0.05.

COMPARATIVE GENETIC VARIANCE IN HEAD AND FACIAL TRAITS 32 1

trait is a result of differential additive effects of alleles for that trait in a population. The term additive effect is a statistical concept, which conce tually means that gene prod-

additively. Dominance and epistasis alter the effects of the alleles acting under strictly additive gene action.

Additive genetic variance of any trait as- sumes major importance in changing or maintaining phenotypes. It is the chief cause of resemblance between relatives and thus the chief determinant of manifested genetic properties of a population. Additive genic action is further accountable to evolution, because selection pressure operates on phe- notypes. Also, it is the additive com onent of

generation to the next. Studies on the enetic basis of quantita-

Cloninger et al., 1979) or genetic nonadditiv- ity is the result of pure dominance (Rao et al., 1982) or urely epistatic interactions be-

al., 1984). Family studies (Susanne, 1975; Weaver and Christian, 1980; Sharma et al., 1984) are enerally consistent with additive models in 8 etermining craniofacial morphol- ogy. Presuming this to be true, the expected value of genetic variance in e uation 2 in

account of additive genetic variance, because dominance and epistatic genetic variance are ne ligible.

indicate greater resemblance in MZ than in DZ twins for head and facial morphology in

ucts act as i P the individual alleles combine

genetic variance that is passed P rom one

tive traits enera P ly assume that all gene effects com t ine additively (Morton, 1974;

tween ad 2 itive genetic deviations (Heath et

Materials and Methods would Il e solely on

In t a e present study, results generally

both samples. In the Indian sample, mean correlation coefficients of all measurements are 0.91 vs. 0.44 in male and 0.85 vs. 0.43 in female twins. Corresponding means are 0.81 vs. 0.40 in male and 0.75 vs. 0.49 in female Belgian twins. The pattern of average corre- lations is drMZ > PrMZ > drDZ 2 PrDz in Indian twins and drMZ > 9 rMz > 0 rDZ > drDz in Belgan twins. An explanation for this trend may be that, in a sample of posta- dolescents and young adults, age changes may lead to an artificially greater similarity in male MZ than in female MZ and in male DZ than in female DZ twins. Susanne et al. (19831, for example, have shown that, be- tween 18 and 25 years of age, regression coefficients for head and face measurements on age are significantly positive more often in males than in females.

Genetic differences among human popula- tions are well known. These genetic differ- ences would lead to patterns of correlations among relatives that would result in differ- ences in heritabilities amon opulations. Environmental and cultura f f actors also have a bearing on heritability estimates. Heritability in a narrow sense refers to the proportion of total phenotypic variance that is attributable to additive effects of genes. Heritability estimates from intraclass corre- lations of MZ and DZ twins are presented in Table 8. In both samples, males show hi her

results are consistent with those from ge- netic variance ratios. Table 8 shows several estimates that exceed one. In all these cases, intraclass correlations for DZ twins are sta- tistically insignificant (P > 0.05), whereas those for MZ twins are highly significant.

heritability estimates than females. T a ese

TABLE 8. Heritability estimates from intraclass correlations

Indian Belgian Measurement Males Females Males Females

Head length 1.02 0.33 0.91 0.62 Head breadth 0.41 0.58 0.88 0.53 Frontal breadth 0.57 1.08' 1.04 0.50 Rizygomatic breadth 0.41 0.87 0.94 0.53 Bigonial breadth 1.32l 0.25 1.29' 0.80 Physiog. face height 1.17' 0.92 0.88 0.72 Morph. face height 1.31' 0.51 0.63 0.36 Nasion-stomion length 0.85 0.72 0.25 0.06 Nasal height 0.95 1.14l 1.36l 0.30 Nasal breadth 1.92l 1.14' 0.57 0.61 Mouth breadth 0.54 0.89 1.45l 0.71 Ear height 1.03l 1.95' 0.46 0.65 Ear breadth 1.03' 0.58 -0.08 0.25

]r,rz > 0.05.

322 K. SHARMA AND C. SUSANNE

Given this situation, it can be said that environmental factors do not operate ran- domly between zygosities. There is evidence of higher environmental covariance in DZ twins than in MZ twins as revealed by the ADZNDZ test. In a Swedish twin study, Fischbein et al. (1990) have reported that MZ twins are more dependent on each other and s end more time together than DZ twins. &mindful of such situations, in the tradi- tional twin studies, the excess MZ correla- tions are interpreted as being genetic in origin. Additional DZ differences created by environmental covariance would lower their intraclass correlations and thus bias the re- sults in favor of high genetic estimates, which are in reality spurious.

By conventional methods, Indian and Bel- gian twins consistently show high and signif-

enetic variability for craniofacial traits. icant 1 ccording to Christian’s methodolo any association between twin type any; trait under study would bias genetic vari- ance estimate for the trait. Furthermore, if means between MZ and DZ twins differ, any further genetic analysis is unwarranted un- der the model. The t‘ tests indicate signifi- cant differences only for bigonial diameter, which ma be attributed to a type I statisti-

nificant genetic variability of this trait should be cautiously drawn, since both In- dian and Belgan twins reveal an association between twin type and the mean of this trait (two of four instances). These differences may be due to developmental differences associated with mandibular growth. On the whole, MZ and DZ twins have similar biolog- ical determinants for head and facial traits.

Variance heterogeneity indicates differen- tial environmental influence between zygos- ities. Indian females show greater variance ine ualities than males. This may be attrib-

males than males. Despite modernization, arental care still favors males more than emales in India, and conse uently males

enjoy preferential treatment. % o such trend is observed in Belgian twins. Results indi- cate that most variables exhibit a lar er within-pair mean square in DZ than in h Z twins and that the among- air mean square is larger in MZ than in 8 Z pairs. Such a picture emerges only when a significant ge- netic variance is present. The results indi- cate that the basic assum tion of equality of

cal error. & owever, any inference about sig-

Ute 1 to greater environmental stress on fe-

!

environmental variance f etween zygosities

is not tenable for about one-third to one- fourth of the traits. A similar situation has been noticed in dental and skinfold traits (Potter et al., 1979; Sharma and Corruccini, 1986; Sharma, 1988). Hence, within-pair ge- netic variance estimates are not valid for some of the craniofacial traits as they are biased. The variance inequalities necessi- tate revised amon -component estimates of genetic variance. %y influencing the vari- ance heterogeneity, sex effects further con- found an already complex situation. Hence, in studies where sex effects are not ac- counted for, biased estimates of genetic vari- ance would result.

Another characteristic feature of the data is that MZ twins manifest higher total phe- notypic variance than DZ twins in six of nine instances in Indian twins and in four of seven instances in Belgian twins. Following Christian’s model, we would reason that this is due to unequal environmental variance rather than greater MZ genetic variance. Kempthorne and Osborne (1961) postulate that ‘competitive forces” are different for the zygosities. Moreover, it may also be associ- ated with developmental peculiarities asso- ciated with the twinning process, since different biological factors are involved in MZ and DZ twinning. Parents of MZ twins have been reported to ex erience more prob-

DZ twins, resumably because of more fre-

, 1990). These factors may roblems among MZ twins

hschbein et ap. contribute to variation among MZ twins. However, these developmental peculiarities and environmental factors generally are not of the magnitude to cause any significant differences in the phenotypic means between the zygosities.

Among-component estimates of genetic variance are generally considered more con- servative. Consequently, average GVRs (sub- stituting among component if valid) are re- duced both in Indian and in Belgian twins. This finding is contrary to the picture re- ported in genetic variance analysis of dental occlusal traits (Sharma and Corruccini, 1986; Corruccini et al., 1986). For occlusal traits, the revised amon -component esti- mates raised the GVR in f ndian twins, and significantly reduced the GVR in American twins. Also, in the present study, the revised among-component estimates in Be1 ‘an twins are significant for mouth breadt a in males and head breadth and bigonial

lems in rearing the chi1 c f ren than parents of

uent heath P

COMPARATIVE GENETIC VARIANCE IN HEAD AND FACIAL TRAITS 323

breadth in females, whereas the within-pair GV estimates are not significant. Thus the generalization that among-component esti- mates of genetic variance are conservative ma not be tenable in all cases.

Jurthermore, four of the 13 significant Indian male genetic variance estimates for craniofacial traits are invalidated by evi- dence of relatively stronger environmental covariances among MZ than among DZ twins as revealed by the ADZNDZ test. A similar picture is evident in Belgian male twins, among whom the estimates are invalidated for three traits. However, the situation in female twins is more stable where lesser evidence of environmental covariance is ob- served in both samples. In Indian female twins, both affected traits (frontal breadth and ear height) already dis lay an insignifi- cant GVR, but, in Bel ian emale twins, the

nificant GVR that is invalidated. The given results reveal a complex but

significant environmental component of variance in several craniofacial traits. Pat- terns of environmental effects operating be- tween sexes within the same population and between Indian and Belgian populations are generally different. Si ificant environmen-

dicates that selection pressures are also operating on craniofacial traits. The most affected are facial and mandibular traits. This is ex ected, however, since there is

facial bones, jaws, and teeth (see Sharma and Corruccini, 1986). For example, Dahl- berg (1964) has reported an adaptive corre- lation between dental function and nose form. Furthermore, the dentition, including occlusion, is responsive to other culturally determined selection pressures particularly associated with chewing habits, food prepa- ration technology, etc. (Garn, 1964; Brace and Montagu, 1965; Corruccini, 1984).

In summary, comparison of means and variances between zygosities permits the de- tection of patterns of association of twin z gosity with mean and variance of a trait. &is in turn ermits the detection of hidden

between zygosities that are not considered under the classical twin model. The influ- ence of sex tends to bias genetic variance estimates if its effect is not accounted for. In both Indian and Be1 'an twins, males mani-

affected trait (mouth % ! readth) shows a sig-

tal determination of p 8" enotypic variance in-

usually a R armonious relationship among

biological an if or environmental inequalities

fest, on average, a 8! igher genetic variance

ratio than females. The study shows that the twin method is still a unique and efficient method for resolving genetic and environ- mental effects. However, oversimplification of the method should be avoided, and only models that provide for testing underlying assumptions should be used.

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