A multivariate procedure for estimating heritable components from twin data was applied to ridge counts obtained from the entire dermatoglyphic system. Covariance matrices of MZ and DZ within-pair differences were used to estimate genetic correlation matrices for 20 finger ridge counts, 6 palmar interdigital counts, 20 toe counts, 4 hallucal counts, and 6 sole interdigital counts. The proportion of genetic variation was found to be greater in ridge counts of patterns than in ridge counts of interdigital areas. On digits, finger counts are more highly heritable than toe counts. Each of the dermatoglyphic areas yielded several independent genetic components, ranging from general to specific. Environmental variation was found to be local and to frequently involve reciprocal interaction between twin pairs.

It is estimated here that, at the time of conception, the mean coital rate of young women who subsequently bear DZ twins is about 6% higher than that of the general population of young married women. This differential seems too small to account wholly for the greater promptness in conceiving DZ twins. Accordingly, it is suggested that coital rate is associated with DZ twinning in two ways. The major link seems to be indirect and mediated by erotic response which causes an increase in gonadotrophin levels and thus in double ovulation. In a minority of cases, frequent coitus may give rise to DZ twins via superfecundation, but it seems that, in human beings, the corpus luteum of one ovum will usually inhibit fertilization of any further ova after a short interval. The evidence presented here, though indirect, seems to suggest that, under particularly erotic conditions, double ovulation is sometimes induced. It seems reasonable to infer that coitus also occasionally provokes or accelerates single ovulations, thus impugning some rhythm methods of contraception. But there seems no very good evidence that rape induces ovulation. Some notes are added on the implications of induced ovulation for estimates of fecundability and of the length of the fertile period.

The statistical model for the analysis of twin-family data of Williams and Iyer is examined. The model uses a large number of redundant parameters. It does not lead to quantitative predictions for new relationships. It allows for epistasisbut not dominance. It makes assumptions about assortative mating which are inconsistent with any biologically plausible mechanism. It assumes that the environmental correlation between parent and offspring is due to the direct effect of the parental genotypes, not phenotypes, on offspring environment. Other models which avoid these problems are more appropriate for the analysis of extended twin-family data.

An individual's functional ability in physiological responsiveness is thought to be an interaction between his heredity and his environment. This hypothesis was tested to determine if different extragenetic influences would alter functional adaptability in a set of MZ triplets. After a 3-month aerobic physical fitness training program varying only in frequency, measured values for the triplets' maximum oxygen consumption (MaxVo2 ) were 59.1, 44.5, and 57.8 ml/min/kg, as compared to pretreatment values of 45.2,45.1, and 49.1 ml/min/kg respectively. These results clearly indicate intrapair differences in functional adaptability, stemming from difference in the training frequency program. The split-triplet design of this study indicates that environmental factors contribute substantially to the intrapair variance found among MZ siblings. Data extrapolation suggests that environmental stimulation of sufficient magnitude is likely to alter the functional adaptability in the individual set by his genotype.

The value of single determinations of serum hCG, hPL, and αFP for the detection of multiple pregnancy was investigated in a consecutive series of 992 women between the 14th and 24th week of pregnancy. With the 90th percentile as referent value, all twin pregnancies (n = 10) were detected by the combined results of the three determinations. Considered separately, hPL proved to be the most useful indicator of multiple gestation; the sensitivity of hPL alone was 70% and the predictive value of hPL concentrations above the 90th percentile was 8.1.

Central pockets were defined as small, loop-enclosed whorls whose quantitative values must not exceed the third part of the quantitative value of the loop or as small, whorl-like patterns in the core of a loop having at least one curved ridge with its convexity towards the opening of the loop. Applying this classification scheme, the frequency of central pockets was found to be 17.5% in 200 males and 17.0% in 200 females, but was significantly higher in a sample of 21 male and 22 female pairs of MZ twins (33.2% and 34.1%, respectively). Twin as well as family data (94 families with 269 children) pointed to a rather weak hereditary influence upon the formation of central pockets. Rudimentary central pockets occurred in 9.5% of males and 10.0% of females. Since no common genetic basis could be established for central pockets and rudimentary central pockets, the latter should not be scored as central pockets.