UNSW Embryology
PLACENTAL DEVELOPMENT
TWINNING, DIZYGOTIC
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TEXTWeinberg (1909)
suggested that hereditary twinning is transmitted
only through the female line, applies only to
dizygotic twins, and is probably recessive.
Observation of multiple births following use of
pituitary gonadotropins suggests a pituitary
mechanism for gene action (Milham,
1964). Wyshak and White
(1965) presented evidence, based on Mormon
records, which they interpreted as supporting
recessive inheritance. Among the children of female
DZ twins, 17.1 twins per 1000 maternities occurred
as compared with 7.9 among children of male DZ
twins. Female sibs of DZ twins had 17 per 1000
twins, whereas male sibs had 13.1 per 1000.
Supposedly the gene is too frequent for one to
expect increased consanguinity in the grandparents
of dizygotic twins. Taylor
(1931) reported multiple sets of dizygotic
twins in 4 generations. In the same family, mother
and daughter did not menstruate until after their
first pregnancies, at ages 20 and 22, respectively.
The differentiation of multifactorial and
monofactorial inheritance of twinning is difficult.
Ethnic differences in the rate of dizygotic
twinning is evidence of genetic factors. In
interracial marriages the rate follows that of the
mother's ethnic group. Furthermore, when the mother
is a racial hybrid, the dizygotic twinning
frequency is that of the race with the lower
frequency (Morton et al.,
1967).
Montgomery et al.
(1993) mapped a gene responsible for multiple
ovulation in sheep to a region of the sheep genome
homologous to 4q in the human. Specifically, they
showed that it was linked to markers in the region
of 4q21-q25. Martin et al.
(1984) described higher concentrations of
follicle-stimulating hormone in mothers of
dizygotic twins compared with mothers of single
births, in agreement with results in the sheep for
ewes carrying the Booroola fecundity gene (FECB;
134720).
Although the FecB mutation in sheep behaves as an
autosomal dominant, it will be worthwhile testing
markers from human chromosome 4q to identify
markers linked to twinning.
The frequency of twin deliveries varies among
human populations. The highest twinning rates for
Caucasian populations have been recorded on the
archipelago of Aland and Aboland, in southwest
Finland, whereas multiple deliveries in adjacent
mainland areas are known to be rarer. Using data
from the pre-industrial era (1752-1850), Lummaa
et al. (1998) compared the lifetime
reproductive success of mothers who had produced
twins with that of mothers of singletons in these
archipelago and mainland sites. When they
restricted their analysis to mothers with a genetic
tendency to produce twins, they found that lifetime
reproductive success was maximized by having twins
on the archipelago, but by having singleton
offspring on the mainland. They considered this
result to be consistent with the difference in
twinning rate being maintained by natural
selection. The 'profitability' of twinning would,
in their model, depend on the level of resources.
The model suggested that predictable resource
levels favored the evolution of increased
reproductive output. In the archipelago, the amount
of food available had traditionally been relatively
high and constant, with total crop failures being
rare and with survival ensured by fishing. In poor
mainland areas, on the other hand, crop failures
and subsequent famines had been common throughout
the centuries. The authors assumed that only
dizygotic, but not monozygotic, twinning has a
genetic component (Parisi et
al., 1983) and considered only the analysis of
dizygotic twinning as relevant to evolution of the
number of children produced at one time driven by
natural selection. Results using only mothers with
twins of different sexes confirmed the evolutionary
inference.
REFERENCES
CLINICAL SYNOPSIS
CONTRIBUTORSVictor A. McKusick - updated : 11/6/1998
CREATION DATEVictor A. McKusick : 6/4/1986
EDIT HISTORYterry : 11/18/1998 |
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