Universlty of Manitoba, ln Partîal Fulfiìlment - MSpace at the ...

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ln general the lncldence of dysraphlsm shows an east-west cllne,
whlch decreases across Nôrth Amer¡ca (Hewitt, 1963) and increases across
the British lsles (Elwood, 1970). Th¡s pattern could be due to genetic
or envlronmental factors, or both.
2.2.6 Ethnìc DistributîoL
There are deÍlonstrable differences in incidence between different
ethnic ArouPs. living within the same region (Naggan and t4acHahon, 1967) '
ln the southern U.S'A. and in South Africa, Negroes show a much
I ot¡er incidence of dysraphism than whites (Alter, 1962; Penrose, 1957) '
Thls may partly be due to under-rePort¡ng.r ðs the ínc¡dence among
Negroes in Kenya is comparable to that in the white Population (Khan,
1965). A similar paradox is seen in lndia where reported dysraphism
ls uncommon except êrnong Sikhs, who show one of the h¡ghest rates in
the vrorld that persists after emigration (Searle, 1959) '
These ethnic differences within a community might also reflect
genet¡c or envîronmental variables, though some insìght into their
relatlve roles is offered by studies of imrnigration. l-ect (t969)
found that immigrant groups in Birmingham showed a change toward the
nBlformation .rates
of the host commun î ty, though marked ethnlc differences
were st¡ll âpparent. Th¡s suggests an envi rorimental modiflcatlon
of underlying genetic d i fferences.
2.2.7 Fami ly Studïes
Neural defects tend to be repeated in a slbship,but the recurrence
pâtterns of dysraphism and congenital hydrocephalus differ from each
other (!4acHahon, Pugh and lngal ls, 1950). The recurrence risk for
dysraphism has been calculated as about 53 after oi¡e affected slbllng