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

6
and about 112 after two (Laure,nce, 1969). These rates fall well short
of the 257. level suggesti,,,e of recessive inheiîtance (Pen'rose, l!å$) .
Twîn studies show a lower concordance than might be expected, with
a risk to the co-twin of about lt% (Record and l"lcKeown, 195]). When a
woman remarries the recurrence risk for maternal half-síbl ings is at
¡east ês great as for fuii sibiings (Yen and l"iact4ahon, 1968).
2.2.8 Maternal Age and Parity
Maternal age and parÍty are difficult to anâlyse separately. Dysraphic
pregnancies are commoner ín primipara and grand multipara than at inter- ..
medìate parities (Record and McKeown, ,|949). Some (though not all)
¡nvest¡gators êlso report a higher frequency in older mothers as an
independent effect (Edwards, t95B; Record, l96l).
2.2,9 Soc io- Ecojgdqlta tus
Poorer fami I íes show a higher incidence of dysraphism (Ceffey and
Jessop, 1957; Edwards, l95B; Pleydell, 1960) though nor of -other
lethal
bîrth defects (Anderson, BaÍrd and Thomson., I95B). This social gradient
however is not shown by Jewish famil ìes (Naggan and HacMahon, i967) and
does not apply to the Negro, populatíon of the southern U.S.A. (Alter,
1962) .
2.2.10 Urbanîzation
u".,1* ,JJreport
ê hígher incidence of dysraphísm in industriat
and urban communities, especially among poor familíes (Anderson, Baird
and Thomson, 1958; Edwards, .t958; Pleydelt, f960).
2.2.11 0o0c Llq íons_
Epidemiological studíes have thus revealed many assocîations but
no clear etîology for neural tube defects. lmportant environmental