The relative influences of “nature” and “nurture” in
human traits have long been a subject of debate. A common method
for assessing the relative influence of genetics and environment involves
the study of twins, which occur with a frequency of approximately
1 in 100 births in Caucasians (the prevalence is slightly lower
in Asians and slightly higher in Africans). Sir Francis Galton,
a cousin of Charles Darwin, realized that monozygotic (or identical)
twins could be compared with dizygotic (or fraternal twins) to shed
light on the nature-nurture question. Monozygotic (MZ) twins, which
arise from the early cleavage of the embryo into two virtually identical
embryos, share 100% of their genes. Dizygotic (DZ) twins,
which are caused by the fertilization of two egg cells by two different sperm
cells, are genetically the same as siblings, sharing 50% of
their genes. Galton reasoned that a trait strongly influenced by
genes should show greater similarity in MZ twins than in DZ twins. For
quantitative traits such as height or blood pressure, this similarity
is typically measured as an intraclass correlation coefficient,
which varies from –1.0 to 1.0. An intraclass correlation
of 1.0 indicates a perfect positive association for a trait in all
twin pairs, and a correlation of –1.0 indicates a perfect
negative association. A correlation coefficient of 0 indicates that
there is no association. For present/absent traits, such
as neural tube defects, a concordance rate is estimated (ie, if
one twin has the trait, how often does the other twin have it?).
For traits in which the prevalence varies according to gender, MZ twins
are compared with like-sexed DZ twins.