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Module 1.1

To better understand these interactions, behavior geneticists conduct studies of genetically
related and unrelated people. Family studies search for traits and diseases that tend to be
shared by family members — for example, among biological siblings, or by a parent and a
child. Twin studies and adoption studies, to which we will now turn, assess in more detail the
effects of shared genes and shared environments.

®
AP Science Practice Check Your Understanding

Examine the Concept Apply the Concept
Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
▶ Explain the difference between heredity and environment. ▶ Were you surprised to learn how genetically similar we are
▶ Explain some effects of small genetic variations within and to our chimpanzee cousins? What was your impression before
between species. reading this section?
▶ Come up with one unique research question or topic that
Copyright © Bedford, Freeman & Worth Publishers.
would be of interest to a behavioral geneticist.
Answers to the Examine the Concept questions can be found in Appendix C at the end of the book.

Twin and Adoption Studies

1.1-3 How do twin and adoption studies help us understand the ef fects and inter -
1.1-3 How do twin and adoption studies help us understand the effects and inter-
actions of nature and nurture?
actions of nature and nurture?
®
AP Science Practice
To scientifically tease apart the influences of environment and heredity, behavior geneticists
could wish for two types of studies. The first would control heredity while varying the home Research
environment. The second would control the home environment while varying heredity. Dramatically manipulating an
Such studies with human infants would be unethical, but nature has done this work for us. infant’s home environment, which
has the potential to cause harm,
would be unethical. Unit 0 has a
Identical Versus Fraternal Twins full discussion of psychology’s

Identical (monozygotic) twins develop from a single fertilized egg that splits. Thus, ethical guidelines for research.
they are genetically identical — nature’s own human clones ( Figure 1.1-2 ). Indeed, they

are clones who share not only the same genes but
also the same conception and uterus, and usually Identical Fraternal Figure 1.1-2
the same birth date and cultural history. Two slight twins twins Same fertilized egg, same
qualifications: genes; different eggs,
different genes
• Although identical twins have the same genes, Identical twins develop from a
they don’t always have the same number of cop- single fertilized egg, fraternal
ies of those genes repeated within their genome, twins from two.
and they sometimes differ in their brain’s tiny
wiring structures. These variations help explain
why one twin may have a greater risk for certain
illnesses and disorders, including schizophrenia
( Lee et al., 2018 ; Maiti et al., 2011 ).
• During prenatal development, most identi-
cal twins share a placenta (the structure that
transfers nutrients and oxygen from mother to
embryo), but one of every three sets has sepa- identical (monozygotic)
rate placentas. One twin’s placenta may provide twins individuals who
developed from a single fertilized
slightly better nourishment, which may contrib- egg that split in two, creating two
ute to a few identical twin differences ( Marceau Same Same or genetically identical organisms.
et al., 2016 ; van Beijsterveldt et al., 2016 ). sex only different sex

Interaction of Heredity and Environment Module 1.1 11

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