The famous Charles Darwin had always wondered
about the “similarity and dissimilarity of twins” (p. 76). Why can pairs of
twins end up being polar opposites, whereas others can be extremely similar
like the twins mentioned on page 75? Shenk discussed the uncanny similarities
between Jim Lewis and Jim Springer, identical twins that met for the first time
when they were thirty-nine years old, yet had almost identical lives. Thomas
Bouchard and his colleagues at the University of Minnesota published research
that seemed to show that genes were actually capable of determining: “60% of
intelligence, 60% of personality, 40-66% of motor skills, and 21% of
creativity” (p. 77). On the other hand,
University of Virginia psychologist Eric Turkheimer’s examination of only poor families showed that intelligence
was “near 0% heritable- demonstrating one and for all that there is no set
portion of genetic influence on intelligence” (p. 79). As we have studied in
our Genetics unit, identical twins have identical DNA and fraternal twins
typically have about 50% of the same genetic material, just like regular
siblings. However, if the possibility of a mass variety of inheritance is true,
then is it just coincidence that Jim Lewis and Jim Springer had nearly
identical lives? Would that also be considered lucky for Mendel when he bred
the pea plants? Is it possible for us to conclude that fraternal twins are the
product of various degrees of dominance as discussed in Chapter 14 Section 3 on
page 271? Additionally, do the vast amount hidden dissimilarities seem to
undermine the results of the twin studies done by Bouchard and Turkheimer if in
taking the Jim twins, “for every tiny similarity between [them], there were
thousands of tiny (but unmentioned) dissimilarities” (p. 81)? Feel free to refer
to Chapter 14 of the AP Biology book to relate to the human upbringing
correlating to interdependence in nature with our environment and/or evolution
or regulation of gene expression affecting genotypes and phenotypes.
Shivani Thakker (shivanithakker1357@gmail.com)
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ReplyDeleteFirst of all, “heritability” does not refer to individual inheritance via genes; as Shenk points out, “Heritability…is a population average, meaningless for any individual person” (79). Genes do not actually “determine” intelligence, personality, and creativity; rather, a heritability of 60% means that variation in a particular sample can be attributed to 60 percent genes and 40 percent environment. Turkheimer’s study implies all of the variation is attributable to the environment. Indeed, heritability is a “statistical phantom” – implying a nature v. nurture distinction that simply does not exist.
ReplyDeleteIdentical twins share 100 percent of their actual DNA; however, the shared DNA is not sufficient for actual shared characteristics of the twins. Identical twins likely spend long periods of childhood in contact with each other and the same surroundings, which is why the characteristics and personalities of those twins are so often shared. With twins that lack those shared surroundings, however, the twins are much less likely to share characteristics or personalities. The interesting case with Jim Springer and Jim Lewis is more an amazing coincidence than anything else. Again, the two boys had a similar environment throughout childhood, so they developed in similar ways – but not the same ways.
“For every tiny similarity between the Jim twins,” Shenk writes, “there were thousands of tiny (but unmentioned) dissimilarities.” (81). Really, twin studies tell us that people reared apart can end up incredibly different; that heritability itself, even when interpreted correctly, is not a real statistic.
All this relates directly to interdependence in nature - that is, the idea that "each organism interacts continuously with its environment, which includes both nonliving factors and other organisms." (Campbell 6) The interaction with the environment is what causes the "identical" twins to develop in such divergent ways when in separate environments. While Gregor Mendel's idea and experiments may seem to indicate that characteristics vary purely based on genes, Mendel was operating in a very controlled environment, where all the pea plants he was using were in the same place. Indeed, according to an article on PNAS, "most monozygotic twin pairs are not identical; several types of phenotypic discordance may be observed, such as differences in susceptibilities to disease and a wide range of anthropomorphic features." (http://www.pnas.org/content/102/30/10604.full?sid=147a4204-ee12-421b-96df-20d7a2c50955). Note that these monozygotic pairs were not reared apart; rather, these are twins raised in the same environment, and differences are observed even there. While the actual genome of the twins is the same, the epigenome is not; there is nothing preventing reared-apart twins from becoming completely different from one another.
Ari Bakke
-Aribakke@gmail.com