I. Terms To Know
   Before we begin to talk about the role of genetics and inheritance in psychology it's important that we get a clear understanding of some of the terms which we will be using.  There are some common misconceptions which should be avoided and we can avoid some of them by knowing the correct meanings of definitions of words that are often tossed around without a full understanding of what they mean..

 

A. Gene
   The definition of gene is one source of confusion because there are two acceptable definitions based on the context being discussed.  While they are similar in meaning they are not exactly the same.  A gene can be an observable inherited trait. Anything from how curly someone's hair is to their blood type or whether or not theyÕre double jointed.  A gene can also mean a specific deoxyribonucleic acid (DNA) sequence which codes for a specific polypeptide or protein.  Sometimes the things we see as a single observable trait may be coded for by more than one DNA sequence.  Some DNA sequences may not result in an easily observable trait.

 

B. Genotype
   The genotype is the DNA content of a cell's nucleus, whether a trait is externally observable or not.

 

C. Phenotype
   The phenotype is the pattern of expression of the genotype or the magnitude or extent to which it is observably expressed.

 

D. Monozygotic
   The term monozygotic means "derived from one fertilized egg."  Identical twins are, therefore, monozygotic twins that share the exact same DNA just as they shared, essentially, the same uterine environment.

 

E. Dizygotic
   The term dizygotic means "derived from two fertilized eggs." Fraternal twins are dizygotic twins and only share half the same DNA but , essentially, the same uterine environment.

 

F. Clone
   Technically speaking, a clone is any organism that shares the same genotype with another.  Monozygotic twins are clones.  So are two garden plants taken from the same cutting.  And so are two bacteria that have just resulted from the division of one bacterium. 

 

II. Common Misconceptions
   There are still more misconceptions that should be addressed so that we can understand the role of genetics on behavior.

 

A. One Gene means ...
   One common misconception arises from an old outdated simplification used in many biology classes: "One Gene = One Protein."  Actually, some genes (DNA sequences) can yield more than one protein.  For instance, one gene found in the brain for, proopiomelanocortin (POMC), yields 5 proteins once it has been fully processed.  Additionally, most observable traits are polygenic (influenced by more than one gene) rather than controlled by a single gene.

 

B. Environment means ...
   Environmental roles can mean more than just those of the immediate surroundings.  It includes a multitude of non-genetic factors such as nutrition, experience and activity, education, as well as culture and society and even exposure to drugs or toxic pollutants.  For example, some early sensory experiences during infancy and childhood are required to properly wire the sensory systems of the brain.  If those experiences aren't encountered in time the resulting deficits are permanent.  Later sensory experiences are not able to undo the damage.

 

C. Clone means ...
   As we have pointed out a clone has the same "genetic fingerprint" as another organism.  But clones are not necessarily completely identical in every way. Identical twins for instance will have different fingerprints.  As we mentioned in chapter one if one identical twin has schizophrenia the other identical twin has a 50%, but not a 100% chance, of also having schizophrenia.  Some influence of environment can change the expression of the clone's genotype.

 

III. Heritability
A. Variance due to genes vs. Total Variance
   Heritability (h2) refers to the observed variation in traits (phenotypic, of course) that can be accounted for by genetics in a population, not in individuals.  To calculate this number an estimate must be made of the total variation seen for a trait in a population (for instance, hair color in a random group of people) which is the variation cause by both genetics and environment influences.  Then an estimate is typically made by looking at related individuals to estimate the variability due to genetics.  The genetic variance is divided by the total variance and the resulting number is heritability, h2.  If we see h2 = 0.8 then that means that 80% of the variability of that trait is due to genetics and 20% is due to environmental factors.

 

B. Caveats
   There are several cautionary things to keep mind, both from a purely biological point of view but also from a psychological point of view.  First, the estimate of variation due to genetics is a difficult one to make and may be wrong.  Second, heritability is a measure of how much a trait may be influenced by genetics for a group, but is meaningless to tell us anything of use about traits within a single unique individual.  Third, because one needs diversity in the genes of a population to be able to be able to detect variance in the first place, a study of clones would actually yield no heritability estimates.  Since the variation due genetics would be zero in a group clones, h2 would be zero since zero divided by any number would be zero.  On the other hand, a diverse environment producing a great deal of variability among a group of mostly unrelated individuals would yield the largest and best estimates of heritablitiy.

 

IV. Inheritance of Psychological Traits
A. Relatedness
   Relatedness refers to how many genes are shared between individuals. For example identical twins share the exact same DNA so they have a relatedness of 1.0 or 100%.  They share half the genes of each parent so they each have a relatedness of 0.5 or 50% with each parent.  Fraternal twins and siblings born at different times also have a relatedness of 50% with each other as well as with their parents.  All children, twins or either typeor not, will have a relatedness of 25% with their grandparents as well as their aunts and uncles.

 

B. Relatedness & Concordance of Schizophrenia
     As we have previously stated, identical twins have a relatedness of 100% but a concordance rate of 50% (if one individual has a trait or condition, the chance of another individual having the same trait or condition is the concordance).  There does seem to be some prenatal environmental influence on schizophrenia as well.  For instance, fraternal twins and regular siblings have a relatedness of 50%.  But fraternal twins, who shared the same uterine environment have roughly twice the concordance rate for schizophrenia as siblings (17% vs. 9%).

 

C. Relatedness & Correlation of I.Q.
   We can also see a strong but not complete role for genetics in intelligence quotient (I.Q.) scores.  Identical twins raised together show I.Q. scores with a correlation coefficient of 0.86 which is quite high but not the perfect correlation of 1.0.  Identical twins raised apart have similar but slightly lower correlation coefficients of 0.72, suggesting some role for environment. This is supported by the observation that biologically unrelated adopted siblings raised together share a small but significant correlation coefficient in I.Q. scores of 0.30.

 

D. Cultural & Racial Studies of I.Q.
   There has historically been a great deal of controversy regarding the intelligence scores of different cultural and racial groups.  However, such of the social and political controversy seemingly ignores what has been long known about the role of environment and education in intelligence testing.  Some groups maintain that since racial and ethnic minorities as a whole score lower on I.Q. tests that they are genetically inferior.  First, regardless of racial makeup, lower socio-economic class children adopted at an early age by middle or upper class families score an average of 14 points higher on intelligence tests than the peers they left behind.  When black children adopted at similarly early ages are adopted by white middle or upper class families, at adolescence their I.Q. scores are higher than the black population overall and not different from the white population overall.  Taken together, these findings indicate that environmental and social factors, such as deprived backgrounds and opportunities, are responsible for poor performances on I.Q. tests by any specific racial or cultural segments in our society. 

 

V. Failures of Genetic Determinism

   Regarding the inheritance of psychological traits, it seems that it is a reasonable view.  Dog lovers can testify that breeds of dogs have been bred for both behavioral and personality traits as well as for physical traits.  Even those of us unfamiliar with the wide variety of dog breed would be able notice differences in personality and intelligence between say a Border Collie and a Basset Hound, between a Great Dane and a Chihuahua, between a Rottweiler and an Irish Setter.  However, when the genetic basis for more complex traits and behaviors are searched for in humans, the record is quite spotty.  Even for such traits as schizophrenia, which we know are inherited, no specific gene or genes have been isolated.  It is important to remember that most traits polygenically controlled so analyzing linkages may be very difficult.  Also unknown environmental factors may influence the degree to which genes are expressed and may prevent their expression altogether.  Further, there is no single environment that is optimal for all genetic combinations and the links between some genes and phenotypic traits are correlational. With those lessons in mind what follows are some notable stories about genetics and behavior. 

 

A. Criminality
   In the 1960's a supposed link was found between a chromosomal aberration called XYY and violent criminal behavior.  Because the aberrations involved an extra Y chromosome and the Y chromosome is the chromosome that determines a fetus to become male which led these individuals to be dubbed XYY supermales.  It was hypothesized that this resulted in exaggerated qualities of aggressiveness and genetically predestined these individuals to a life of crime. However, further studies showed that 96% of men with the XYY condition led relatively normal lives.  Of those that do have problems, most are mildly mentally retarded and may have some irregular facial abnormalities.

 

B. Alcoholism
   In 1990 researchers at the UTSA Health Science Center reported a link between a dopamine D2 receptor variant and alcoholism.  However, no other laboratory in the world has been able to reproduce their findings.

 

C. Manic-Depression
   Two 1987 studies examining relatively genetically limited populations, Israeli and Amish subjects, each reported finding a different gene as linked to bipolar disorder (manic depression). Both studies were later retracted.

 

D. Alzheimer's Disease
   Genetics plays a role in Alzheimer's.  It runs in families and one form of early onset Alzheimer's has been linked to a specific gene called Apo E. In total 4 different genes have been associated with early onset Alzheimer's but those account for less than 2% of cases.  Furthermore, 75% of AlzheimerÕs cases occur in individuals with no family history or known genetic abnormality.