PSY 250 Unit 2 (Text Chapter 2) -- Genetics and heritability, prenatal development, birth

Pages 46-53

Genotype refers to the hereditary characteristics inherited from one's ancestors.  Phenotype refers to the outward, observable (behavioral or physiological) expression of the genotype.  Note that phenotypes are almost always (to a greater or lesser extent, see below) strongly influenced by environmental as well as genetic considerations.  In the simplest case, a phenotypic characteristic or trait is controlled by a single pair of genes.  However, most characteristics of interest to psychologists, such as intelligence or temperament, are pleiotropic (determined by multiple gene pairs acting interactively or in concert).

Except for the germ cells (ova and sperm), all normal human cells contain 23 pairs of chromosomes (= 46 chromosomes total), the so-called diploid number.  These somatic cells reproduce exactly through the process of mitosis.  Germ cells, however, are produced through a different process of meiosis, which ensures that the resulting cells have only one chromosome from each pair (the haploid number).  Do you see why this is necessary?  (What would happen if germ cells were produced through the process of mitosis?)

The different gene possibilities that might be found at a given gene location (all of which potentially govern the same trait) are called alleles.  In the simplest situation, the genotype for a given trait consists of two alleles, one from each parent.  (Exceptions are noted below.)

In the simplest, conventional situation, there are two possible alleles, one dominant and the other recessive.  The dominant allele will be expressed phenotypically if even one allele is present;  the recessive allele is expressed phenotypically even if no dominant alleles are present.  For instance, the dominant gene for the ability to curl your tongue might be denoted C (capital letters conventionally indicate dominant alleles), and the recessive gene for the inability to do so might be denoted c.  There are thus three possible genotypes:  CC or homozygous dominant, Cc or heterozygous, and cc or homozygous recessive.  (If the two alleles were equally prevalent in the gene pool, we would expect 75% of humans to be able to curl their tongue.  Do you see why?  Construct a genetic transmission diagram as discussed in lecture.)

In some cases, there are more than two alleles.  Usually in such cases, one allele is recessive, but the other two may be co-dominant (neither is dominant over the other, so that both are phenotypically expressed to some extent when one of each is present).  For instance (simplifying the real case to some extent), eye color is controlled by three possible alleles:  B = brown, G = green, b = blue.  The BG genotype results in the "hazel" eye color.  (What other possible genotypes exist, and what phenotype would be associated with each?  See if you can figure that out.)

Of the 23 chromosome pairs, the first 22 are autosomes (same gene locations are found on the two sister chromosomes of the pair).  The 23rd pair are the sex chromosomes, because they control the sex of the individual.  The two types are called X and Y chromosomes:  XX means a female, XY means a male.  (Do you see why the YY pattern is impossible?)  The X chromosome contains some gene locations that are not duplicated anywhere on the Y chromosome;  thus, some (not all) of the traits governed by gene locations on the sex chromosomes are sex linked (are transmitted differently for male vs. female children, since males and females have different genotypes for these traits).  For these traits, females have two alleles (one on each X chromosome), while males have only one allele (on their X chromosome), with a nonexistent gene location or blank on the Y chromosome.  For instance, pattern baldness is controlled by two alleles, B = bald and b = nonbald.  Possible female genotypes are BB, Bb, bb, but possible male genotypes are B0, b0.  (Do you see why this explains the greater prevalence of pattern baldness in men vs. women?)  A female with the heterogeneous genotype is known as a carrier:  she does not express the recessive trait phenotypically but can pass it on to her offspring.  

Click on this hyperlink to see some practice genetics problems of the type that will be presented on the exam.  

Pages 53-57

Heredity does not rigidly fix behavior or other phenotypic traits, but may establish a range of possible responses or expressions (the range of reaction).  In some cases, the range of reaction is wide (environment accounts for most of the variance in the trait), and we then say that the heritability of the trait is low.  In other cases, the range of reaction is narrow (heredity accounts for most of the variance in the trait), and we then say that the heritability of the trait is high.  The relative role of heredity vs. environment can be quantitatively expressed as a heritability quotient that can range from a low of 0 (heredity plays no discernable role) to a high of 1 (heredity completely determines the trait).  A trait with an extremely narrow range of reaction is sometimes said to be canalized.  Some representative heritability quotients:

• Reasoning or IQ:  about 0.70
• Extraversion:  about 0.36
• Negative Emotionality or Neuroticism:  about 0.33
• Conscientiousness:  about 0.32
• Openness:  about 0.26
• Agreeableness:  about 0.21

Since genotypes themselves can influence the environment in a number of ways, the two kinds of influences can interact and are not wholly separate.  Three ways in which this influence can take place are:

• Passive interaction is when the child's parents have a genetic trait which causes them to act in ways that produce a given environment for the child.  Due to the fact that the child also shares the genetic heritage of the parents, this environment is likely to be a good fit to the child's genetic predispositions as well.
• Evocative interaction is when the child acts in a certain way (or manifests a trait or quality) due to genetic influences, but then the parents, responding to this, alter the child's environment, thus reinforcing the trait. 
• Active interaction is when genotypically driven motivational patterns cause children to directly seek out environments compatible with those predispositions, also known as niche picking.

Research methods used to study heritability include adoption studies and twin studies.  In the former, the trait similarity between a child and her/his adoptive parents (representing environmental influences) in compared to that between the child and her/his biological parents (representing hereditary influences).  To avoid confounds, children adopted at birth are studied, but since adoptions are not made at random, confounds can rarely be eliminated entirely.  In the latter, similarities between identical (monozygotic) twins are compared to those between fraternal (dizygotic) twins.  Hybrid designs can also be used (e.g., studies of identical twins separated at birth).  Potential confounds include niche picking and the fact that two children raised in the same home experience many nonshared environmental factors.

Pages 57-74

A philosophical preamble

Click here for my reflections and speculations on some important issues related to bioethics.  I've separated this material from the main lecture notes, but that doesn't mean it's unimportant.  In fact it's very important for you to think deeply about these issues... the culture you save might be your own.

Stages of prenatal development

Biologically, there are no clear-cut dividing lines or boundary points in the sequence of development that begins at conception and ends some nine months after birth (neurologically speaking, the process of physiological development is only about 50% done at the time a baby is born).  But for reasons of convenience, it's conventional to divide prenatal development into three phases:

• The germinal phase, beginning with conception and ending with implantation (roughly two weeks).  During this phase, the developing organism is conventionally called the zygote (though, of course, people of my philosophical ilk tend to object to the possible subtext of terms like this, which tend subliminally to dehumanize the unborn child;  since these are the traditional terms and since they have some biological utility, we'll use them, but without any specific value implications).
• The embryonic phase, beginning with implantation and ending with the structural differentiation of the major organ systems (roughly three months). 
• The fetal phase, comprising the remaining (roughly) six months.  Viability -- the ability of the infant to survive on its own if born prematurely -- occurs roughly halfway through the fetal stage (of course, recent medical advances continue to push the point of viability well into the second trimester). 

Your textbook does an excellent job of summarizing the biological details of prenatal development -- there's no need for me to paraphrase an entire chapter for you here.  (Read the book, in other words.)  To understand the stem cell controversy, however, it's necessary to understand some basics about cellular differentiation as it takes place during the germinal and embryonic stages.

• Stem cells are those which retain the ability to produce a wide variety of daughter cells, as described below.  Technically, the term for this is cellular potency.
• Within the first several hours after conception, the fertilized egg or zygote divides into two cells, then four, and so on until, by the fourth day, it consists of a solid ball of cells called the morula.  Each cell of the morula is generally (not completely) identical;  very little cellular differentiation has yet occurred, though there is evidence of some minimal differentiation as early as the third cycle of cell division (an eight-celled organism).  When we talk about cellular differentation, of course, we are talking about differences in the cytoplasm of the cell;  all somatic cells of the body, as we learned in Unit 3, are genetically identical, but cells differ from one another in terms of which genetic instructions are, or can be, activated.  Cells at this point are said to be totipotent:  any cell, tissue, or organ system of the body could be derived from them (all parts of the genetic code can be activated within the cytoplasm).
• Over the next 8 to 10 days, the cells continue to divide, folding in on themselves, until they form a hollow ball or blastula.  The blastula has an outer layer of cells called the trophoblast and an inner layer called the embryoblast.  By this point, cellular differentiation has reached the point that only cells of the embryoblast (but not the trophoblast) could form cells of the body proper.  The trophoblast, in contrast, will develop into structures that are not part of the embryo (later, the fetus):  the placenta, the amniotic sac, and the umbilical cord.  Because these structures are necessary to fetal development, no cell of the blastula, if isolated, could be turned into a human being:  the cells are no longer totipotent, but they are pluripotent because any desired cell line (tissue type) of the body could be derived from them.
• Throughout the next two weeks, cells of the embryoblast further differentiate into three distinct germ layers:  the ectoderm, mesoderm, and endoderm.  From each germ layer, distinct groups of tissues and organ systems develop:  the brain and nervous system, skin, hair, nails, and most of the sensory systems from the ectoderm;  the heart, muscles, bones, tendons, and other parts of the circulatory and urogenital systems from the mesoderm;  and the digestive and respiratory systems and related internal organs from the endoderm.  Because each germ layer can only generate a limited range of tissue types, none of the embryonic cells at this stage are pluripotent, but they are multipotent.
• What of cells of the adult human being?  Most of them have lots nearly all of their potency:  they can only duplicate exactly (producing cells of the exact same kind), and some (such as nerve cells) can't even do much of that.  (However, some specialized adult cells retain a significant degree of multipotency if not pluripotency, and these are the adult stem cells.  The jury is still out regarding the question of whether adult stem cells retain enough potency to make the ethically debatable practice of harvesting embryonic stem cells in any way necessary.  Naturally, given my values as described above, I'm personally opposed to any form of stem cell research that involves the destruction of human embryos.  Again, your agreement or disagreement is your own affair -- but do understand the biology, and the nature of the value controversy, involved.)

Teratogenic agents and effects

A teratogen is an agent (e.g., ingested substance, disease organism, or other environmental influence) that can negatively impact prenatal development -- causing birth defects or other behavioral or physiological abnormality -- by means of its ability to cross the placental barrier that separates the maternal and fetal bloodstreams.  See the text for an extremely important list of potential teratogens and their likely effects.  It's disturbing to note that the average American woman consumes about 10 different drugs (prescription and non-prescription) over the course of pregnancy;  over 80% of pregnant women consume at least some alcohol, and over 30% smoke, despite clear evidence that both alcohol and nicotine have clear teratogenic effects.

The most dangerous time within prenatal development for serious, structural teratogenic effects is, of course, the embryonic phase (most of the first trimester), since this is the time that the major organ systems of the body are being differentiated.  However, during the fetal phase, more subtle, functional teratogenic effects (such as various forms of minimal brain dysfunction or organically based personality disturbances) can occur, as well as some structural effects.

Because maternal stress produces hormonal imbalances and most hormones can cross the placental barrier, stress can be thought of as an indirect teratogen.  Hyperactivity, irritability, and a range of other behavioral problems after birth have all been correlated with maternal stress during pregnancy.

Gender development and components of gender

We tend to think of gender as a clear-cut, either/or human attribute, but in fact, there are at least five different aspects to or components of gender.

• As discussed in Unit 3, chromosomal gender is determined at conception.  The XX pattern means a chromosomal female, the XY pattern a chromosomal male.
• By 3-4 months, the presence or absence of a Y chromosome alters the hormonal environment of the developing organism.  Under normal circumstances, the Y chromosome stimulates the production of the male hormone androgen, which "pushes" embryonic and fetal development along a male pathway.  In the absence of androgen, the "default" (female developmental pathway) is taken.  Since the reproductive system and external genitalia are formed at this time, anatomical gender is established at this time.
• The last trimester (7-9 months) is marked by intensive brain development, and the hormonal environment shapes the formation of brain pathways having to do with emotion, motivation, and cognition along either male or female lines, again depending on the presence or absence of androgen.  This is biocortical gender.
• How parents and others label, treat, and react to the child after birth is its social gender.
• How the child eventually comes to think of her/himself as being either male or female is its cognitive gender (gender identity).

Normally, all these gender variables or gender markers point in the same direction (all male, or all female).  But hormonal abnormalities (due to various kinds of maternal or fetal medical conditions) can cause things to go awry:  a child, for instance, who looks male (anatomical gender) but feels himself to be a female in terms of self-concept and gender identity (stemming, probably, from biocortical gender).  This may help explain transsexualism (not the same as sexual preference, by the way, which is only loosely correlated with any of the above).

Pages 75-80

For untold centuries prior to 1800, birth was a social event held in the home.  Female birth helpers (friends and family, as well as semiprofessional midwives) attended the birth, and men were generally banned from playing any substantial role. 

In the early 1800's, with the growth of the (still all-male) medical profession, as medicine became increasingly scientific and less purely traditional in character, what amounted to a turf battle between female midwives and male doctors began to emerge.  It began to become fashionable among upper-class women to be attended by a physician, and over the rest of the century this trend worked its way down the socioeconomic ladder in the well-known phenomenon of social drift (as people copied the behavior of those one rung higher on the status ladder).  Thus, a class distinction emerged, with those who could afford to do so seeking the services of a male doctor, and those who could not continuing to utilize a female midwife.  The physicians' argument (or marketing stance) was that birth is a medical matter;  doctors are medically trained;  therefore, women need doctors at birth.  This was the beginning of a shift away from a wholistic perspective of birth (birth is part of the normal process of life) toward a mechanistic one (birth is a medical matter, implicitly analogous in many respects to a form of illness, even if a desired one).  The mechanization of birth increased with the emergence of obstetrics as a medical specialization toward the end of the nineteenth century.

By 1900, emerging government regulation and licensing of trades and professions dealt a final blow to midwifery, and for six decades or so midwives were essentially driven underground.  By 1920, the next step in the cultural change surrounding birth came with a move from home based births to hospital births.  In 1900, only 5% of babies were hospital born;  by 1935, the figure had risen to 75%, and by 1960 to nearly 99%.  Whether this really reduced infant mortality as claimed by the medical profession can be disputed;  deaths due to birth complications plunged appreciably, but increases in iatrogenic (hospital-induced) bacterial infections exploded.  However, giving birth in the hospital came to be seen as fashionable and upscale, again with the upper classes leading the way.

While the rise of anesthetics led to the promise that childbirth could become a totally painless process, in reality the only real effect during the first half of the century was to reduce women to a purely passive role;  as late as 1950, many hospital births involved the use of "twilight sleep", induced by a mixture of morphine, chloroform, and scopolamine (an amnesiac).  If this sounds barbaric, it was (but due to the amnesiac effects of the mixture, participants could not disconfirm the claims of "painless birth"). 

The tide away from wholly mechanistic birth practices began to turn in the 1960's with the "birth reform" movement.  A new emphasis on pain management through nonmedical techniques emerged, partly due to the mistrust of the scientific establishment that was part of the countercultural ethos of the day.  Motivated in part by marketing pressures, hospitals began to develop "alternative birthing centers";  midwifery again become fashionable and, slowly, legal;  two distinct alternatives, one high-tech and the other high-touch, emerged, but with little integration.  (See John Naisbitt, Megatrends, for a more general discussion of the high-tech/high-touch polarity in society.  Twenty years after its publication, the book remains astonishingly current and cogent.)  Hospital births became more high-tech, with impressive electronic fetal monitors, epidural anesthetics, and a sixfold (!) increase in cesarean births between 1970 and 1990.

Since 1990, legal and economic forces have collided to produce the managed care revolution, which has obviously added an additional layer of bureaucratic complexity to the birth process.  A continuum of birth alternatives exists today, from the very mechanistic to the very wholistic.

Study Guide

1.  How do genotype and phenotype differ?  What, other than genotype, has an influence on phenotype?  What are pleiotropic characteristics?

2.  How do the diploid and the haploid number differ?  What kinds of cells have each, respectively?  How do mitosis and meiosis differ?

3.  How do gene locations and alleles differ?  How are alleles related to genotype?

4.  How do dominant, co-dominant, and recessive alleles differ?  How do homozygous and heretozygous genotypes differ?  Give examples, and be able to work simple genetics problems such as those provided in the hyperlinked material.

5.  How do autosomes and sex chromosomes differ?  What are two types of sex chromosomes, and how do they differ?  How does this fact determine the transmission of sex-linked traits?  What is a carrier?

6.  What is the relationship between heritability, range of reaction, and canalization?  How is heritability quantified by means of the heritability quotient?

7.  Describe there different forms of heredity-environment interaction.  Give (or be able to recognize) examples of each.  What is niche picking? 

8.  Describe how adoption studies and twin studies are used to estimate heritability.  What are some possible confounds in such studies?  What is meant by the notion of nonshared environment? 

9.  Explain what is meant by the fact-value distinction and its role in science.

10.  What are three stages of prenatal development, and how do they differ?

11.  Explain what is meant by cellular differentiation and the distinction between totipotent, pluripotent, and multipotent cells.  How does this relate to the stem cell controversy?

12.  What are three germ cell layers?  Which organ systems of the body develops from each?

13.  What is a teratogen?  What are two different kinds of teratogenic effects, and when is each most likely?  Why?

14.  What are five gender variables or gender markers?  Use these to explain the nature of gender differentiation and how it might go wrong.

15.  Summarize the social history of birth in terms of (a) the wholistic-mechanistic distinction, (b) the phenomenon of social drift.

Back to Unit 1

On to Unit 3

Back to Main Page