Chapter 2- Genetics and Experience/Behaviour
Mendel studied the inheritance in pea plants. He decided to study the dichotomous traits (starting with
crossing the true-bred offspring lines)
Dichotomous traits- traits that occur in one form or the other, but never in combination
Ie. Seed colour (every pea plant has either brown or white seeds)
True Breeding Lines- breeding lines in which interbred members always produce offspring of the same
trait generation after generation
In his early experiment, he studied the inheritance of seed colour, brown or white. Began by cross
breeding the offspring of a line of pea plants that were true-bred white. The offspring of this all had
brown seeds. Then, he bred the first generation offspring with one another and found that ¾ of the
second generation had brown seeds and ¼ had white.
Dominant trait- appeared in all of the first generation offspring
Recessive trait- appeared in about ¼ of the second generation offspring
In humans, brown eyes are dominant and blue eyes are recessive
Somehow, in his experiment, the recessive trait (white seeds) was passed on to ¼ of the second
generation pea plants by first generation pea plants that did not have those traits
Phenotype- physical, observable traits
Genotype- traits that it can pass on to its offspring through genetic material
His theory then consisted of four points.
1) 2 kinds of inherited factors for each dichotomous trait (called genes)
2) Each organism possesses two genes for each of its dichotomous traits (each pea plant possesses
either 2 brown genes, 2 white, or one of each) (the two genes that control the same trait are
called alleles) (organisms that posses two identical genes for a trait are homozygous for that
trait, those that possess two different are heterozygous).
3) One of the two kinds of genes for each dichotomous trait dominates the other in heterozygous
4) For each dichotomous trait, each organism randomly inherits one of its “father’s” two factors
and one of its “mother’s” two factors Chromosomes: Reproduction and Recombination
Genes are located on chromosomes- threadlike structures in the nucleus of each cell
Chromosomes occur in matched pairs (humans have 23). The two pairs of genes (alleles) that control
each trait are situated at the same location, one on each chromosome of a particular pair.
The process of cell division that produces gametes- egg and sperm cells, is called meiosis- the
chromosomes divide and one chromosome of each pair goes to each of the two gametes that results
from the cell division. As a result, each gamete has only half the usual number of chromosomes. When a
sperm and egg cell combine during fertilization a zygote- fertilized egg cell, with the full complement of
chromosomes is produced.
During the first stage of meiosis, the chromosomes line up in their pairs. Then, the members of each pair
break apart at the points of contact and exchange sections of themselves. As a result, each of the
gametes that formed the zygote developed into you contained chromosomes that were unique, spliced
together a recombination of chromosomes from your mother and father.
All other cell division occurs by mitosis. Prior to mitotic division, the number of chromosomes doubles
so that when the cells divide, both daughter cells end up with the full complement of chromosomes.
Chromosomes: Structure and Replication Each chromosome is a double stranded molecule of deoxyribonucleic acid (DNA). Each strand of DNA is
a sequence of nucleotide bases attached to a chain of phosphate and deoxyribose.
Four nucleotide bases:
These make up the genetic code. The two strands that compose each chromosome are coiled around
eachother and bonded T-A and G-C.
Replication is a critical process of the DNA molecule. The two strands of DNA start to unwind. Then the
exposed nucleotide bases on each of the two strands attract their complimentary bases (floating in the
nucleus fluid). Therefore, when unwinding is complete, the two double stranded DNA molecules (both
identical to the original) have been created.
In chromosome replication, errors and mutations can occur.
Mutations- accidental alterations in individual genes.
In most cases, mutations disappear from the gene pool within a few generations because the organisms
that inherit them are less fit. In rare occasions, mutations can increase fitness (rapid evolution)
Sex Chromosomes and Sex Linked Traits
The typical chromosomes, which come in matched pairs, are called autosomal chromosomes. One
exception is sex chromosomes- the pair that determines and individual’s sex. Two types of sex
chromosomes, X and Y, the two look different and have different genes. Females have two X
chromosomes and males have one X and one Y.
Sex-linked traits- traits influenced by genes on sex chromosomes.
All sex linked traits are controlled by genes on X chromosomes because the Y chromosome is too small
and carries few genes. Traits controlled by genes on the X chromosome occur more frequently in females. If the trait is
dominant, it occurs more frequently in females, they have twice the chance of inheriting because they
have twice the number of X chromosomes. Recessive sex linked traits occur more frequently in males (ie
Genetic Code and Gene Expression
Structural genes are genes that contain the info necessary for protein synthesis.
Proteins- long chains of amino acids, they control the physiological activities of cells and are important
components of cellular structure.
All the cells in the body contain exactly the same genes.
The stretches of DNA that lack structural genes are known to have portions called enhancers.
Enhancers- stretches of DNA whose function is to determine whether particular structural genes initiate
the synthesis of proteins and at what rate.
The control of gene expression by enhancers is an important process, because it determines how a cell
will develop and how it will function at maturity.
Transcription factors- proteins that bind to DNA and influence the extent to which genes are expressed
In gene expression, first, the small section of chromosome that contains the gene unravels and the
unraveled section of one of the DNA strands serves as a template for the transcription of a short strand
of ribonucleic acid (RNA). Remember RNA has uracil instead of thymine and has a phosphate and ribose
backbone instead of a deoxyribose. This strand of transcribed RNA is called messenger RNA because it
carries the genetic code out of the nucleus. Once it has left the nucleus, the messenger RNA attaches
itself to one of the many ribosomes in the cytoplasm. The ribosome then moves along the strand of
messenger RNA, translating the genetic code as it proceeds.
Each group of three consecutive nucleotide bases along the messenger RNA strand is a codon. Each
codon instructs the ribosome to add one amino acid to the protein that it is constructing. Each kind of
amino acid is carried to the ribosome by transfer RNA- as the ribosome reads a codon, it attracts a
transfer RNA molecule that is attached to the appropriate amino acid. This occurs until the ribosome
reaches a stop codon, then the protein is released into the cytoplasm.
Gene expression occurs in two steps:
1) transcription of base sequence code to an RNA base sequence code
2) translation of the RNA base sequence code into a sequence of amino acids 1) The DNA molecule partially unravels, exposing the structural gene that is to be transcribed
2) A strand of messenger RNA is transcribed from one of the exposed DNA strands and carries the
genetic code from the nucleus into the cytoplasm of the cell
3) In the cytoplasm, the strand of messenger RNA attaches itself to a ribosome. The ribosome
moves along the strand, translating each successive codon into the appropriate amino acid,
which is added to the lengthening protein by a molecule of transfer RNA.
4) When the ribosome reaches the end of the messenger RNA strand, a codon instructs it to release
the completed protein
Not all DNA is in the nucleus, also in mitochondria.
Human genome project- purpose to compile a map of the sequence of all 3 billion bases that compose
human chromosomes. Completed in 2001.
We humans have a relatively small number of genes (20,000). The discovery that genes compose only
2% of human DNA led to the rapid growth of field of research, epigenetics- focuses on mechanisms that
influence the expression of genes without changing the genes themselves.
Four influential lines
1) Active Nongene DNA- Those portions of DNA that did not directly participate in the synthesis of
proteins were though to be nonfunctioning evolutionary remnants (pseudogenes or junk DNA)
2) MicroRNAs- short single strands of RNA that have a major effect on gene expression through
their actions on enhancers and mRNA
3) Monoallelic Expression- one of the two alleles is inactivated and the other is expressed
4) Alternative splicing- occurs when some strands of mRNA are broken apart and the pieces are
spliced to new segments. This allows a single gene to encode more than one protein
Behavioral Development: Interaction of Genetic Factors and Experience
Ontogeny- the development of individuals over their life span
Phylogeny-evolutionary development of a species through ages
Tryon did a study (cross fostering control procedure) with maze-bright and maze-dull rats, selectively
breeding them to assess how well they would do in the mazes if they had maze-bright parents or maze-
dull parents. By the eighth generation, the maze-bright offspring made few errors (even if they were
reared by maze-dull rates) and maze-dull offspring made many errors (even if they were reared by
maze-bright rats) (reared meaning cared for).
Selective breeding studies have shown that genes influence the development of behavior, but this
doesn’t conclude that experience does not.
Phenylketonuria (PKU)- single gene metabolic disorder causing distinct odor in urine of mentally
retarded children, also showing symptoms of vomiting, seizures, hyperactivity. This gene is recessive and
only develops in homozygous individuals (develop from both their mother and father)
Development of Bird Song
The first stage of this behavior is sensory phase- begins several days after hatching. The young birds
form memories of the adult songs they hear, usually sung by their own male relatives, that later guide
the development of their own singing. They cannot acquire songs of other species or songs that they did
not hear in the sensory phase. Second phase is sensorimotor phase- begins when the juvenile males begin to twitter subsongs (several
months old). The rambling vocalizations of subsongs are gradually refined until they resemble the songs
of the adults. Auditory feedback is necessary.
Most songbirds are age limited learners- adult songs, once perfected, remain unchanged for the rest of
the bird’s life.
Others are open ended learners- they are able to add new songs to their repertoire throughout their
The descending motor pathway
descends from the high vocal center on each side of the brain to the syrinx (voice box) on the same side
and mediates song production.
The left descending motor pathway plays a more important role in singing than the right motor pathway
(which duplicates the left hemisphere dominance for language in humans). The high vocal center is four
times larger in male canaries than females. Each spring, as the male canary prepares its new repertoire
of songs for the summer seduction, the song control structures of the brain double in size and shrink
back in the fall (triggered by elevated levels of the hormone testosterone that results from increasing
daylight). The seasonal increase in size of the song control brain structures results from the growth of
new neurons (not increase in size of existing ones).
Genetics of Human Psychological Differences
To assess relative contributions of genes and experience to the development of differences in
psychological attributes, often compare monozygotic twins (identical) to dizygotic twins (fraternal) who
have been separated at infancy by adoption. MINNESOTA STUDY OF TWINS REARED APART.
In general, adult identical twins were substantially more similar to one another on all psychological
dimensions than fraternal, whether or not they were raised in similar environments.
Heritability estimates- not about individual development, but it is a numerical estimate of the
proportion of variability that occurred in a particular trait in a particular study as a result of the genetic
variation of that study. Tells us about the contribution of genetic differences to phenotypic differences
and nothing to say about the relative contributions of genes and experience to the development of
individuals. Multiplier effect- when a particular gene encourages a developing individual to select experiences that
increase the behavioural effects of the gene
-all human behavioral traits are highly heritable
-being raised in different family environments contributes little to the diversity of behavioral traits
-experiences other than the family environment contribute significantly to behavioral diversity
Neural Conduction and Synaptic Transmission
Resting Membrane Potential
Membrane potential- is the difference in electrical charge between the inside and outside of a cell
To record this, you need to have the tip of one electrode in the neuron and the other tip outside in the
extracellular fluid (microelectrode)
When both electrode tips are in the extracellular fluid, the voltage difference between them is zero. A
steady potential when inside the neuron and in the extracellular fluid is -70mVolts. This indicates that
the potential inside the resting neuron is 70mV less than the outside of the neuron.
Resting potential- -70mV (said to be polarized at this point)
The salts in neural tissue separate into positive and negative charged ions. The resting potential results
from the ratio of negative to positive charges is greater inside the neuron than outside.
The first two homogenizing factors is random motion. The ions in neural tissue are in a constant random
motion and the particles in random motion tend to become evenly distributed because they are more
likely to move down their concentration gradients than up them: this is because they are more likely to
move from areas of high concentration to low concentration.
The second factor that promotes even distribution of ions if electrostatic pressure. Any acculumation of
charges in one area tends to be dispersed by the repulsion among the like charges in the vicinity and the
attraction of opposite charges concentrated elsewhere.
No single class of ions is distributed equally on the two sides of the neural membrane. Four kinds of ions
contribute significantly to the resting potential: Na+, K+, Cl- and various negative ions. The
concentrations of Na+ and Cl- ions are greater on the outside of a resting neuron where K+ is greater on
the inside. The negatively charged protein ions are synthesized inside the neuron and stay there.
Two properties of neural membrane are responsible for unequal distribution of ions. One is passive
(doesn’t involve the consumption of energy). The other is active (involves energy consumption). The
passive property of neural membrane that contributes to unequal disposition is its differential
permeability to those ions. In resting neurons, K+ and Cl- ions pass readily through the neural
membrane. Na+ passes through with difficulty and the negative protein ions do not pass through at all.
Ion channels- specialized pore where ions pass through the neural membrane which are each
specialized for the passage of particular ions.
There are specialized mechanisms in the cell membrane to counteract the influx (input) of sodium ions
by pumping out sodium ions rapidly as they pass in and counteract efflux (outflow) of potassium ions by
pumping them in. This input and output of ions is not an independent process. Such ion transporter is performed by
energy consuming mechanisms in the cell membrane that continually exchange three sodium ions inside
the neuron for two potassium ions outside. These transporters are commonly referred to as sodium-
Generation and Conduction of Postsynaptic Potentials
When neurons fire they release from their terminal buttons chemicals called neurotransmitters which
diffuse across the synaptic clefts and interact with specialized receptor molecules on the receptive
membranes of the next neurons in the circuit. When neurotransmitter molecules bind to postsynaptic
receptors, they typically either depolarize (decrease the resting membrane potential from -70mV to -67
mV) the receptive membrane or hyperpolarize it (increasing the resting membrane potential from -70
Postsynaptic depolarizations are called excitatory postsynaptic potentials (EPSPs) because they
increase the likelihood that the neuron will fire. Postsynaptic hyperpolarizations are called inhibitory
postsynaptic potentials (IPSPs) because they decrease the likelihood. These are both graded responses
(the amplitudes are proportional to the intensity of the signals that elicit them: Weak signals elicit small
postsynaptic potentials and vice versa)
EPSPs and IPSPs travel passively from their sites of generation at synapses, usually on the dendrites or
cell body, in much the same way that the electrical signals travel through a cable.
First, the transmission of postsynaptic potentials is very rapid (rates of EPSPs and IPSPs are the same,
duration differs). Second, the transmission of these is decremental: EPSPs and IPSPs decrease in
amplitude as they travel through the neuron. They don’t travel very far along the axon (couple mm).
Integration of Postsynaptic Potentials and Generation of Action Potentials
The post synaptic potentials created at a single synapse typically have little effect on the firing of the
postsynaptic neuron. Whether or not a neuron fires depends on the balance between the excitatory and
inhibitory signals reaching its axon. Action potentials are generated at the location adjacent to the axon
hillock. The EPSPs and IPSPs created by the action of neurotransmitters at particular receptive sites on a
neuron’s membrane are conducted instantly and detrimentally to the axon hillock. If the sum of the
depolarization and hyperpolarization at the axon adjacent to the axon hillock at any time is sufficient to
depolarize the membrane to a level reffered to as threshold of excitation (about -65mV ) an action
potential is generated near the hillock. The action potential is a massive, momentary reversal of the
membrane potential from about -70 to +50mV. Action potentials are not graded responses, their
magnitude is not related to the intensity of the stimuli that elicit them. They are all or none responses.
In effect, all t