Psychobiology
Chapter 2: Genetics and Evolution
Introduction
The branch of genetics is very important to know more about humans, illnesses and
disorders. We know a lot, but there is more that we don’t know.
There are many new developments in genetics, like the Human Genome Project which
focuses on the molecular structure of genes, but there is a long history in this branch due to
which we now have a view of it.
There is also a new branch for this in philosophy: bioethics.
In the Beginning – Evolution
To answer “the big questions” about the world and life, scientists face many problems, just
like Charles Darwin did when writing about evolution and the origins of species against the
views of pro-creationism.
Now, this is a fact in science, a field that hasn’t many facts, more theories. This was back
then also just a theory, but biologists were convinced by it, due to his evidence-based
approach. Evidence is crucial for theories.
Darwin’s Revolutionary Theory – The Theory of Evolution
His theory was that all organisms have a common ancestor and we’re all adaptations. This
happened through natural selection, in which those with the best characteristics for an
environment survive and reproduce.
Genetics are passed on via reproduction and who survives and reproduce depends on the
“survival of the fittest”.
Nowadays, we have 2 processes that look like natural selection:
- Selective breeding, which already goes a lot faster than natural selection and it’s an
intentional plan to pass some characteristics on.
- Genetic modification, which is even quicker and in which genes are added or deleted.
Evidence: The Case In Support of the Theory of Evolution
One piece of evidence came from biogeography. This handled the question whether some
organisms could live in a specific environment, and others couldn’t. This is because each
organism has specific characteristics for its own environment. Hard to study, usually it takes
a long time, but luckily there are also fast changes in some environments (plants) and they
are noted.
The second piece of evidence was about fossil records. This lets us make a time course. We
find fossils and different geological layers.
Another piece of evidence is the structural similarity between species. This has to mean they
have a common ancestor.
Also, the development of embryos were compared in embryology. Apparently, there are
similar stages. The similarities disappear soon though.
And last, molecular biology, which looks at the actual elements of genes. Species share
common strands of genetic material. This is new information, but it supports Darwin’s fossil
evidence.
The Organism and the Environment
Genetics are the basis of the old nature-nurture debate. However, the Darwinian account
shows evidence for the nurture side; that environment changes biology and behavior.
According to Johnston and Edwards, behavior is determined by genetics, experiences and
perceptions of the environment. The latter is not absolute; it depends on how we see it.
,The Evolution of Humans
What are the steps of evolution, that led to humans?
First, there was the earth.
Second, there were water-dwelling organisms.
Third, there were organisms with nerves, surrounded by bones (with vertebrae, spinal cord).
Fourth, some came out of the water, onto the land. This started the development of
amphibians. They got access to better water and good food, due to which they good survive
and reproduce. They evolved and got legs and lungs.
Fifth, reptiles are a branch of these amphibians. Because of their dry skin, they could hold
water and live further away from the water.
Sixth, from these reptiles, mammals evolved. They could breastfeed their kids. They
developed further and this lead to them being able to be pregnant: growing the kid inside
itself. Humans usually only have 1 or 2 kids. This is because pregnancy is more safe than
growing inside an egg. They are more in danger, so usually there were more kids there.
However, a foetus in the uterus also has some dangers, but different.
Humans are part of the primates family Hominines, from the genera Homo (the other is
Australopithecus). We are Homo Sapiens. It is believed that the first Homo Sapiens are from
Africa, and after many years, they migrated out of it (‘Out of Africa’ hypothesis). Where
they came, determined how they ‘looked’. Depended on the environment.
Brain volume has changed, increased, over the years. Possibly due to the use of tools.
We are closely related to chimpanzees, but the reason we are very different is probably
because of changes in transcription and translation of the genetic codes.
There are important changes in the protein sequence in the human genome, together with
duplication and deletion of genes. Also, we depend less on smell and more on vision.
Another difference is our brain: much bigger. However, within species that doesn’t matter.
Differences between regions are more important.
This organ is an evolutionary development of the vertebrate’s spine. Some structures in the
brain are the same between species, but especially new parts, like the cerebrum, are more
developed. It is also bigger and has more convolutions (sulci).
Evolutionary Psychology
Evolutionary psychology is very theoretical and it tries to answer the question why some
behaviors change, why some behaviors are maladaptive, and why they used to be there.
From Evolution To Genetics
When Darwin came with his theory, he didn’t have all the info we do know about the
mechanics of evolutions. Our modern information started with Mendel.
Types of heritable characteristics:
- Qualitative traits: influenced by a single gene, following a simple pattern, like
Mendelian inheritance. You either have it, or you don’t.
Unaffected by environment, usually. Ex.: Huntington’s disease.
Seems pretty straightforward, but doesn’t have to be. The pattern might change, due
to mutations, or there is low penetrance when an allele doesn’t lead to the
disorder. There might be a case of Mosaicism, like with Turner’s syndrome.
However, diseases can’t only occur due to genetics. The environment, especially in
the uterus, can also have an impact. Sometimes, even the same impact as some
genes have (phenocopies).
- Quantitative trait: influenced by multiple genes, thus it can also lead to polygenetic
disorders. Don’t follow a simple pattern less predictable. The phenotypic
expression doesn’t only depend on genes, like for the other traits, but also on the
environment. Quantitative trait loci (QTL) describes how multiple genes at different
locations on the chromosome contribute to this trait.
Also, using linkage analysis, QTLs can be investigated to look for common
occurrence among individuals.
,Mendelian Genetics
Mendel found out about the differences between conspecifics and the inheritance of
behavioral, physiological and anatomical characteristics.
For his experiments, he used pea plants. For one, he examined dichotomous traits using
the trait for color of its seeds:
- When mixing 2 plants with the same color, the offspring got that color.
- When mixing 2 plants with different colors, only 1 trait/color appeared in the 1st
generation cross dominant trait.
The recessive trait wasn’t expressed, because it was overridden.
- In the second-generation, though, some plants did express the recessive trait, even
though it wasn’t in the first generation. This means that the 1st generation plants were
carriers of the recessive trait, but it wasn’t noticeable because of the dominant trait.
So, there is a difference between observed traits (phenotype) and genetic traits
(genotype). This info can be used for eye-colors. 2 genes for 1 trait alleles.
Mendelian Genetics and Eye Color
Sperm
Brown blue
Eggs Brown (d) BB Bb
blue (r) Bb bb
Brown – a lot of melanin; blue – lack of melanin (pigment)
Genotype Phenotype
BB (homozygous) Brown eyes
Bb (heterozygous) Brown eyes
bb (homozygous) Blue eyes
The gametes (ovum and sperm) can only carry one factor: it either has brown, or blue eyes,
depending on the combination above.
When someone has 2 eyes with both a different color, it is due to either:
- Heterochromia iridium
Reading of the genetic code is at fault
- Heterochromia iridis (Trauma)
Mendelian Inheritance: Huntington’s Disease – Dominant Allele in Action
HD can be explained using the principles of Mendelian genetics.
It is a case of a dominant allele having weakening effects.
Symptoms are motor disturbances and cognitive decline.
When the offspring gets 2 recessive h alleles, it won’t get HD.
However, if a kid gets a H and a h, there is a 50% chance.
Mendelian Inheritance: PKU – A Recessive Allele in Action
PKU: delayed cognitive maturation due to a recessive allele.
These patients miss an important enzyme needed to break down an essential amino acid.
phenylalanine builds up in the body harmful to the brain and CNS
For it to be expressed, a patient needs 2 RECESSIVE alleles, no dominant one.
Chromosomes and Inheritance
Genes are located on chromosomes which are placed in the nuclei of cells. Chromosomes
are made out of strands of the DNA molecule. They come in matched pairs. Every person
has 23 sets a diploid set, so 46 individual chromatids.
A haploid set is when a cell only has one set, no pair in the sperm or ovum.
They combine diploid set.
1 chromosomes is the largest (categorized according to size).
First 22 chromosomes are the autosomes; number 23 is the sex chromosomes.
Complete set is the karyotype. Architecture of a chromosome:
- Centromere is the middle, or the junction, without genetic info.
- From these centromere, you have 1 short arm (p arm) and 1 long arm (q arm)
- Telomeres: genetic sequences at the end of the arms.
- When using staining techniques, stripes appear: cytogenetic bands. They help
locating genes.
, How does the cellular division work? This is a process called meiosis:
- A human has 23 pairs of chromosomes
- Chromosomes duplicate
- The cell starts to divide into 2. Now, it differs between men and women:
- MALE: duplicated chromosomes in each cell
FEMALE: one cell dies
- MALE: the cell divides again
- MALE: gametes created from division with 23 chromosomes
- FEMALE: the sperm fertilizes the egg
- FEMALE: the cell divides
- FEMALE: the cell without the sperm dies
- FEMALE: the 2 gametes combine to create a cell with 23 pairs of chromosomes
Zygote.
Now, there is a simple cell. To get a multicellular organism, there has to be further cellular
division, or mitosis:
- Process continues until the organism is complete
- 23 pairs of chromosomes in the fertilized cell
- Chromosomes duplicate
- Chromosomes separate
- Chromosomes divide into separate cells
However, cells aren’t that identical. One reasoning is crossing over:
- Crossing over occurs during meiosis
- Chromosomes duplicate
- One chromosome cross over to the other
- The point at which the chromosomes cross over breaks and the 2 parts swap sides
(cross over).
- As meiosis continues, the cells carry different info that isn’t a direct replication of the
initial cell.
DNA
DNA is the building block of life. It has 2 functions:
- It replicates itself to make new cells
- It makes proteins and determines the functions of the cell
What is DNA?
2 DNA strands 1 chromosome:
- The cell contains a nucleus with 23 pairs of chromosomes
- The chromosomes contain the genetic code: the DNA
DNA is made up of:
o Carbono
o Sugar (ribose)
o Nitrogenous base
Adenine, cytosine
Guanine and thymine
- DNA is formed from a double helix consisting of a series of nucleotide bases
Nucleotides are part of the nucleic acid.
- As the double helix unravels, the code created by the nucleotides is evident
- The DNA molecules unwinds and the loose strands attract complementary bases:
Two identical molecules are created once the whole strand of DNA has unwound
- Nucleotides from cytoplasm
DNA strand from nucleus
- Resulting long chain of nucleotides consists of one end labelled three prime (3’) and
the other end labelled five prime (5’). These ends connect.
