Hoorcolleges behaviour genetics in psychology
Week 1:
Behavioral genetics is expanding from psychology and psychiatry into other areas, like
neuroscience, economics, political science, education, and sociology.
- This field is growing because it's now easier, cheaper, and more accurate to include genetic
data in research. Scientists can simply collect saliva samples to get genetic information, and
the costs of analyzing genes have dropped. Plus, improved tools—like genome-wide
association studies (GWAS) and polygenic scores—help researchers better understand how
genes may influence behaviors across different areas of life.
The field of behavior genetics aims to answer two major questions: What makes us similar? and What
makes us different? By looking at how traits pass from parents to children, researchers try to
understand why we share certain characteristics and what causes individual differences.
Early Theories of Inheritance: in ancient times, people believed that parents passed down traits to
their children, though their ideas were often inaccurate. For example:
Pythagoras believed that fathers provided the essential traits, while mothers supplied only
the material for growth.
Aristotle thought that children formed from a combination of purified blood and menstrual
blood.
The Microscope and Preformationism
- Two Dutch scientists, Antonie van Leeuwenhoek and Nicolaas Hartsoeker, advanced
understanding by inventing the microscope. They developed the Theory of Preformationism,
suggesting that every human began as a tiny version of themselves inside the sperm (known
as a “homunculus”).
Key Figures: Francis Galton and Charles Darwin
Francis Galton (1822-1911): Known as the founder of behavior genetics, Galton studied twins
and family trees to understand genetic influences. He argued that “nature,” or genetic
inheritance, is more powerful than “nurture,” or environment, in shaping people. His studies
on twins showed striking similarities, reinforcing his views on the power of genetics.
Charles Darwin (1809-1882): Darwin’s theory of evolution showed that all species evolved
through natural selection, where small, beneficial traits increase survival and reproduction.
However, he mistakenly thought traits from every part of the body contributed to the next
generation, leading to the Theory of Pangenesis and “blending inheritance” (the idea that
offspring are a simple mix of parental traits).
Gregor Mendel and the Basics of Genetics: Gregor Mendel was a scientist who studied how traits
(like eye color or height) are passed from parents to children. His experiments helped us
understand how genetics work.
Mendel’s First Law: The Law of Segregation: Mendel discovered that:
- Each trait (like flower color or seed shape) has two "elements" (what we now call genes) in
each person.
- During reproduction, these two elements separate, so a child gets one gene from each
parent.
This law of segregation showed that genes don’t just “blend” but separate and come
together in different combinations.
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,Predicting Traits with the Punnett Square: to understand how traits might show up in children, we
use the Punnett Square. This is a chart that shows possible gene combinations from two parents.
Homozygosity means a person has two of the same genes (for example, "SS" or "ss").
Heterozygosity means they have two different genes (like "Ss").
Dominant and Recessive Alleles: Mendel learned that some traits are dominant, meaning they
show up even if there’s only one copy of the gene. For example:
Dominant allele (written as a capital letter like "S") will show its effect if it's paired with
another dominant or a recessive (for example, "SS" or "Ss").
Recessive allele (written as a lowercase "s") only shows up if there are two copies (like "ss").
Mendel’s Second Law: The Law of Independent Assortment
- Mendel also studied how different traits (like seed color and seed shape) are passed
down independently of each other.
This means that one trait, like fur color, doesn’t affect the chance of inheriting another trait,
like tail length. Each trait is inherited separately.
Mendelian Traits & Disorders: Mendelian traits are characteristics controlled by a single gene. If
there’s a mutation in that gene, it can lead to a specific disorder. Two examples:
1. Huntington’s Disease
2. Phenylketonuria (PKU)
Huntington’s Disease is a serious brain disorder caused by a mutation in one gene.
- Symptoms: It leads to uncontrollable movements, balance problems, slurred speech, thinking
issues, and personality changes.
- Progression: It gets worse over time, causing major brain cell damage and eventually leads to
death. Unfortunately, there is no cure or effective treatment.
- Discovery: Named after George Huntington, who described it in 1872. In 1993, scientists
found its genetic cause.
Genetics of Huntington’s Disease: It’s a dominant trait, meaning a person only needs one
mutated gene from one parent to develop the disease.
Phenylketonuria (PKU): PKU is a metabolic disorder that affects how the body processes protein.
Without proper treatment, it can cause nervous system damage and lead to intellectual disability. A
special diet can prevent symptoms by avoiding certain proteins. Genetics of PKU: PKU is
a recessive trait, meaning both parents must carry the gene for a child to be affected.
Exceptions to Mendel’s Laws: while Mendel’s laws apply to many traits, there are some exceptions:
1. Genes Close Together: Mendel’s Second Law (the idea that different genes are inherited
separately) doesn’t apply if two genes are close together on the same chromosome. These
genes tend to be inherited together.
2. X-Linked Inheritance: For genes on the X or Y sex chromosomes (like color blindness):
- In males: A single copy of a recessive X-linked gene can cause the trait.
- In females: They would need two copies to show the trait, since females have two X
chromosomes.
Complex Traits: some traits, like height, are not controlled by a single gene but by multiple genes.
These are called quantitative traits and don’t follow Mendel’s laws exactly. They are also influenced
by the environment, making them more complex to predict.
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, Mendel’s experiments with pea plants revealed that traits are inherited through "factors" (now
called genes), with each plant receiving one factor per trait from each parent. He established the Law
of Segregation, showing that these factors separate during reproduction, and the Law of
Independent Assortment, demonstrating that most traits are inherited independently.
- Mendel’s findings also introduced the concepts of dominant and recessive traits, which
explained why certain traits could skip generations, laying the groundwork for modern
genetics.
Mendelian Genetics Overview: Mendelian genetics is the study of how traits are passed down from
parents to offspring. Gregor Mendel discovered that each trait is controlled by "elements" (now
called genes), and individuals have two versions (alleles) for each gene, one from each parent. These
alleles separate during reproduction, with one being passed to each offspring. Mendel’s Laws
1. Law of Segregation: Each parent gives one of their two alleles to their offspring.
2. Law of Independent Assortment: Traits are inherited separately from each other, unless the
genes are close on the same chromosome.
Punnett Squares: punnett squares are diagrams used to predict possible gene combinations in
offspring. For example:
- Dominant Allele (represented as uppercase) will show its trait if present (like "S" for short
tail).
- Recessive Allele (lowercase, like "s") only shows if paired with another recessive allele.
Punnett squares help predict how traits might be inherited, showing possible combinations
for traits like eye color, height, or diseases.
Complex Traits in Genetics: complex Traits vs. Simple Traits
Some traits and disorders, such as ADHD, autism, and schizophrenia, do not follow the
straightforward patterns of inheritance that Gregor Mendel described. These complex traits
are influenced by multiple genes and environmental factors, making them more challenging
to understand.
Mendelian Inheritance: Mendel's laws of inheritance apply to simple traits that are controlled by one
or a few genes, like Huntington’s disease and hemophilia. These traits have clear genetic causation
and are often deterministic, meaning a specific genetic mutation will lead to the disorder.
Continuous Traits and Genotype vs. Phenotype: complex traits can show a continuous range of
phenotypes (observed traits) even though they are influenced by many different alleles (genotypes).
The number of possible genotypes increases exponentially with the number of genes, but the
number of phenotypes is often fewer because of how alleles can combine.
- Genotype = Combination of alleles for a trait
- Phenotype = Observable characteristic
For example, skin color is a complex trait affected by multiple genes, leading to a variety of
skin tones among different populations.
Additive Genetic Effects: additive genetic variance occurs when the effects of multiple alleles
combine to influence a trait. Each allele contributes a small effect, leading to a gradual change in
phenotype. This is different from nonadditive effects, where gene interactions can complicate how
traits are expressed.
Influences on Complex Traits: complex traits are shaped by:
- Many genes with small effects
- Environmental interactions
- Probabilistic causation (meaning genetics play a role, but it's not guaranteed)
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