Ch 1/2: Mattering & History
Three laws of behavior genetics:
1. First law: All human behavioral traits are heritable.
2. Second law: The effect of being raised in the same family is smaller than the effect
of genes.
3. Third law: A substantial portion of the variation in complex human behavioral traits is
not accounted for by the effects of genes or families
It is not possible for one gene to be responsible for complicated behaviors, such as Intelligence.
Understanding how genetics + environment contribute to individual variations in behavior.
As time passed... it became:
Easier to add genetics in a research study (saliva from participants).
Cheaper [costs of genotyping]
More Accurate [statistical tools improved].
Ancient World:
→ Main idea: Parents pass on characteristics from the offspring.
Pythagoras: proposed a theory where the father contributes the essential characteristics or
"form," while the mother provides the material building blocks – Suggests a division of roles in
the transmission of traits from parents to offspring.
Aristotle: In his work "Generation of Animals," proposed that children were formed from a
combination of "purified blood from the testes" (semen) and menstrual blood. The mention of
menstrual blood in Aristotle's theory reflects the understanding prevalent in ancient times, even
though it might seem unusual from the modern scientific standpoint.
Antonie van Leeuwenhoek and Nicolaas Hartsoeker: These two dutch men were responsible for the
creating microscopes + proposed the “Theory of Preformationist”.
The idea was that within the sperm or egg, a fully-formed miniature version of the
organism (a homunculus) already existed, and development was essentially the
unfolding or growth of this preexisting form.
Suggested humans are fully developed at conception.
This theory has already been disregarded.
The theory was eventually replaced by the “Theory of epigenesis”.
Which claims that organisms develop progressively from an
undifferentiated state during embryonic development.
Charles Darwin (1809-1822)
, Voyage of the Beagle (1831-1836): Cruised around the world with a ship.
Darwin was impressed by all the diversity he had witnessed during the world trip.
Therefore, he proposed the “Theory of Evolution”.
Darwin’s theory of natural selection as an explanation for the origin of species made a
major impact on scientific thinking.
All species arose through the natural selection of small, inherited variations that
increase the individual's ability to survive and reproduce.
→ While the concept of heredity had thus been found, there was no knowledge yet on how it works...
One theory of heredity proposed by Darwin was the Provisional hypothesis of Pangenesis:
Suggested that cells of the body release gemmules, which are miniature replicas of the parent
cells.
Each part of an organism is thought to produce gemmules, and these particles circulate
throughout the body.
Gemmules carry information from all parts of the body, including acquired traits, to the
next generation.
During reproduction, gemmules aggregate and multiply, contributing to the development of
new organs in the offspring.
They collect in the reproductive organs and play a role in transmitting information to the
next generation.
This idea lead Darwin to development of the “blended inheritance”, as an attempt to explain how
variation works [Although it was a misconception about the mechanism behind variation].
Darwin developed the theory of blended inheritance, suggesting that variation occurs because
offspring are blended versions of their parents.
However, If variation only occurred because the offspring was a blended version of the parents,
variation would eventually cease to exist.
For example, if one parent were tall and the other short, the offspring would be average height
and eventually everyone would be this average height.
However, an environmental condition might induce a systematic change.
,Lamarckian theory of use and disuse: Darwin ended accepting the Lamarckian theory to suggest that
acquired characteristics can be inherited — According to this perspective, traits that are actively used
by an organism will be maintained and developed over time (strengthen), while unused traits will
gradually disappear (vanish).
Francis Galton (1822) : A 19th-century polymath and cousin of Charles Darwin.
Galton was the first to study the inheritance of mental characteristics and to suggest using twins and adoptees
to study nature-nurture problems.
Galton was particularly interested in the study of individual differences and the measurement of
human traits, such as intelligence.
He realized the importance of proper assessment and developed apparatus and procedures
for measuring several types of human capabilities.
Turned to Statistics to measure psychological attributes
Pioneered in the development of median, percentiles and correlations.
Also responsible for creating “Psychometrics”
Founded: Behavior Genetics and Differential Psychopathology.
Galton introduced the use of twins to assess the nature versus nurture debate.
Galton also wrote a book called “Hereditary Genius”.
Claimed Nature prevails enormously over Nurture
In his book, one of the key findings he presented was the observation that "Eminent status" or
achievements were more likely to appear in close relatives.
The likelihood of eminence decreased as the degree of relationship became more remote.
CH3 – Mendel’s Laws
Gregor Mendel (1822-1884) = Mendel was an Austrian scientist and Augustinian friar who is often
referred to as the father of modern genetics.
He conducted groundbreaking experiments with pea plants in the mid-19th century.
Studies aimed to understand how traits are passed from one generation to the next.
Mendel employed a combination of in-breeding and cross-breeding techniques in his
experiments.
In-breeding involved matings between individuals or organisms that are closely
related genetically, helping him create pure-breeding lines.
Cross-breeding, on the other hand, allowed him to study the inheritance of traits
between different lines.
, Mendel started by crossing pea plants that were
pure-breeding for specific traits: one set always
produced smooth seeds, and the other set always
produced wrinkled seeds.
Pure-breeding means that these plants, when
self-fertilized, consistently produced offspring
with the same trait.
The first filial generation (F1) resulted from
crossing the pure-breeding smooth and wrinkled
plants – Surprisingly, all the seeds in the F1
generation were smooth.
This contradicted the prevailing theories of
the time, which expected a blend or mixture of
traits in the F1 generation when pure-breeding
parents with contrasting traits were crossed.
Mendel then intercrossed the plants of the F1
generation among themselves.
The majority of the resulting seeds in the F2
generation were smooth, resembling the trait
dominant in the F1 generation. However,
approximately 1/4 of the seeds were wrinkled
The key observation was the reappearance of the
wrinkled trait in the F2 generation (25%) , even
though it was absent in the F1 generation.
Mendel's findings challenged the blending theory,
and he proposed the concept of discrete hereditary
units (later known as genes) that remain unchanged
through generations.
This led to the formulation of Mendel's laws,
particularly the Law of Segregation and the
Law of Independent Assortment, which
explained the patterns of inheritance
observed in his experiments.
Mendel’s Law explained through different inheritance patterns “Huntington's disease” & PKU”
(1) Huntington’s Disease:
HD shows a consistent pattern of heredity.
It is an autosomal dominant disorder.
Afflicted individuals have at least one parent who also has the disease.
