Source: General Psychology, The Ohio State University, Chapter 3
Tags: Mendel, genetics, phenotype, genotype, allele, heritability, breeder's equation, twin studies, mature adaptations, developmental adaptations, nature vs nurture
Difficulty: Introductory to Intermediate | Prerequisites: Chapter 1 notes (nature vs. nurture debate from behaviourism)
This chapter connects the nature-vs.-nurture debate from Chapter 1 to actual genetic mechanisms. It covers how genes influence behaviour (Mendelian and polygenic inheritance), how heritability is measured and what it really means, and the distinction between mature and developmental biological adaptations. If you have not reviewed the basic research methods material from Chapter 2, do that first, because the twin-study designs here rely on those concepts.
Genes influence behaviour through both single-gene (Mendelian) and many-gene (polygenic) pathways. Heritability (H²) tells you how much of the variation in a trait within a group is due to genetic differences rather than environmental ones, and the Breeder's Equation (R = h²S) predicts how much a trait shifts across a single generation under selection. Biological adaptations come in two flavours: mature adaptations (instincts, hardwired) and developmental adaptations (instinct to learn, shaped during the organism's lifetime).
Phenotype
The observable trait or characteristic of an organism (e.g., brown hair, tall stature). In simple terms, it is what you can see or measure about an organism.
Genotype
The genetic makeup underlying a trait. The actual combination of alleles an organism carries. In simple terms, it is the genetic instruction set, whether or not the trait is visible.
Allele
A different form of the same gene. Roughly a third of genes have multiple allele variants. Think of it as different "versions" of the same recipe.
Mendelian inheritance
A pattern in which a single gene controls a trait. Follows predictable ratios (e.g., 25/50/25 in hybrid crosses).
Polygenic inheritance
A pattern in which many genes contribute to a single trait. Most behavioural traits are polygenic. Think of it as many small genetic contributions adding up, rather than one gene flipping a switch.
Heritability (H²)
A statistic describing how much of the variation in a trait within a population is attributable to genetic differences, as opposed to environmental differences. In simple terms, H² answers "of all the differences we see in this group, what fraction comes from genes?"
H² = 0 means none of the variation is genetic
H² = 1 means all of the variation is genetic
Familiality
When family members share a characteristic for any reason, genetic or environmental. Familiality does not prove heritability. In simple terms, families can be similar because they share genes, because they share environments, or both.
Breeder's Equation
R = h²S, where S is the selection differential (the difference between selected individuals and the general population on a given trait), h² is the heritability, and R is the predicted change in the trait between the offspring and the general population.
Environmentality
The proportion of variation in a trait that is due to environmental (non-genetic) differences.
Mature adaptation
A biological adaptation that meets an organism's needs in adulthood. These are instincts: fixed in the nervous system, unmodified by learning, triggered by a stimulus.
Developmental adaptation
A biological adaptation that prepares an organism to attain adult behaviours during its lifetime. These are "instincts to learn": the learning itself is biologically prepared, but the specific behaviour develops through experience.
Mendel crossed hybrid (F1 generation) peas and tracked traits such as round vs. wrinkled and yellow vs. green
In hybrid crosses, the phenotypic ratio was approximately 25% / 50% / 25% (homozygous dominant / heterozygous / homozygous recessive)
This demonstrated that traits are inherited in predictable patterns governed by discrete units (genes)
Applying Mendelian ratios to humans
Two brown-haired parents who are both carriers of a recessive allele (e.g., for blond hair) can produce children with blond hair, following the same 25/50/25 logic
Scott and Fuller crossed basenji and cocker spaniel dogs and confirmed that Mendelian ratios held regardless of the environment the dogs were raised in
Mendelian vs. polygenic
Some traits are controlled by a single gene (Mendelian): clear-cut dominant/recessive patterns
Most behavioural traits are polygenic: many genes each contribute a small effect, producing a continuous distribution rather than discrete categories
Core concept
Heritability is about variation within a group, not about individuals
It asks: of the differences we observe in this population, how much is explained by genetic differences vs. environmental differences?
Heritability is not familiality
Familiality means family members share a trait. This can happen for genetic reasons, environmental reasons, or both
Heritability arises only when the similarity comes from shared genotypes, not merely from shared environments
Measuring heritability in animals
Tryon's rats: selectively bred "maze-bright" and "maze-dull" rats. Dumb rats raised with smart rats were still dumb, and vice versa. This showed intelligence in rats was a heritable trait.
Measuring heritability in humans
In humans, you cannot selectively breed, so researchers use twin studies
To estimate environmentality: compare identical twins raised apart to those raised together. Differences between the two groups reflect environmental influence
To estimate heritability: compare identical twins raised together to fraternal twins raised together. If identical twins are more similar, the extra similarity is attributed to their shared genes
The Breeder's Equation worked example
R = h²S
If heritability of intelligence is 0.5, average IQ of the general population is 100, and only individuals with IQ of 130 or higher are allowed to reproduce:
S = 130 - 100 = 30 (the selection differential)
R = 0.5 × 30 = 15
The average IQ of children in the next generation would be 100 + 15 = 115
Mature adaptations (instincts)
Meet an organism's biological needs in adulthood
Fixed in the nervous system, unmodified by learning, triggered by environmental stimuli
Non-human example: birdsong mechanisms may already be present in the brain and are simply triggered by the environment so adults can find mates
Human example: reflexive withdrawal from pain, the startle response
Developmental adaptations (instinct to learn)
Meet an organism's need to acquire adult behaviours during its lifetime
The mechanism develops within the organism's lifetime; the behaviour is learned, but the capacity for that learning is biologically prepared
Non-human example: birdsong may develop within the bird's lifetime, where the song is learned during a critical period but the learning itself is biologically guided
Human example: language acquisition, where the ability to learn language is innate but the specific language learned depends on the environment
"Heritability of 0.5 means 50% of your intelligence comes from genes." Heritability describes variation within a population, not the contribution of genes to any one individual's trait. It is a group-level statistic.
"If something is heritable, the environment does not matter." A trait can be highly heritable and still be influenced by the environment. Heritability only tells you about sources of variation under current environmental conditions. Change the environment and the heritability estimate can change.
"Familiality proves genetic influence." Families share environments as well as genes. Similarity among family members does not, on its own, demonstrate that genes are responsible.
Students sometimes confuse mature adaptations with developmental adaptations. The key difference: mature adaptations are ready-made in the nervous system; developmental adaptations require learning, but the capacity to learn them is biologically prepared.
⚠️ Know the Breeder's Equation (R = h²S) and be ready to calculate R given values for h² and S. The exam question about IQ = 130 selection with h² = 0.5 is a classic.
⚠️ Be able to explain the difference between heritability and familiality. This distinction is frequently tested.
⚠️ Understand how twin studies separate genetic from environmental contributions. Know which comparison estimates heritability (identical vs. fraternal, both raised together) and which estimates environmentality (identical twins raised together vs. apart).
⚠️ Be able to give a human and a non-human example of both mature and developmental adaptations.
⚠️ Know Mendel's pea experiment well enough to explain how two brown-haired parents can have a blond-haired child.
True or False: Heritability of 1.0 means environment plays no role in a trait for any individual.
Fill in the blank: R = h²S is known as the __________ equation.
True or False: Identical twins raised apart who are still similar on a trait suggest that the trait has high heritability.
Fill in the blank: A biological adaptation that is fixed in the nervous system, unmodified by learning, and triggered by stimulus is called a __________ adaptation.
True or False: Polygenic traits follow simple dominant/recessive Mendelian ratios.
Answers: 1. False (it means all measured variation in the population is genetic, but the environment could still matter in a different population or context). 2. Breeder's. 3. True. 4. Mature. 5. False (polygenic traits involve many genes and produce continuous distributions).
Q: Describe Mendel's experiment with peas and explain how it accounts for the distribution of hair colour when both parents have brown hair.
A: Mendel crossed hybrid (F1) peas and observed a 25/50/25 phenotypic ratio in offspring. If both parents are heterozygous carriers of a recessive allele (e.g., for blond hair), approximately 25% of children can be homozygous recessive and display the recessive trait (blond hair), even though both parents show the dominant phenotype (brown hair).
Q: What is heritability, and how does the Breeder's Equation predict trait change across a generation?
A: Heritability (H²) is the proportion of trait variation within a population that is attributable to genetic differences. The Breeder's Equation, R = h²S, predicts the shift in a trait over one generation. If H² = 0.5 and only individuals with IQ 130+ reproduce (S = 30), then R = 15, meaning the next generation's average IQ would be 115.
Q: What is the difference between mature and developmental adaptations? Give an example of each for humans and non-humans.
A: Mature adaptations are instincts fixed in the nervous system, unmodified by learning, and triggered by stimuli (non-human: birdsong mechanisms already present in the brain; human: pain withdrawal reflex). Developmental adaptations are biologically prepared capacities to learn specific behaviours during the organism's lifetime (non-human: birdsong learned during a critical period; human: language acquisition).
Q: How do twin studies help separate genetic from environmental influences on behaviour?
A: Comparing identical twins raised together to fraternal twins raised together estimates heritability (identical twins share 100% of genes, fraternal share ~50%; if identical twins are more similar, the extra similarity is genetic). Comparing identical twins raised together to identical twins raised apart estimates environmentality (same genes, different environments; differences reflect environmental influence).
This chapter's genetic concepts connect forward to Chapter 5 (neuroscience), where neurotransmitter function and neural development are partly under genetic control. The nature-vs.-nurture debate introduced by behaviourism in Chapter 1 gets its empirical tools here (twin studies, heritability estimates). The concept of biological adaptations connects to the motivation and drives material in Chapter 6, where regulatory drives (hunger, thirst) are themselves adaptations shaped by evolution.
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