Heritability and individual differences: Twins

How many protein-coding genes do humans have approximately?

20,000–25,000.

What are monogenetic traits?

Traits controlled by a single gene; rare; direct genotype–phenotype link (e.g., cystic fibrosis).

What are complex traits?

Traits influenced by many genes (polygenic) plus environmental factors; common.

What do quantitative genetic studies measure?

The relative contributions of genetic and environmental factors.

What do molecular genetics studies aim to do?

Identify specific genes associated with traits.

What is heritability (H²)?

The proportion of variation in a trait explained by genetic differences (0–100%).

Is heritability fixed?

No — it can change across the lifespan due to gene–environment interactions.

What are monozygotic (MZ) twins?

Identical twins sharing ~100% of their genes; always same sex.

What are dizygotic (DZ) twins?

Fraternal twins sharing ~50% of their segregating genes; can be same or opposite sex.

How do twin studies estimate heritability?

By comparing concordance (correlation) of traits between MZ and DZ twins.

What does higher concordance in MZ twins suggest?

A genetic contribution to the trait.

What did meta-analysis show about heritability across traits?

Average heritability ~49%.

What is zygosity and how is it determined

Whether twins are identical (MZ) or fraternal (DZ).

• using either DNA markers or zygosity questionnaires

What is one major limitation regarding genes studies involving twins

Twin studies cannot identify specific genes involved.

What is the “equal environments assumption”?

Assumes MZ and DZ twins experience equally similar environments.

• MZ twins may have more similar pre and post natal environments

What does “assortative mating” refer to?

Non-random mating (e.g., people choosing partners with similar traits), which can bias estimates.

• this can effect genetic variation

Do twin studies explain individual differences?

No — heritability is a population-level statistic.

How does heritability of BMI change across development?

Increases during childhood and peaks in late adolescence.

• due to gene-environment correlations

What is passive gene–environment correlation?

Parents provide both genes and environment to a child.

What is evocative gene–environment correlation?

A person’s genetically influenced behaviour elicits specific responses from the environment.

What is active gene–environment correlation?

A person seeks out environments aligned with their genetic predispositions.