Heritability - Chapter 7

Heritability

The proportion of variation in a particular trait within a population that can be attributed to genetic differences among individuals

Heritability is WHAT-specific. It can change across different WHAT and WHAT

Heritability is POPULATION-specific. It can change across different POPULATIONS and ENVIRONMENTS

Heritability applies to WHAT, not WHAT. You cannot use heritability to predict how “WHAT” a particular person is

Heritability applies to GROUPS, not INDIVIDUALS. You cannot use heritability to predict how “GENETIC” a particular person is

WHAT and WHAT always work together. Heritability does not mean WHAT doesn’t matter

GENES and ENVIRONMENT always work together. Heritability does not mean ENVIRONMENT doesn’t matter

Heritability is about WHAT between people in a trait

No WHAT = heritability is not WHAT

Heritability is about DIFFERENCES between people in a trait

No VARIATION = heritability is not MEANINGFUL

Heritability = about WHAT in a population, not WHAT for individuals

Heritability = about VARIATION in a population, not DESTINY for individuals

Quantitative genetics produces one number - the WHAT, from 0 to 1 - meant to capture WHAT' influence on a trait

Quantitative genetics produces one number - the HERITABILITY COEFFICIENT, from 0 to 1 - meant to capture GENETICS' influence on a trait

Roughly: it measures how strongly differences among individuals are WHAT to differences among their WHAT

Roughly: it measures how strongly differences among individuals are RELATED to differences among their GENES

you cannot have a WHAT heritability

you cannot have a PERSONAL heritability

Turkheimer

howed heritability changing with the environment you sample

In the poorest families ≈ WHAT

• Heritability of cognitive ability was near WHAT. Almost all the variation among children tracked shared WHAT - the household and neighbourhood conditions the twins had in WHAT.

In the poorest families ≈ 0

• Heritability of cognitive ability was near ZERO. Almost all the variation among children tracked shared ENVIRONMENT - the household and neighbourhood conditions the twins had in COMMON.

In the most affluent families ≈ WHAT

• The pattern reversed. Most of the variation tracked WHAT, and the shared-environment contribution SHRANK toward nothing.

In the most affluent families ≈ 0.6+

• The pattern reversed. Most of the variation tracked GENETIC DIFFERENCES, and the shared-environment contribution SHRANK toward nothing.

The heritability of cognitive ability changed depending on which families you looked at. When the environment is harsh and WHAT variable, WHAT does the explaining. When it is uniformly WHAT, whatever variation is left is WHAT by default.

The heritability of cognitive ability changed depending on which families you looked at. When the environment is harsh and HIGHLY variable, ENVIRONMENT does the explaining. When it is uniformly ADEQUATE, whatever variation is left is GENETIC by default.

Three classic mistakes

• WHAT

• WHAT

• WHAT

Three classic mistakes

• Reading a population number as a personal one

• Reading Heritable as fixed

• Treating it as a property of the trait

Reading a population number as a personal one

• “Height is 80% heritable” does not mean 80% of your height came from your WHAT and 20% from your dinners. It means that in the group studied, most of the difference between people tracked WHAT. There is no such thing as your WHAT heritability

Reading a population number as a personal one

• “Height is 80% heritable” does not mean 80% of your height came from your GENES and 20% from your dinners. It means that in the group studied, most of the difference between people tracked GENETIC DIFFERENCE. There is no such thing as your PERSONAL heritability

Reading heritable as fixed

• A high heritability says nothing about whether a trait can be WHAT. PKU is caused by a single gene and is completely managed by diet. Vision is highly heritable and corrected with glasses. Heritability describes where variation WHAT comes from, not WHAT it could come from.

Reading heritable as fixed

• A high heritability says nothing about whether a trait can be CHANGED. PKU is caused by a single gene and is completely managed by diet. Vision is highly heritable and corrected with glasses. Heritability describes where variation CURRENTLY comes from, not WHERE it could come from.

Treating it as a property of the trait

• Heritability belongs to a trait in a WHAT in a WHAT. Move to a population with less WHAT variation and heritability WHAT, with no WHAT change at all. Ask “heritable where, and among whom?”

Treating it as a property of the trait

• Heritability belongs to a trait in a POPULATION in a CONTEXT. Move to a population with less ENVIRONMENTAL variation and heritability GOES UP, with no GENETIC change at all. Ask “heritable where, and among whom?”

This does not mean there is a gene for watching television. It means that heritable WHAT - temperament, activity level, sociability, impulse control - shape the situations people end up in. Genes reach outcomes by long, indirect, WHAT routes

This does not mean there is a gene for watching television. It means that heritable DISPOSITIONS - temperament, activity level, sociability, impulse control - shape the situations people end up in. Genes reach outcomes by long, indirect, ENVIRONMENTAL routes