behavioural genetics 1

Question 1

what is heritability?

Answer 1

• an estimate of how much variance in some characteristic within some population is due to differences in heredity
• varies between 0 and 1
  –> i.e. 0 and 100%
• 0.5 intermediate
• not related to individual differences due to genes
• variation in ENTIRE POPULATION due to genes
• lower heritability suggests other factors that cause variance

Question 2

the use of twins

Answer 2

• behavioural genetics mainly look at twin studies
• like to compare MZ and DZ twins

Question 3

heritability in MZ twins

Answer 3

• all traits are in and around 50%
  –> about 50% due to genes and about 50% due to environment
• positive correlations
• not 100%

Question 4

MZ twin predictions

Answer 4

• we can see how similar pairs of twins are for a particular trait
• if a trait was completely heritable the twin pairs would score exactly the same as one another
  –> in MZ twins who share 100% of genes
• MZ twins tend to have positive correlations

Question 5

the use of DZ twins

Answer 5

• fraternal twins
• non-identical
• shared 50% of genes
  –> same has any other full siblings
• 2 eggs
• can compare to MZ twin results

Question 6

problem with using anecdotal studies

Answer 6

• subjective
• can spot similarities by chance accidentally
• can spot differences by chance accidentally

Question 7

problems with just studying identical twins (solution?)

Answer 7

• in modern studies it is unlikely that the twins were reared a part
• therefore they share an environment
• solution = compare MZ twins with DZ twins
  –> likely to both share environments
  –> therefore variance can be due to genes

Question 8

heritability of psychological disorders

Answer 8

• higher heritability in MZ twins for most disorders
  –> e.g. schizophrenia and childhood fatigue
  –> likely to have genetic influence
• higher correlation coefficient links to genes
• DIFFERENCE between DZ and MZ links to genes
• similarities in MZ and DZ link to influence of environment
  –> e.g. bulimia is relatively low and similar for DZ and MZ twins

Question 9

how do we calculate heritability using behavioural genetics?

Answer 9

• heritability is calculated by comparing the correlation coefficients of identical and non-identical twins for a particular trait
• we know that identical twins share all their genetic variation
• we know that non-identical twins share 50% of their genetic variation
• heritability is twice the difference between the correlations for identical minus non-identical twins

Question 10

formula for variance

Answer 10

Vp= A squared + C squared + E squared

Question 11

what do the letters in the heritability formula stand for?

Answer 11

• V is the variance of a trait
• A is the genetic component
• C is the common or “shared” environment
  –> anything in environment that makes traits similar across twin pairs
• E is the “non-shared” environment
  –> anything in the environment that makes the traits dissimilar across twin pairs

Question 12

formula for MZ correlation coefficient

Answer 12

Rmz = A squared + C squared

Question 13

formula for DZ correlation coefficient

Answer 13

Rdz = 1/2 A squared + C squared

Question 14

easy formula for a heritability (A squared)

Answer 14

• A squared = 2 x (Rmz - Rdz)
• heritability is twice the difference between the correlations for identical minus non-identical twins

Question 15

slight issues with behavioural genetics

Answer 15

• heritability is ‘specific’ to the population in which it is calculated
• it looks at variance (differences) between individuals
• high heritability does not imply the environment doesn’t alter that trait
• particularly for the individual
• it can change over time?

Question 16

heritability and the environment

Answer 16

• high heritability does NOT imply that environment does NOT play a role
• you can have 100% heritability in plants but create different environments and if plants grow differently, environment will be playing a role
• if you didn’t have 2 plants with 100% heritability, you might think it’s due to genes, when really it’s due to the environment

Question 17

heritability and cognitive ability

Answer 17

• heritability of general cognitive ability increases linearly from childhood to young adulthood
  –> in the same cohort of twins
• could be due to environments
  –> i.e. similarities in people from the same geographical are (or more variance)
  –> or due to the multiplier effects

Question 18

complexities with heritability: multiplier effect

Answer 18

• if Genetic or prenatal influences produce even a small increase in some activity, the early tendency will change the environment in a way that magnifies that tendency
• i.e. people who think they’re good at something do more things to get better, hence they change the environment
  –> genetic predisposition is therefore multiplied by the environmental/behavioural changes

Question 19

complexities with heritability: tobacco example

Answer 19

• heritability estimate of 65%
• far fewer people smoke tobacco in modern times than they did in the 90s
• but heritability estimate is still 65%
  –> how can it be genetic then?
  –> clearly environment has played a role
  –> unlikely for genes to change

Question 20

summarise behavioural genetics and heritability

Answer 20

• heritability is an estimate of how much variance in some characteristic within some population is due to differences in heredity
• varies between 0 and 1 (i.e. 0 and 100%)
• it is calculated by comparing the correlations or concordances of identical and non-identical twins
• it is specific to the population in which it is calculated
• it can ‘change’ over time