1. Matching Possible answers:-

moderate retardation          Down syndrome        mild retardation            dementia

severe retardation               reading disorder         fragile X syndrome           PKU

dyslexia                          Alzheimers disease               Lesch-Nyhan syndrome

additive gene effects        non-additive gene effects        genotype x environment interaction

assortative mating              random mating                         inbreeding depression


Subaverage intellectual functioning, IQ in the range 35-50 moderate retardation

Subaverage intellectual functioning, 85% of the retarded mild retardation

Metabolic disorder causing moderate retardation if untreated PKU

Second most common genetic cause of retardation fragile X

Category comprising 80% of learning disorders reading disorder

Severe cognitive decline with age dementia

Specific type of reading disorder dyslexia

Lowering of mean phenotypic score often seen in the offspring

of related individuals inbreeding depression

Disease causing 50% of dementia cases Alzheimers disease

A cause of correlation between spouses assortative mating

Effect seen when changes caused by the environment depend

on the genotype of the individual genotype/environment interaction

  1. Autism has a prevalence of about 4-5 per 10,000. A study of 233 siblings of autistic children found that 7 of the siblings (3%) were autistic and that 226 were not. Does this result suggest little family resemblance or considerable family resemblance for autism? Strong family resemblance

Give reasons for your answer. Prevalence of autism is only 0.045% (4 or 5 per 10,000) but risk is 3%. Risk is therefore 60 times prevalence, indicating strong family resemblance family members are 60 times more likely to have autism than members of the general population. 

  1. Assuming adequacy of sample sizes, measurements and the twin study method, and basing your answers on the following correlations, what can you conclude about the genetic and environmental influences on the following conditions?

Condition                MZ correlation               DZ correlation

ADHD                           0.62                                 0.25

Depression                     0.44                                  0.19

Autism                           0.91                                  0.44



 moderate heritability of 62% ( from MZ twin correlation)

 non-shared e accounts for 38% of variation (from MZ correlation)

 no evidence for shared e since DZ correlation is not more than half the MZ correlation

 some evidence for non-additive gene effects since DZ correlation is less than half MZ correlation 


 lower heritability than ADHD, h2= 44% (from MZ correlation)

 biggest influence on variation = non-shared e at 56% (from MZ correlation)

 no evidence of shared e (MZ correlation> DZ correlation x2)

 DZ correlation indicates may be some non-additive gene effects.


 highest heritability of the 3 disorders here, heritability = 91%

 very little non-shared e (9% of variance)

 no evidence for shared e or non-additive gene effects since MZ correlation = 2 x DZ correlation 


  1. i. Name a chromosomal abnormality that is a cause of mental retardation

____any from class eg Down syndrome, Turners_syndrome___________

ii Name a single gene disorder that is a cause of mental retardation

_____any from class eg PKU, Duchenne muscular dystrophy etc__________

iii Is there any evidence that the genes involved in these disorders have an effect on cognitive ability in the normal range? Give reasons for your answer.

 depends on which disorders you chose as examples

eg Down syndrome yes, there does appear to be at least one gene for cognition on chromosome21, in the DownCriticalRegion1, recognized so far

PKU no, the PKU locus does not appear to be a gene that normally functions in cognitive processes in the brain.










  1. Brief account of the genetics of general cognitive ability.

What is g? general measure of cognitive ability derived by factor analysis from scores from tests of various more specific abilities whose scores intercorrelate

Explain the evidence for a genetic influence on g from the following:

  1. family studies

parent/offspring correlations are in the range of 0.42 0.47

indicates level of cognitive ability does run in families

but this could be due to shared e or shared genes or both 


  1. adoption studies

adopted apart P/O and sib correlations = 0.24, indicating heritability of 48%

adoptive P/O and sibs correlate in range 0.19 0.32, indicating shared e component of 19 32% of the variation.


  1. twin studies

MZ correlation = 0.86 DZ = 0.60

indicates heritability of 52%, shared e (34%) and non-shared e (14%) as influences also








Summarize the factors influencing g in childhood

 all data is consistent with heritability of about 50%, shared e = 25% and non-shared e (and error) = 25% of variance during childhood


How do these influences change over time?

 shared e influence does not persist and heritability gets bigger:

60+% of variation in adulthood is genetic, rest of variation is non-shred e (and error)