Download Cognitive Disabilities - University of Western Ontario

Cognitive Disabilities
Mental Retardation
• APA Diagnostic and Statistical Manual of Mental
Disorders-IV (DSM-IV):
– Sub-average intellectual functioning
– Onset before 18 years
– Limited adaptive (life) skills
Four Levels of Mental Retardation
1. Mild: IQ 50-70
-about 85% of all MR individuals
-most can hold a job and live independently
-familial (runs in families)
2. Moderate: IQ 35-50
-about 8% of all MR individuals
-most institutionalized or group homes
-good self-care and simple language skills
-not familial
Four Levels of Mental Retardation
3. Severe: IQ 20-35
-about 4% of all MR individuals
-require considerable supervision
-some self-care and language comprehension skills,
limited speech
-not familial
4. Profound: IQ < 20
-about 2-3% of all MR individuals
-most institutionalized
-limited if any language comprehension, usually no speech
-not familial
Two Types of Mental Retardation
1. Cultural/Familial:
Attributable to the combined effects of the multiple
genetic and environmental factors that contribute to
variation in IQ throughout its normal range
2. Organic:
Attributable to a major traumatic event that
causes irreversible damage to the developing CNS
IQ Distribution
Evidence for Two Types of MR
Evidence for Familiality of Mild MR
Reed & Reed (1965):
-studied 80,000 relatives of 289 individuals with MR
-20% of children with one mildly MR parent were mildly MR
-about 50% of children with two mildly MR parents were
mildly MR
Nichols (1984):
-studied IQs of 15 MZ and 23 DZ pairs in which at least 1
twin had mild MR
-concordance for MR: MZ = 75%, DZ = 46%
Other twin studies:
-low IQ is at least as heritable as is IQ in the normal range
Factors Contributing to Organic MR
(% are very approximate)
Specific Environmental Factors:10-30%
-teratogens (alcohol)
-peri- and post-natal trauma (Asphyxia)
-fetal infections (Rubella)
-head injuries
Specific Genetic Factors: 50-60%
-chromosomal disorders (Down Syndrome)
-genetic disorders (PKU, RETT, Lesch-Nyhan)
Down Syndrome
• single most important cause of MR: occurs in about 1/1000 births
• average IQ: 55 but a wide range exists
-top 10% have IQ’s in the lower end of normal range
• many of those who reach age 45 will have some form of dementia
• caused by a trisomy of chromosome 21
(i.e. 3 rather than 2 copies of chromosome 21 are inherited)
• occurs more often in offspring of older mothers, presumably due
to activation of eggs that have been dormant for decades
Normal Set of Chromosomes (Female)
Chromosomes of a Male with Trisomy 21
Phenylketonuria (PKU)
• occurs in about 1/10,000 births
• if untreated, IQ’s often < 50 though some near normal
• PKU is a single-gene recessive disorder
• easily detected (at birth) and treated by a diet low in phenylalanine
(found in red meats and other foods)
• even with treatment, PKU individuals are somewhat below
average IQ
• excellent example of something which is entirely ‘genetic’ being
largely treatable by ‘environmental’ intervention
Rett Syndrome
• second most common cause of MR in females
(after Down Syndrome)
• occurs in about 1/10,000 females
• shows few effects in infancy
• by age 5, most affected girls cannot stand, talk, or use hands and
will show autistic-like behavior
• due to these symptoms, an accurate assessment of intelligence is
essentially impossible
Other Genetic or Chromosomal Disorders
Contributing to MR
More than 100 genetic disorders include MR among their
symptoms:
Fragile X Syndrome
Duchenne Muscular Dystrophy
Neurofibromatosis Type 1
Angelman Syndrome
Williams Syndrome
XXY Male Syndrome
XXX Female Syndrome
XYY Male Syndrome
XO Female (Turner’s)
Fragile X Syndrome
• second most common cause of MR after Down Syndrome
• twice as common in males (1/1250) than females (1/2500)
• most fragile X males have moderate MR, many only mild,
some normal IQ
• only about half of fragile X females are affected because one of
the two X chromosomes inactivates
• typically 6 to 54 CGG triplet repeats on the X chromosome
• fragile X chromosome may have > 200 such repeats
- ‘fragile’: this many repeats literally makes it break easily.
• neurobiological research may one day counteract its effects, which
molecular genetic research is beginning to understand
Duchenne Muscular Dystrophy
• an X-linked recessive disorder found in 1/3500 males
• average IQ of affected males is 85
• verbal abilities typically more impaired than nonverbal
• neuromuscular disorder that usually leads to death by 20 years
• it is not known how the DMD gene affects IQ
Sex Chromosome Abnormalities
• an X-linked recessive disorder found in 1/3500 males
• average IQ of affected males is 85
• verbal abilities typically more impaired than nonverbal
• neuromuscular disorder that usually leads to death by 20 years
• it is not known how the DMD gene affects IQ
Sex Chromosome Abnormalities
XXY Male Syndrome
-occurs in about 1/750 male births
-somewhat lower than average IQ, poor academic
-achievement, language problems
XXX Female Syndrome
-occurs in about 1/1000 female births
-average IQ=85
-speech therapy often required
Sex Chromosome Abnormalities
XYY Male Syndrome
-occurs in about 1/1000 male births
-often require speech therapy, academic problems, higher
incidence of juvenile delinquency
XO Female (Turner’s) Syndrome
-occurs in about 1/2500 female births (but 99% miscarry)
-surviving females have just one X chromosome
-short stature, abnormal sexual development, often infertile
-normal verbal IQ but performance IQ is below average
Reading Disability (Dyslexia)
-the primary disability in about 80% of children with a
diagnosed learning disorder
-Dyslexic children read slowly and with poor comprehension
-familial and likely heritable:
-among 250 twin pairs in which at least one was dyslexic,
MZ concordance = 66%, DZ = 36%
-DF extremes analysis (group heritability)
-the first QTL for any human disorder was reported and
replicated for reading disability
Group Heritability
• An estimate of the extent to which the difference between
average and extreme scoring individuals is attributable to
genetic and/or environmental factors
• Example: Reading disabilities
GROUP HERITABILITY
Reading Disabilities
GROUP HERITABILITY
Reading Disabilities
group h2 = 0
x
RD
x
MZ & DZ
GROUP HERITABILITY
Reading Disabilities
group h2 = 1.0
x
RD & MZ
x
DZ
GROUP HERITABILITY
Reading Disabilities
Defries & Fulker (1985)
group h2 = .28
x
RD
-2.8
x
MZ
-2.0
x
DZ
-.1.6
Estimating Group Heritability
1. Compute ratio of reading scores:
MZ = 2.0/2.8 = .71
DZ = 1.6/2.8 = .57
2. Group Heritability
= 2(MZ - DZ)
= 2(.71 - .57) = .28
3. Therefore, 28% of the difference between RD and general
population is attributable to genetic factors
 .71 - .28 = 43% of the difference is attributable to shared
environmental factors
 the remaining 29% is attributable to nonshared
environmental factors
Comparing Heritability and Group Heritability
 (individual) h2 for reading ability = .50
 group h2 for reading disability = .28
Since these differ, this indicates that the genetic etiology of
reading disabilities is different from that of the normal
distribution of reading scores