GN 301 Module 7: Intellectual Disabilities

St. Thomas Aquinas defined intelligence as the ability to combine and separate.

IQ

  • IQ is assumed to be distributed as a normal distribution

  • What is the average IQ? 100

  • What is the normal range?  85-115

  • IQ below 70 is considered intellectually disabled

  • About 3% of the population falls in the intellectually disabled range

Genetic Causes of Intellectual Disability

Single gene examples (Simple Mendelian)

  • PKU (autosomal recessive)

    • Cannot properly metabolize phenylalanine

  • Galactosemia  (autosomal recessive)

    • Toxic levels of galactose in blood

  • Hurler Syndrome (autosomal recessive)

    • Missing lysozomal enzyme allows chemical buildup leading to neurological degeneration
    • Often death by age 10
    • Enzyme therapy

  • Tay-Sachs Disease (autosomal recessive)

    • inherited lysosomal storage disorder – missing enzyme
    • fatty substance builds up in the brain and destroys nerve tissue
    • Convulsions, blindness, paralysis, death usually by 5 yrs

  • Rett Syndrome (X-linked dominant)

  • Polygenic: Possible to get the “bad luck” alleles of several polygenes that contribute to intelligence

    • See familial inheritance patterns of some intellectual disabilities
    • More than 75% of cases come from less than 10% of families
    • If both parents have IQ values in the 50 – 70 range, there is a 60-70% chance that their child will also have a subnormal intelligence

Chromosomal Examples

  • Trisomic Situations
    • Trisomy 21 (Down Syndrome)
    • Trisomy 18 (Edward Syndrome).
    • 50% die in utero Of those who live, 90% will not live longer than 1 yr.
    • About 1 in 6000 live births 
    • Trisomy 13 (Patau Syndrome).
    • 1 in 12,000 live births
    • Fragile X Syndrome.
    • Many copies of repeated CGG sequence 
    • William Syndrome.
    • Deletion of 26 genes on chr #7
Environmental factors for intellectual disability

  • Prenatal: pre-birth health conditions

    • Infections (measles, malaria, HIV, herpes)
    • Small enough to get to fetus through placenta
    • Medicines, Drugs, Alcohol, Smoking
    • Maternal Health
    • High Blood Pressure 
    • Diabetes 
    • Nutrition

  • Perinatal: at delivery

    • oxygen deprivation at delivery is the main cause from environmental factors
    • Premature delivery (low birth weight) (<5.5 pounds)
    • Low birth weight correlated with lower IQ

  • Preterm vs Premature

    • "Preterm" is a medical term used by healthcare professionals to refer to a baby that is born before the 37th week of pregnancy, regardless of whether the baby is born early or not.
    • "Premature," on the other hand, is a more general term that refers to any baby that is born before the due date, regardless of how early they are born. In other words, a baby that is born at 36 weeks would be considered preterm, but not necessarily premature, while a baby that is born at 32 weeks would be considered both preterm and premature.

  • Postnatal: after delivery

    • early nutrition is critical!
    • Protein deficiency causes damage to central nervous system