Genetics and Intellectual disability

Question 1

Heredity and Environment

Answer 1

• Genotype –a particular set of genes that a person inherits from his or her parents
• Phenotype –the behavioural product of the interaction of a person’s genotype with the environment
• need the environment to express those genes
• genotype and phenotype don’t always match

Question 2

How we reproduce: Germ cells

Answer 2

• Ovum-the female germ cell
• Sperm- the male germ cell

Question 3

Body vs. germ cell division

Answer 3

• Meiosis -cell division of germ cells
• more complex; more chances of something going wrong
• Mitosis –cell division of body cells
• e.g., skin, hair, nails, muscles

Question 4

Chromosomes

Answer 4

• Chromosomes-threadlike structures located in the nucleus of the cell that carry genetic information
• 23 from each parent
• Researchers are now able to collect DNA through cheek cell swabs via the mail

Question 5

Chromosomes, Genes and DNA

Answer 5

• Each of us has billions of cells
• Inside the nucleus of each of these cells is one’s unique set of 46chromosomes
• Chromosomes hold, as a compact coil, an incredibly long molecule of DNA (deoxyribonucleic acid)
• DNA stores all the coded information that influences what you look like, your body functions and psychological functions
• Genes are sections of DNA which contain complete messages. They can be thought of as ‘words’ along the DNA ‘sentences’, with about 100,000 of these ‘words’ in each cell

Question 6

Inheritance

Answer 6

• Inheritance – transmission of maternally and paternally derived genes
  – Certain characteristics may be due to the influences of mainly one chromosome or gene
  – Polygenic inheritance- due to multiple genes that determine complex behaviours such as intelligence, sociability; mostly polygenic

Question 7

Chromosomal atypicalities that affect development

Answer 7

• Sex linked inheritance –genes that are carried on the X chromosome
  – Fragile X –damage to the X chromosome causes intellectual disability and associated attention and social difficulties
• damage means that there’s less genetic info

Question 8

Fragile X Inheritance

Answer 8

• non-symptomatic parents
• each child has a 50% chance of inheriting the defect
• since girls also inherit an X from the father they may not be as severely affected
• son is more likely to be affected because they only have one X chromosome

Question 9

Non-Inherited gene mutation

Answer 9

• Mutations – not inherited they are due to problems during meiosis (e.g., Down syndrome)
• Chromosomal atypicality due to failure to separate pair 21 and leads to extra genetic material (three instead of 2 chromosomes 21) causes intellectual disability, motor and language difficulties

Question 10

Meiosis in Down syndrome

Answer 10

nondisjunction - chromosomes don’t separate properly

Question 11

Genotype-Environment Correlation

Answer 11

Parents give their biological offspring both their genes and a home environment, but both are a function of the parents’ own genes, providing a built-in correlation between the individual’s genotype (inherited characteristics) and his or her early home environment

Question 12

Reactive Interaction

Answer 12

Different persons exposed to the same environment experience it, interpret it, and react to it differently

Question 13

Evocative Interaction

Answer 13

• Each individual’s personality evokes distinctive responses from others
• how other people react to you

Question 14

Proactive Interaction

Answer 14

• Different individuals select and construct different environments
• As a child gets older and begins to select and construct environments of his or her own, the effect of the built-in genotype-environment correlation diminishes and the influence of proactive interaction increases
• Reactive and evocative interactions remain
  important throughout life
• e.g. choosing one major over another

Question 15

Examples of genes and environments working together to influence development

Answer 15

• Reactive Interaction: Different persons exposed to the same environment experience it, interpret it, and react to it differently
• may be bc of self-esteem, temperament, age, sensitivity

Evocative Interaction: Each individual’s personality evokes distinctive responses from others
– Siblings with different temperament
* Inhibited vs. exuberant
- parent may want to play more sports with exuberant child
- parents may be more protective of inhibited child

Proactive Interaction: Different individuals select and construct different environments.
- As a child gets older and begins to select and construct environments of his or her own, the effect of the built-in genotype-environment correlation diminishes and the influence of proactive interaction increases
- Reactive and evocative interactions remain important throughout life
- a child with lower SES may not have the same opportunities to express themselves
- start to choose who you hang out with; environment becomes more narrow as you grow older

Question 16

Shared genes and environments: Impact on behaviour

Answer 16

• Twin studies
  – Identical –share genetic material and home environment
  – Fraternal- do not share genetic material -uterine environment and family environment
  – Identical twins reared apart- share genes but not home environment

– Concordance rates –the percentage of cases in which a characteristic (e.g., ID) is exhibited by one member of a twin pair is also exhibited by the other twin
* Identical 46%
* Fraternal 14%

Question 17

Bi-directional influences in gene-environment
interactions

Answer 17

• Environment (physical, social, cultural)
• Behaviour
• Neural activity
• Genetic activity

Question 18

Reaction range: A dynamic developmental construct

Answer 18

• The notion that human being’s genetic makeup establishes a range of possible developmental outcomes (phenotypes)
• Within these boundaries it is environmental factors that largely determine how the person develops over time
• if given good nutrition then everyone reaches full height potential
• dwarf won’t reach potential of others bc have constrains

Question 19

Reaction Range

Answer 19

likely has down syndrome - not as much improvement bc likely have genetic constrains

• Example: Child with Down syndrome
  – Genetic effects on the organism
  – Environment deprived vs. enriched

Question 20

Deprived environment alone does not cause ID

Answer 20

• Little evidence that deprived environments alone cause intellectual disability
• the environment makes things harder; still need a genotype that is less ideal
• Most children in psychosocial adverse environments do not have ID but they are at higher risk for low IQ
• Possible undetected genetic anomalies in certain individuals with mild ID
• In combination with family pedigree those exposed to psychosocial adversity had a much greater rate of ID

Question 21

Environment may directly impact the function of genes

Answer 21

• Some argue that environmental factors can affect genetic processes. Genes are not immutable!

– There is evidence that epigenetic processes are at play and that these may place organisms at risk for developmental conditions such as ASD

– Although the genes remain the same (what you inherit) environment/lifestyle may change the function of those genes. That is when and how they work
- genes remain the same but function changes

Question 22

Epigenetics

Answer 22

• refers to external (to genes) modifications to DNA that turn genes “on” or “off”
  – modifications do not change the DNA sequence, instead, they affect how cells “read” genes
• One study found that maternal exposure to hurricanes and tropical storms in Louisiana at mid-gestation resulted in the highest prevalence of autism
• environment affects the health of our genes

Question 23

Epigenetic mechanisms

Answer 23

• Examples of epigenetic mechanisms
  – DNA methylation — the addition of a methyl group, or a “chemical cap,” to part of the DNA molecule, which prevents certain genes from being expressed

– histone modification.- histones are proteins that DNA wraps around.
-Without histones, DNA would be too long to fit inside cells. the DNA cannot be “read” by the cell. Modifications that relax the histones can make the DNA accessible to proteins that “read” genes

• Effects may be passed down to next generation if occur in the germline

Question 24

Possible genetic mechanisms of ID
Iarocci & Petrill (2012)

Answer 24

• 3 theoretical mechanisms

– Major genetic effects that are rare and large (example Down Syndrome, William’s Syndrome, Fragile X)
- contribute to these conditions
- something very significant happens (extra genes, missing genes)
- effects are so significant that many other things are also affected
- have many obstacles to overcome (medical problems, attention problems, anxiety, etc.)

– Aggregation of mild deleterious mutations –rare but effects are small and subtle (example unknown causes of ID)
- subtly affect multiple things but accumulate
- don’t know cause of ID (polygenic)

– Genes for cognitive ability that are common variants that act as risk factors across the range of ability (example genes for intelligence that contribute to low IQ-familial type ID)
- ID depends on environment

Question 25

Strategies for studying genetic basis of DD: Top-down approaches

Answer 25

– Observes behavioural characteristics to give clues to genes

– E.g., extreme behaviours such as overly social behavior in William’s syndrome
- behaviours that are really different than in normal population
* Staring at faces
* Social disinhibition

Question 26

Strategies for studying genetic basis of DD: bottom-up approaches

Answer 26

– Begin with genes to cells and then behavior
– E.g., number of gene deletions is related to severity of William’s syndrome
- not the same in all children
- some have more deletions; more affected

Question 27

Strategies for studying genetic basis of DD: endophenotypes

Answer 27

• from Greek endon meaning within
  – In between genes
  and behaviour
  – E.g., study performance on attention, social cognition (theory of mind), executive function tasks