ch3 genetics
TRANSCRIPT
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Genetics
Natalie Tien
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Mechanisms of Inheritance
Cell Division
Chromosome
DNA
Mendels Studies-principles of genetic transmission
Revisions of Mendels Principles
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Cell Division
Cells have 3 major subdivisions:
Nucleus
Cytoplasm
Cell membrane
Human Chromosomes:
23 pairs (46 in all)
For each pair: one from mother, one from father
22 pairs of autosomes
1 pair of sex chromosome
Cell Type:
- body cells
-germ cells
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Mitosis
Body cells
Three Phases First phase: cell duplicates itself
Second phase: line up at the equator, move to opposite poles
Third phase: cell divides into two
2N 4N 2N
Each new cell contains 46 chromosomes
Genetically identical to the parental cell.
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Meiosis
Germ cells
Five phases: First phase: cell duplicate themselves into two identical strands
Second phase: chromosomes pair up with their partners
Third phase: cross over (two strands no longer identical) Fourth phase: line up at the equator
Fifth phase: cell divides
2N 4N 2N 1N
Result:- Four gametes posses 23 chromosomes.
- Each chromosomes are genetically unique.
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DNA
Chromosome DNA genes
Double helix
Base-pairing rules:
(A) Adenine (T) Thymine (C) Cytosine (G) Guanine
AT
GC
This sequence of the base-pairs determines the codedinformation carried by the gene.
98% of human DNA is also found in the DNA of thechimpanzee
Alleles: genes for the same trait located in the same place on apair of chromosomes.
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Human Genome Project
The possibility of altering humans hereditary code
through genetic engineering.
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How do genes affect behaviour?
We know it is complex, not yet entirely understood fornow.
There are 2 kinds of genes:
Structural genes Regulator genes
Structural gene: guide the production of proteins.
Regulator gene: controls the activities of the structural genes.
Can selectively suppress the production of protein Turn structural genes on and off at different points in
development
The mechanism works depend on the environment.
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Principles of Genetic Transmission
Gregor Mendel(1822-1884)
Interested by the process of hereditary transmission.
Phenotype: the expressed or observable trait (purple flower, whiteflower)
Genotype: the underlying genes (PP, pp, Pp) Principle of Dominance:
Dominant gene
Recessive gene
Principle of Segregation
Principle of Independent Assortment
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Revision of Mendels Principles
Polygenetic inheritance
Height, weight, skin color
In humans, most behavioural traits are also affected by multiplegenes (temperament, intelligenceetc.)
Incomplete dominance Some traits result in neither entirely dominant nor entirely
recessive.
Example: sickle-cell anemia
Codominance Both genes of a trait are dominant
Both characteristics are expressed completely
Example: Blood Type AB
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Revision of Mendels Principles
Genomic Imprinting
A trait does not follow any of the unusual laws ofinheritances, rather, it matters whether the mother orfather provides a particular gene. Example: Asthma
The relevant alleles are biochemically marked such thatone of them is imprinted or silenced, and only the
other allele finds expression in the phenotype.
Environment can play a crucial role in the expression ofgenes.
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Genetic Disorders
What is driving force behind evolution?
Variations, mutations
When the errors turn out to be adaptive, they result in
improvements in the creation of a new species.
Majority of mutations are maladaptive.
About 90% of all genetic abnormalities result inmiscarriage rather than live births.
Only about 1% of all babies have detectable
chromosomal abnormalities.
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Hereditary Disorders
Dominant traits
Huntingtons disease
Neurofibromatosis
Recessive traits
Tay-Sachs disease
Phenylketonuria (PKU)
Sickle-cell anemia (SCA)
Cystic fibrosis
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Family Studies
Asks whether the phenotypic similarity on some trait follows from thegenotypic similarity among the people being compared.
Kallikak family study
Henry Goddard
1912 An illegitimate son by a tavern maid with a developmental
disability.
Later married a normal intelligence woman
Limitation: Distinguish between hereditary and environmental
influences.
Lots of things are shared in a same family but not all of them aredetermined by our genes.
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Adoption Studies
Address the issues of the difficulty in interpreting that either genes orenvironment could account for the patterns we find in FamilyStudies.
Provide the evidence about the origins of problem in development.
Colorado Adoption Project(1975): Longitudinal study (since as preschooler)
250 families with adopted child v.s 250 families with biologicalchild
Focus: intelligence, intellectual ability, cognitive skills
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Adoption Studies: Findings Support
Role of genetic process
Stronger correlations have been found between the scores of biologicalsiblings than between the children and their adoptive parents
Finding indicates: children inherited their intellectual abilities at least tosome degree
Provide the evidence about the origins of problem in development
Behaviours can be inherited
A positive relation between the adoptive parents disciplinary practices and
the biological parents history of psychiatric problems.
The children initially inherited their behaviour problems from their biological
parents. These problems then evoked responses (harsh discipline) from their adoptive
parents.
The adoptive parents disciplinary practices then served to maintain the
childrens problem behaviours.
Findings suggest diseases and psychological disorders can be inherited.
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Twin Studies
Identical twins
- Develop from the same fertilized egg
- Monozygotic twins (MZ)
Fraternal twins- Develop from two different eggs and are called dizygotic twins
(DZ)
Researchers look for particular traits and behaviours indifferent sets of twins
- IQ Test: finding correlation between IQ test scores
- MZ twins showed higher correlations than did DZ twins
- Personality test
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Do Genes Influence Temperament?
Answer: Yes, they do. (at least to some extent)
Results indicate that genes are strongly involved in theaspect of temperament at both ages and in bothlocations.
Developmental Pacing :
The rhythm of the childrens life events
Psychologists assume that developmental pacing is guided bythe regulator genes, which turn the structural genes on and off at
different points in development and thus control proteinproduction.
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Problem of Twin Studies?
How do we know that a family treats a set of fraternaltwins as similarly as a set of identical twins?
Expectations of treating identical twins alike mayinfluence how people behave toward the children, andmay affect how the children themselves behave.
Method to avoid the problem:
- Study twins who are separated early in life and raised in different
adoptive homes.
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Combined Twin-Study and Adoption-Study Approaches
Minnesota Study of Twins Reared Apart
135 pairs of twins
The Study provides strong evidence for a genetic contribution todifferences in IQ
Even when reared apart, identical twins correlate substantially inIQ
Although this combined approach is the most desirable,it is also the most difficult to use.
Because, so few twins are raised apart
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Models of Gene-Environment Interaction
How much of a role do genes play in these sorts ofdifferences?
How genes and environment exert their effects?
What processes do these factors operate ? How do they interact with one another?
Researchers in the field of Behaviour genetics interested
Contemporary psychologists believe that genes (nature)and environment (nurture) interact to determine humanbehaviour, but they differ as to which factor they feel hasmore influence.
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Limit-Setting Model
Irving Gottesman(1974)
Suggested that genes interact with the environment by settingthe upper and lower limits of our development.
The Reaction Range
The genes have set the limits on potential skills on specific task,regardless to their environment.
The reaction ranges overlap
Either poor genes or a poor environment can be somewhatovercome by strength in the other.
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Niche-Picking Model
Sandra Scarr
Genes play a much more active role in development than simplysetting limits within which the environment can operate.
In the model, genes are not one determinant of the kind of
environment the child experiences. Children play a role in producing their own environments, which
occurs through the genes
Childs genes operate to produce a correlation between the
childs genotype and the childs environment
Ensure that both genes and environment push development inthe same direction.
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Niche-Picking Model
Passive gene environment correlation During infancy
i.e. Musically inclined parents are likely both to give birth to musically inclined
children and provide a musical home environment for them.
Evocative gene environment correlation As children get older
i.e. A child who speaks and reads early (behaviours that likely have somegenetic basis) may prompt parents to provide a rich language environment(books, storytelling, educational games). In this way, the childs genes help
create an environment that is compatible with the childs genetically set
predispositions.
Active gene environment correlation As children gain more independence
Childrens genes can operate through a more active way to produce
environments that suit them.
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Niche-Picking Model Evaluation
Does not ignore the importance of the childs family
environment.
Views the environmental less as a direct cause of thechilds development and more as a vehicle that enables
the genes to guide the child along a particulardevelopmental path.
Contends that genetic influences actually grow strongerwith age, as children become better able to activelyrecruit the environments their genes bias them toward.
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Environmental Genetics Model
Robert Plomin(1994,2000a)
Childrens developmental outcomes are related to their family
environments
His model is not limited to parent-child interactions
Extends to other family members
Non-shared environment
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Bioecological Model
Urie Bronfenbrenner and Stephen Ceci
Assign much more importance to the environment than do manybehaviour genetics
The model adds a more explicit description of how and when this
occurs The Proximal Processes
Urie Bronfenbrenner and Stephen Cecibelieve that
proximal processes are valuable to the child in bothhigh- and lower-quality environments, but in differentways.