Download - Ch3 Genetics

8/2/2019 Ch3 Genetics

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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.

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