Transcript SR6e Chapter 3

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Zygote: Union of sperm & ovum at
conception
 Contains 23 pairs of chromosomes
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 One pair from each parent
 Each pair influences a characteristic
Chromosomes: thousands of genes
containing DNA
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Meiosis: process producing sperm, ova
Mitosis: cell-division process creating all
other cells throughout life
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Massive genome analysis projects
999/1000 human base chemicals: identical
1/1000 accounts for differences between us
Humans/Chimps share 96% genetic material
Gene variants evolved in recent centuries
◦ Adaptations to food sources, diseases,etc.
Findings also useful to identify genes
associated with disease, drug treatments
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Monozygotic (MZ) twins: 100% related
◦ Single zygote divides
◦ 2 genetically identical individuals
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Dizygotic (DZ) twins: 50% on average
◦ 2 ova fertilized by 2 sperm
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Siblings: 50% on average
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Parent & Child: 50% related, shared
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Males: XY; Females: XX
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Genes: instructions for development
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Genotype: genetic makeup a person inherits
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◦ Characteristics like eye color, height, IQ
(potential) e.g., genes for tallness
Phenotype: actual/expressed trait (height)
Regulator genes turn gene pairs on/off at
different times
◦ Turned on for adolescent growth spurt
◦ Turned off in adulthood
Always influenced by environmental factors
also
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Single gene-pair inheritance
◦ Dominant gene = dominant trait
◦ Recessive genes
 Trait expressed if paired with a similar
gene (Homozygous)
 Trait not expressed if paired with
dissimilar gene (Heterozygous)
◦ Recessive traits: homozygous recessive
◦ Dominant traits: hetero or homozygous
gene pair
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About 9% affected in US
◦ Homozygous recessive
Heterozygous are “carriers”
◦ Can transmit gene to offspring
◦ If both parents carriers: 25% chance
Example of incomplete dominance
◦ Offspring may have sickling episodes
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Single genes located on sex chromosomes
Actually X-linked
Males - no counterpart on Y chromosome
◦ Only needs one to be color-blind
Females - counterpart on 2nd X chromosome
◦ Usually for normal color-vision (dominant)
◦ Must inherit on both to be color-blind
Also Hemophilia, Duchene MS, others
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X-Linked Inheritance
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For most important human characteristics
◦ Height, intelligence, temperament, etc.
Trait influenced by multiple pairs of genes
These traits are normally distributed
◦ I.e., found in the same proportion in all
populations
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A change in gene structure/arrangement
Produces a new phenotype
More likely in sperm than in ova
May be harmful or beneficial
Can be inherited by offspring
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Errors in chromosome division during
meiosis
◦ Too many or too few chromosomes result
◦ Most spontaneously aborted
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Down Syndrome: Trisomy 21
◦ Physical characteristics
◦ Mental retardation
◦ Related to age of both parents
◦ Often develop Alzheimer’s in middle age
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The rate of Down syndrome births increases steeply as the mother’s age
increases.
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Turner’s syndrome: 1/3000 females
◦ Single X chromosome (XO) : small, unable
to reproduce, stubby fingers, webbed neck
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Klinefelter syndrome: 1/200 males
◦ XXY: Sterility, feminine traits
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Fragile X syndrome: one arm on X is fragile
(Leg of X barely connected)
◦ Usually males (sex-linked inheritance)
◦ Most common heredity cause of MR
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Helps people understand and adapt
Prenatal diagnosis: techniques include
◦ Amniocentisis, preimplantation
Human genome project yielded much info
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Tay-Sachs disease
◦ Cause: recessive gene pair (European Jews/French
Canadians)
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Huntington’s disease
◦ Deterioration of nervous system
◦ Single dominant gene
◦ One affected parent = 50% chance in
offspring
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Genetic/environmental cause of traits
Heritibility estimates proportion of
phenotypic variation in a population that is
attributable to genetic variation among
individuals.
Experimental and selective breeding
◦ Tryon’s maze-bright rats
Twin, adoption, family studies
◦ Reared together or apart
◦ Concordance rates--twins are concordant
if they both display a trait of interest
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Genetic similarity
◦ Degree of trait similarity
Shared environmental influence
◦ Living in the same home
Non-shared environmental influences
◦ Unique experiences
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Analysis of genes and their effects
◦ May compare humans with other animals
eg. Alzheimer’s disease
◦ Most common form of old age dementia
◦ Twin studies show heritability
◦ Possible genetic links being tested
◦ Environmental factors also being tested
 High cholesterol, head injury
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Correlations highest in identical twins
◦ Genetic factors determine trait
Correlations higher if twins reared
together
◦ Environmental factors
◦ Non-shared experiences influential
Identical twins more alike with age
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Temperament Correlations
◦ Identical twins = .50 to .60
◦ Fraternal twins = 0 (even reared together!)
Personality Correlations Similar
◦ Shared environment unimportant
◦ Genetic inheritance important
◦ Non-shared experiences important for
differences
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Correlations between the traits of identical twins raised apart in Minnesota Twin
Study.
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Schizophrenia concordance rates
◦ ID twins: 48%
◦ Fraternal twins: 17%
◦ Affected parent increases risk even if
adopted at birth
Inherited predisposition
◦ Environmental factors (triggers)
◦ Prenatal exposure to infection suspected