transmission of genes from generation to generation chapter 3
TRANSCRIPT
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Transmission of Genes From Generation to Generation
Chapter 3
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3.1 Heredity: How are Traits Inherited?
Why do we begin examining inheritance by discussing Gregor Mendel and pea plants?
Before Mendel there was no clear understanding of how traits were inherited and passed from one generation to the next
Mendel used experimental genetics to uncover the fundamental principles of inheritance
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3.2 Mendel’s Experimental Design
Mendel’s experiments defined the ideal properties of an experimental organism: It should have a number of different traits
that can be studied The plant should be self-fertilizing and
have a flower that reduces accidental pollination
Offspring of self-fertilized plants should be fully fertile so more crosses can be made
http://www.youtube.com/watch?v=Mehz7tCxjSE
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ANIMATION: Crossing garden pea plants
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Mendel’s Experiments in the Monastery Garden
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Traits Selected for Study by Mendel
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3.3 Crossing Pea Plants: Single Traits
Mendel’s initial crosses studied the inheritance of a single trait such as seed shape or seed color When plants with smooth peas were crossed with plants
having wrinkled peas, the offspring all produced smooth peas
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3.3 Crossing Pea Plants: Single Traits P1: Smooth X wrinkled
F1: Smooth
Self-fertilize F1 plants
5,474 Smooth
1,850 wrinkledF2:
Total peas in F2: 7,324
• When the plants from these smooth seeds were crossed with each other, the offspring yielded 7,324 peas
5,474 were smooth
and 1,850 were wrinkled
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ANIMATION: Monohybrid cross
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INTERACTION: Test Cross
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Mendel’s Terminology
P1 = parental generation F1 = first filial (generation after parent generation)
generation F2 = second generation
So in this experiment P1: smooth x wrinkled F1: offspring all smooth F2: offspring smooth (5,474) and wrinkled (1,850)
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Mendel’s Conclusions
Only one of the parental traits was expressed in the F1 plants Genetic factors can be hidden (unexpressed) Despite identical appearance, P1 and F1 plants
must have different genetic factors F1 plants must carry hidden genetic factors
The trait not expressed in the F1 plants reappeared in 25% of the F2 plants F1 plants must carry two factors, one from each
parent. (these factors are now referred to as genes)
Traits were not blended as they passed though parents
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Traits
Dominant Trait - Trait expressed in the F1 (heterozygous) condition
Recessive Trait - Trait repressed in the F1 but re-expressed in some members of the F2 generation
Phenotype - Observable properties of an organism
Genotype - The specific genetic constitution of an organism
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Phenotype and Genotype
P1 :
Genotypes SS ss
Meiosis
Phenotypes Smooth wrinkled
Gametes S S s s
Fertilization
F1 :Genotype Ss
Phenotype Smooth
X
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Mendel’s Principle of Segregation
Pairs of factors (genes) separate from each other during gamete formation and exhibit dominant/recessive relationships
https://www.youtube.com/watch?v=-deWjUJBxGk
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Gene Pairs Segregate during Gamete Formation
Genes - Fundamental units of heredity
Segregation - Members of a gene pair separate from each other during gamete formation
Ss SsSs Ss
Smooth SmoothS s S s
F1 cross Gamete formation by F1 parents
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Combinations of Gene Forms (Alleles)
Allele - One possible alternative form of a gene Usually distinguished by its phenotypic effects
Homozygous - Having identical alleles for one or more genes
AA or aa
Heterozygous - Having two different alleles for one or more genes
Aa
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ANIMATION: Genetic terms
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Punnett Square
Ss Ss S sSs Ss
SSmooth Smooth
S s S s s
F1 cross Gamete formation by F1 parents
Set up Punnett square
Genotype PhenotypeS s
1 SS 3/4 Smooth2 Ss S
sS Smooth
Ss Smooth
1 ss1/4
wrinkleds
F2 ratioGamete combinations
represent random fertilization
ss wrinkled
SS Smooth
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3.4 The Principle of Independent Assortment
Members of one gene pair separate or segregate independently of other gene pairs
https://www.youtube.com/watch?v=QyY8yb9S--s
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Mendel’s Principle of Independent Assortment
Independent assortment - The random distribution of alleles into gametes during meiosis Leads to formation of all possible combinations of
gametes with equal probability in a cross between two individuals
http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html
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3.4 The Principle of Independent Assortment
P1 cross
Smooth Yellow wrinkled green
F1: All Smooth Yellow
F1 x F1: Smooth YellowSmooth Yellow
F2:9/16 Smooth
Yellow3/16 Smooth
green
3/16 wrinkled Yellow
1/16 wrinkled green
x
x
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ANIMATION: Dihybrid cross
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A Dihybrid Cross
F1: Smooth Yellow X Smooth Yellow
F2: Of all offspring Of all offspring Combined probabilities
if Smooth
3/4 are Yellow( 3/4 )( 3/4 ) = 9/16
Smooth Yellow3/4 are Smooth
1/4 are green ( 3/4 )( 1/4 ) = 3/16 Smooth green
and if wrinkled
1/4 are green ( 1/4 )( 1/4 ) = 1/16 wrinkled green
1/4 are wrinkled3/4 are Yellow
( 1/4 )( 3/4 ) = 3/16 wrinkled Yellow
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Punnett square Illustrates Independent Assortment in a Dihybrid Cross
P1 cross
SSYY ssyy ssYY SSyy
Smooth Yellow xwrinkled green wrinkled Yellow Smooth green
Gamete formation
Gamete formation
SY sy sY SyFertilization Fertilization
SsYy
F1 = Smooth Yellow
F1 cross
SsYy
SsYy
P1 cross
x
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Punnett square Illustrates Independent Assortment in a Dihybrid Cross
SY Sy sY sy
SY SSYY Smooth Yellow
SSYy Smooth Yellow
SsYY Smooth Yellow
SsYy Smooth Yellow
Sy SSYy Smooth Yellow
SSyy Smooth green
SsYy Smooth Yellow
Ssyy Smooth green
F2
GeneratIon
sY SsYY Smooth Yellow
SsYy Smooth Yellow
ssYY wrinkled Yellow
ssYy wrinkled Yellow
sy
SsYy Smooth Yellow
Ssyy Smooth green
ssYy wrinkled Yellow
ssyy wrinkled green
F2 Genotypic ratios F2 Phenotypic ratiosSSYY
9/16 Smooth YellowSSYySsYY
4/16 SsYy
1/16 SSyy 3/16 Smooth green2/16 Ssyy
1/16 ssYY 3/16 wrinkled Yellow2/16 ssYy
1/16 ssyy 1/16 wrinkled green
SsYyF1 cross SsYyx
1/16
2/162/16
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Exploring Genetics: Evaluating Results: The Chi Square Test
A statistical test to determine whether the observed distribution of individuals in phenotypic categories is as predicted or occurs by chance
X2= ∑ d2/E
Acceptable probability depends on the number of phenotypic classes
df=n-1
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Chi-Square Analysis of Mendel’s Data
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Mendel’s Contribution
Mendel’s principle of segregation and principle of independent assortment are fundamental to our understanding of the science of heredity (genetics)
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3.5 Meiosis Explains Mendel’s Results
Genes pairs (alleles) are located on chromosome pairs The position occupied by a gene on a chromosome is
referred to as a locus The behavior of chromosomes in meiosis causes
segregation and independent assortment of alleles
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ANIMATION: Independent assortment
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Genes, Chromosomes, and Meiosis
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A a a A Aa a
Meiosis I
B B b b bB B
A A a a A A a a
Metaphase II
B B b b b b B B
Gametes
A B A B a b a b A b A b a B a B
A
b
Segregation and Independent Assortment in Meiosis
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3.6 Mendelian Inheritance in Humans
Inheritance of certain human traits is predictable based on Mendelian inheritance patterns
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Aa Aa
Aa Aa
A a A a
Genotype Phenotype
A a
1 AA3/4 normal
2 AaA
AA normal
Aa normal
1 aa 1/4 albinoa
Aa normal
aa albino
Segregation of Albinism
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AaDd AaDd
AD Ad aD ad
AADD Pigment Hearing
AADd Pigment Hearing
AaDD Pigment Hearing
AaDd Pigment Hearing
AD
AADd Pigment Hearing
AAdd Pigment
Deaf
AaDd Pigment Hearing
Aadd Pigment
DeafAd
AaDD Pigment Hearing
AaDd Pigment Hearing
aaDD Albino
Hearing
aaDd Albino
HearingaD
AaDd Pigment Hearing
Aadd Pigment
Deaf
aaDd Albino
Hearing
aadd Albino Deaf
ad
Independent Assortment of Two Traits Can be Followed
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Pedigree
A diagram listing the members and ancestral relationships in a family
Used in the study of human heredity Uses a standard form and symbols Uses family history to show how a trait is inherited and
estimate risk for family members Provides genetic counseling for those at risk of having
children with genetic disorders
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Symbols Used in Pedigree Construction
Male Aborted or stillborn offspring
FemaleDeceased offspring
Mating
Mating between relatives (consanguineous)
or Unaffected individual
IParents and children. Roman numerals symbolize generations. Arabic numbers symbolize birth order within generation (boy, girl, boy)
or Affected individual
II or1 2 3
Proband; first case in family that was identified
P P
Monozygotic twins or Known heterozygotes
Carrier of X-linked recessive traitDizygotic twins
Offspring of unknown sex Infertility
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Numbering System used in Pedigrees
Each generation is identified by a Roman numeral
Each individual within a generation is identified by an Arabic number
Proband: The first affected family member who seeks medical attention for a genetic disorder
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Following a Trait Through a Pedigree
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ANIMATION: Pedigree diagrams
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3.8 Variations from Mendel
Alleles can interact in ways other than dominant/recessive Incomplete dominance Codominance Multiple alleles
Different genes can interact Epistasis
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Incomplete Dominance
The expression of a phenotype that is intermediate to those of the parents.
An example is the inheritance of flower color in snapdragons: R1R1 (red) x R2R2 (white) = R1R2 (pink)
x
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INTERACTION: Incomplete dominance
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Codominance
Full phenotypic expression of both members of a heterozygous gene pair Blood Types - If a person is type AB this means that
both the A allele and the B allele are equally expressed and are therefore codominant.
Roan fur in cattle. Many breeds of cattle are dominant for red hair or white hair
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Multiple Alleles
Genes that have more than two alleles The inheritance of the ABO blood types in humans Rabbit coat color
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Multiple Alleles
A B
A A B B
A A B B
A IAi IA or IAi
B IBIB or IBi
A
B B
A A
AB IAIB
B O ii
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Genes Can Interact to Produce Phenotypes
Epistasis - A form of gene interaction in which one gene masks or prevents expression of another gene. Lab coat color Albino gene is an epistatic gene. When the albino
condition occurs, the genes that determine skin color are present, but not expressed.