Biology 201 Chapter 12 PowerPoint
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Transcript of Biology 201 Chapter 12 PowerPoint
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1
Chapter 12
Lecture Outline
Patterns of InheritanceChapter 12
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Mystery of heredity
• Before the 20th century, 2 concepts were the basis for ideas about heredity– Heredity occurs within species– Traits are transmitted directly from parent to
offspring• Thought traits were borne through fluid and
blended in offspring• Paradox – if blending occurs why don’t all
individuals look alike?
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Early work
• Josef Kolreuter – 1760 – crossed tobacco strains to produce hybrids that differed from both parents– Additional variation observed in 2nd generation
offspring contradicts direct transmission• T.A. Knight – 1823 – crossed 2 varieties of
garden pea, Pisum sativa– Crossed 2 true-breeding strains– 1st generation resembled only 1 parent strain– 2nd generation resembled both
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Gregor Mendel
Chose to study pea plants because:1. Other research showed that pea hybrids could be produced2. Many pea varieties were available3. Peas are small plants and easy to grow4. Peas can self-fertilize or be cross-fertilized
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Mendel’s experimental method
• Usually 3 stages1. Produce true-breeding strains for each trait
he was studying2. Cross-fertilize true-breeding strains having
alternate forms of a trait – Also perform reciprocal crosses
3. Allow the hybrid offspring to self-fertilize for several generations and count the number of offspring showing each form of the trait
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StigmaStyle
Anthers (male) 1. The anthers are cut away on the purple flower.
PetalsCarpel (female)
4. All progeny result in purple lowers.
3. Pollen is transferred to the purple flower.
2. Pollen is obtained from the white flower.
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
9
Monohybrid crosses
• Cross to study only 2 variations of a single trait
• Mendel produced true-breeding pea strains for 7 different traits– Each trait had 2 variants
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F1 generation
• First filial generation• Offspring produced by crossing 2 true-
breeding strains• For every trait Mendel studied, all F1 plants
resembled only 1 parent– Referred to this trait as dominant– Alternative trait was recessive
• No plants with characteristics intermediate between the 2 parents were produced
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F2 generation
• Second filial generation• Offspring resulting from the self-
fertilization of F1 plants• Although hidden in the F1 generation, the
recessive trait had reappeared among some F2 individuals
• Counted proportions of traits– Always found about 3:1 ratio
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Purple White
Yellow Green
Round Wrinkled
Green Yellow
1. Flower Color
2. Seed Color
4. Pod Color
Dominant Recessive
3.15:1X
X
X
X
3.01:1
2.96:1
2.82:1
F2 Generation
705 Purple:224 White
6022 Yellow:2001 Green
5474 Round:1850 Wrinkled
428 Green:152 Yellow
3. Seed Texture
Inflated Constricted
X2.95:1
Axial Terminal
Tall Short
6. Flower Position
7. Plant Height
X
X
3.14:1
2.84:1
882 Inflated:299 Constricted
651 Axial:207 Terminal
787 T all:277 Short
5. Pod Shape
Dominant Recessive F2 Generation
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3:1 is actually 1:2:1
• F2 plants– ¾ plants with the dominant form– ¼ plants with the recessive form– The dominant to recessive ratio was 3:1
• Mendel discovered the ratio is actually:– 1 true-breeding dominant plant– 2 not-true-breeding dominant plants– 1 true-breeding recessive plant
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Parent generation
Self-cross Self-cross Self-cross Self-cross
Cross-fertilize
Self-cross
True-breedingPurpleParent
True-breeding
WhiteParent
PurpleOffspring
F1 generation
F2 generation(3:1 phenotypicratio)
F3 generation(1:2:1 genotypicratio)
PurpleDominant
PurpleDominant
PurpleDominant
WhiteRecessive
True-breeding
Non-true-breeding
Non-true-breeding
True-breeding
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Conclusions
• His plants did not show intermediate traits– Each trait is intact, discrete
• For each pair, one trait was dominant, the other recessive
• Pairs of alternative traits examined were segregated among the progeny of a particular cross
• Alternative traits were expressed in the F2 generation in the ratio of ¾ dominant to ¼ recessive
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Five-element model
1. Parents transmit discrete factors (genes)2. Each individual receives one copy of a
gene from each parent3. Not all copies of a gene are identical
– Allele – alternative form of a gene– Homozygous – 2 of the same allele– Heterozygous – different alleles
4. Alleles remain discrete – no blending5. Presence of allele does not guarantee
expression– Dominant allele – expressed– Recessive allele – hidden by dominant allele
• Genotype – total set of alleles an individual contains
• Phenotype – physical appearance
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Principle of Segregation
• Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization
• Physical basis for allele segregation is the behavior of chromosomes during meiosis
• Mendel had no knowledge of chromosomes or meiosis – had not yet been described
Punnett square
• Cross purple-flowered plant with white-flowered plant
• P is dominant allele – purple flowers• p is recessive allele – white flowers• True-breeding white-flowered plant is pp
– Homozygous recessive• True-breeding purple-flowered plant is PP
– Homozygous dominant• Pp is heterozygote purple-flowered plant
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P
P
p
p pp
P
P
p
p pp
Pp
P
P
p
p pppP
P
P
p
p pp
Pp
pP
PpPP
a.
1. p + p = pp. 2. P + p = Pp.
3. p + P = pP. 4. P + P = PP.
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p
P
p
PPp
Pp
Pp
Pp
White parent pp
b.
P
P
p
ppp
Pp
PurpleparentPP
PurpleheterozygotePp
Purpleheterozygote Pp
F1 generation
PP
pP
F2 generation 3 Purple:1 White(1PP: 2Pp :1pp )
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Human traits• Some human traits are controlled by a single
gene– Some of these exhibit dominant and recessive
inheritance• Pedigree analysis is used to track
inheritance patterns in families• Dominant pedigree – juvenile glaucoma
– Disease causes degeneration of optic nerve leading to blindness
– Dominant trait appears in every generation
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21
2 3 4 51
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Dominant Pedigree
Generation I
Generation II
Generation III
Key
affected female
affected male
unaffected female
unaffected male
3
• Recessive pedigree – albinism– Condition in which the pigment melanin is not
produced– Pedigree for form of albinism due to a
nonfunctional allele of the enzyme tyrosinase– Males and females affected equally– Most affected individuals have unaffected
parents
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1 2
1 2
1 2
3
3
1 2 3
4
4
5
5 6 7
Recessive Pedigree
Generation I
Generation II
Generation III
Generation IV
Heterozygous
Homozygous recessive
Keymale carrier
female carrieraffected female
affected male
unaffected female
unaffected male
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One of these personsis heterozygous
Mating betweenfirst cousins
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Dihybrid crosses
• Examination of 2 separate traits in a single cross
• Produced true-breeding lines for 2 traits• RRYY x rryy• The F1 generation of a dihybrid cross
(RrYy) shows only the dominant phenotypes for each trait
• Allow F1 to self-fertilize to produce F2
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F1 self-fertilizes•RrYy x RrYy•The F2 generation shows all four possible phenotypes in a set ratio
– 9:3:3:1– R_Y_:R_yy:rrY_:rryy– Round yellow:round green:wrinkled
yellow:wrinkled green
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Cross-fertilization
RY Ry rY ry
Meiosis Meiosis
rr yy
Parent generation
RR YY
Rr Yy
F1 generation
Meiosis(chromosomes assort independently
into four types of gametes)
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RY Ry rY ry
RR yy Rr yy
Rr yy rr yy
9/16
3/16
3/16
1/16
round, yellow
round, green
wrinkled, yellow
wrinkled, green
RY
Ry
rY
ry
F1 X F1 (RrYy X RrYy)
F2 generation
RR YY RR Yy Rr YY Rr Yy
RR Yy Rr Yy
rr Yy
rr Yy
rr YYRr YyRr YY
Rr Yy
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Principle of independent assortment
• In a dihybrid cross, the alleles of each gene assort independently
• The segregation of different allele pairs is independent
• Independent alignment of different homologous chromosome pairs during metaphase I leads to the independent segregation of the different allele pairs
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Probability
• Rule of addition– Probability of 2 mutually exclusive events
occurring simultaneously is the sum of their individual probabilities
• When crossing Pp x Pp, the probability of producing Pp offspring is – probability of obtaining Pp (1/4), PLUS
probability of obtaining pP (1/4)– ¼ + ¼ = ½
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• Rule of multiplication– Probability of 2 independent events occurring
simultaneously is the product of their individual probabilities
• When crossing Pp x Pp, the probability of obtaining pp offspring is– Probability of obtaining p from father = ½ – Probability of obtaining p from mother = ½ – Probability of pp = ½ x ½ = ¼
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Testcross
• Cross used to determine the genotype of an individual with dominant phenotype
• Cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp)
• Phenotypic ratios among offspring are different, depending on the genotype of the unknown parent
35
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P
p
P P
p
p
Heterozygousdominant
Homozygousrecessive
Alternative 2:Half of the offspring are white and the unknown
flower is heterozygous (Pp)
PP or Pp
thenIf Pp
DominantPhenotype(unknowngenotype)
If PPthen
Alternative 1:All offspring are purple and the unknown
flower is homozygous dominant (PP)
Homozygousrecessive
Homozygousdominant
PpPp Pp pp
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Extensions to Mendel
• Mendel’s model of inheritance assumes that– Each trait is controlled by a single gene– Each gene has only 2 alleles– There is a clear dominant-recessive
relationship between the alleles
• Most genes do not meet these criteria
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Polygenic inheritance
• Occurs when multiple genes are involved in controlling the phenotype of a trait
• The phenotype is an accumulation of contributions by multiple genes
• These traits show continuous variation and are referred to as quantitative traits– For example – human height– Histogram shows normal distribution
38
30
20
10
00 5′6″ '6′0″5′0″
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Num
ber o
f Ind
ivid
uals
(top): From Albert F. Blakeslee, “CORN AND MEN: The Interacting Infl uence of Heredity and Environment—Movements for Betterment of Men, or Corn, or Any Other Living Thing, One-sided Unless Th ey Take Both Factors into Account,” Journal of
Heredity, 1914, 5:511-8, by permission of Oxford University Press
Height
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Pleiotropy
• Refers to an allele which has more than one effect on the phenotype
• Pleiotropic effects are difficult to predict, because a gene that affects one trait often performs other, unknown functions
• This can be seen in human diseases such as cystic fibrosis or sickle cell anemia– Multiple symptoms can be traced back to one
defective allele
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Multiple alleles
• May be more than 2 alleles for a gene in a population
• ABO blood types in humans– 3 alleles
• Each individual can only have 2 alleles• Number of alleles possible for any gene is
constrained, but usually more than two alleles exist for any gene in an outbreeding population
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• Incomplete dominance– Heterozygote is intermediate in phenotype
between the 2 homozygotes– Red flowers x white flowers = pink flowers
• Codominance– Heterozygote shows some aspect of the
phenotypes of both homozygotes– Type AB blood
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Parent generation
1 : 2 : 1
CR CW
Cross-fertilization
CWCWCRCR
F1 generation
CRCW
CRCWCRCR
CR
CW
CRCW CWCW
CRCR: CRCW: CWCW
F2 generation
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43
Human ABO blood group
• The system demonstrates both– Multiple alleles
• 3 alleles of the I gene (IA, IB, and i)– Codominance
• IA and IB are dominant to i but codominant to each other
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Alleles
AB
NoneO
GalactosamineA
GalactoseB
BloodType
SugarsExhibited
Donates andReceives
Receives A and ODonates to A and ABReceives B and ODonates to B and ABUniversal receiverDonates to ABReceives OUniversal donor
Both galactose andgalactosamine
IAIA, IAi(IA dominant to i)
IBIB, IBi(IB dominant to i)
IAIB
(codominant)ii
(i is recessive)
Environmental influence• Coat color in
Himalayan rabbits and Siamese cats– Allele
produces an enzyme that allows pigment production only at temperatures below 30oC
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© DK Limited/Corbis
Temperaturebelow33º C, tyrosinaseactive, dark pigment
Temperature above33º C, tyrosinaseinactive, no pigment
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Epistasis
• Behavior of gene products can change the ratio expected by independent assortment, even if the genes are on different chromosomes that do exhibit independent assortment
• R.A. Emerson crossed 2 white varieties of corn– F1 was all purple
– F2 was 9 purple:7 white – not expected
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AB Ab aB ab
AABB AABb AaBB AaBb
AABb AAbb AaBb Aabb
AaBB AaBb aaBB aaBb
AaBb Aabb aaBb aabb
9/16 Purple: 7/16 White
AB
Ab
aB
ab
Cross-fertilization
a.
b.
Parentalgeneration
F1 generation
F2 generation
Pigment(purple)
EnzymeB
EnzymeAPrecursor
(colorless)Intermediate(colorless)
White(aaBB)
White(AAbb)
All Purple(AaBb)