Transcript Document

Patterns of Inheritance
Chapter 12
1
Terms
• Allele – variations of a gene
• Every individual has 2 alleles for the same gene
• On homologous chromosomes (1came from mom, 1 came
from dad)
• Homozygous – same allele
• Example – BB or bb
• Heterozygous – different alleles
• Example - Bb
• Dominant – trait that will be expressed
• Example – B
• Recessive – trait that will only be expressed when in homozygous
form
• Example - b
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Mystery of heredity
• Before the 20th century we knew:
– Heredity occurs within species
– Traits are transmitted directly from parent to
offspring
• Heredity thought to be fluid and blended
• Problem: If blending occurs why don’t all
individuals look alike?
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Gregor Mendel
Chose to study heredity in pea plants because:
1. Other research showed that pea hybrids could
be produced
2. Many pea varieties were available
3. Peas are small plants and easy to grow
4. Peas can self-fertilize or be cross-fertilized
1.Self-fertilization – male and female parts on same flower so
there will be self fertilization if flower not disturbed
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• Mendel's Method:
1. Produce true-breeding strains for each trait
• Example – pea plants with all purple flowers
• OR
• Pea plants with all white flowers
2. Cross-fertilize true-breeding strains
• Example – cross purple flower pea plant with white flower
pea plant
3. Allow the hybrid offspring to self-fertilize for several generations…
…Then count the number of offspring showing each form of the trait
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6
Monohybrid crosses
• Cross to study 2 variations of a single trait
• Example – pea color (yellow or green)
• Mendel produced true-breeding pea
strains for 7 different traits
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Mendel's 7 pea traits
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Dominant
Recessive
F2 Generation
Dominant
Recessive
F2 Generation
5. Pod Shape
1. Flower Color
882 Inflated:
299 Constricted
705 Purple:
224 White
X
X
2.95:1
3.15:1
Purple
Inflated
White
Constricted
6. Flower Position
2. Seed Color
6022 Yellow:
2001 Green
651 Axial:
207 Terminal
X
X
3.01:1
Yellow
3.14:1
Green
Axial
Terminal
3. Seed Texture
7. Plant Height
5474 Round:
1850 Wrinkled
787 T all:
277 Short
X
2.96:1
Round
X
Wrinkled
2.84:1
Tall
4. Pod Color
Short
428 Green:
152 Yellow
X
2.82:1
Green
Yellow
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F1 generation
• First filial generation
• Offspring produced by crossing 2 truebreeding strains
• Example – cross pea plant that has green peas
with one that has all yellow peas
• For every trait Mendel studied, all F1
plants resembled only 1 parent
– Referred to the trait shown in F1 as dominant
– Alternative trait was recessive (hidden)
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F2 generation
• Offspring resulting from the self-fertilization of F1 plants
• The recessive trait had reappeared among some F2
individuals
• Counted proportions of traits
– Always found about 3:1 ratio
• (3 dominant to 1 recessive)
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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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Truebreeding
Purple
Parent
Truebreeding
White
Parent
Parent generation
Cross-fertilize
Purple
Offspring
F1 generation
Self-cross
Purple
Dominant
Purple
Dominant
Purple
Dominant
White
Recessive
Truebreeding
Non-truebreeding
Non-truebreeding
Truebreeding
Self-cross
Self-cross
Self-cross
Self-cross
F2 generation
(3:1 phenotypic
ratio)
F3 generation
(1:2:1 genotypic
ratio)
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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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This results in Mendel's model:
1.
Parents transmit discrete factors (genes)
2.
Each individual receives one copy of a gene from each parent (total per
indiv.= 2 copies)
3.
Not all copies of a gene are identical
– Different versions of a gene are different ALLELES of that gene
• Homozygous – 2 of the same allele
• Heterozygous – different alleles
Alleles remain discrete – no blending
4.
5.
Presence of allele does not guarantee expression
•
If Dominant allele – expressed
•
If Recessive allele – can be hidden by dominant allele
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• Genotype – The set of alleles an
individual contains
• Phenotype – physical appearance
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Principle of Segregation
• Two alleles for a gene separate 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
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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
1. p + p = pp.
P
P
p
pP
3. p + P = pP.
a.
P
p
P
Pp
p
pp
2. P + p = Pp.
p
P
p
Pp
P
PP
Pp
pp
p
pP
pp
4. P + P = PP.
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White parent pp
p
p
Purple
parent
PP
P
Pp
Pp
Pp
Pp
P
F1 generation
Purple
heterozygote Pp
p
P
Purple
heterozygote
Pp
P
PP
Pp
pP
pp
p
F2 generation 3 Purple:1 White
(1PP: 2Pp :1pp )
b.
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• This type of inheritance pattern is SIMPLE DOMINANCE
– One primary gene controls phenotype
– Dominant or recessive alleles
• Pedigree analysis is used to track inheritance patterns in
Human families
• Example: juvenile glaucoma (dominant disease)
– Disease causes degeneration of optic nerve leading to
blindness
– Dominant trait appears in every generation
• Unless completely removed from a lineage
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21
• Example of a Recessive disease 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 (autosomal)
– Most affected individuals have unaffected parents
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Dihybrid crosses
• Examination of 2 separate traits in a single cross
• To test if the 2 traits are connected
• First, Mendel bred 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
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The F1 self-fertilizes
• RrYy x RrYy
• The F2 generation shows all four possible
phenotypes in a set ratio
– 9:3:3:1
R is Round (dominant)
– 9 Round Yellow
– 3 Round green
– 3 wrinkled Yellow
– 1 wrinkled green
r is wrinkled (recessive)
Y is Yellow (dominant)
y is green (recessive)
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Principle of independent assortment
• In a dihybrid cross, the alleles of two
different genes assort independently of
one another
• 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 either of 2 exclusive events
occurring is the sum of their individual
probabilities
• The probability of getting this OR that.
• When crossing Pp x Pp, the probability of
producing PP or 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 – most useful in genetics
– Probability of 2 independent events occurring
simultaneously is the product of their individual
probabilities
• The probablility of getting this AND that.
• 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
– But the genotype is unknown (Pp or PP?)
• 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
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unknown parent
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Homozygous
dominant
Homozygous
P
recessive
Dominant
Phenotype
(unknown
genotype)
P
p
Pp
Pp
Alternative 1:
All offspring are purple and the unknown
flower is homozygous dominant (PP)
Heterozygous
dominant
Homozygous
recessive
If PP
If Pp
then
then
PP or Pp
P
p
p
Pp
pp
Alternative 2:
Half of the offspring are white and the unknown
flower is heterozygous (Pp)
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Extensions to Mendel
• Mendel’s model of inheritance assumes
SIMPLE DOMINANCE
– Each trait is controlled by a single gene
– Each gene has only 2 alleles
– There is a clear dominant-recessive
relationship between the alleles
• However, 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
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Number of Individuals
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30
20
10
0
0
5′0″
5′6″
Height
6′0″
'
(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
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Pleiotropy
• Refers to an allele which can effect more than one
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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CRCR
CWCW
Parent generation
Cross-fertilization
CRCW
F1 generation
CR
CW
CRCR
CRCW
CRCW
CWCW
CR
F2 generation
CW
1:2:1
CRCW: CWCW
CRCR:
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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
IAIA, IAi
(IA dominant to i)
IBIB, IBi
(IB dominant to i)
IAIB
(codominant)
ii
(i is recessive)
Blood
Type
Sugars
Exhibited
Donates and
Receives
A
Galactosamine
Receives A and O
Donates to A and AB
B
Galactose
Receives B and O
Donates to B and AB
Both galactose and
galactosamine
Universal receiver
Donates to AB
None
Receives O
Universal donor
AB
O
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Environmental influence
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• Coat color in
Himalayan
rabbits and
Siamese cats
– Allele
produces an
enzyme that
allows
pigment
production
only at
temperatures
below 30oC
Temperaturebelow
33º C, tyrosinase
active, dark pigment
Temperature above
33º C, tyrosinase
inactive, no pigment
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© DK Limited/Corbis
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
• One gene has effect on another gene if it’s
present
• Coat color in Labrador retreivers
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