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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?
3
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
7
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
11
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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
14
• 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
17
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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.
18
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
34
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
35
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
36
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
37
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
39
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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:
40
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
41
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
42
Environmental influence
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• 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
43
© 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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