23.1 Mendel’s Laws Chapter 23 Gregor Mendel Patterns of ...
Transcript of 23.1 Mendel’s Laws Chapter 23 Gregor Mendel Patterns of ...
1
Chapter 23Patterns of Gene
Inheritance
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23.1 Mendel’s Laws
• Gregor Mendel
– Investigated
inheritance at the
organism level
(1860’s)
– Concluded that plants
transmit distinct
factors to offspring
• Now called genes found
on chromosomes
23.1 Mendel’s Laws
• In humans, chromosomes come in pairs called
homologous chromosomes
• One member of the pair is inherited from the
mother, while the other member is inherited from
the father
• Homologous chromosomes
– Both members have same length and centromere
location
– Both carry the same genes for the same traits in the
same order
– Alleles – alternate forms of a gene
Homologous ChromosomesCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
G
R
S
t
g
r
s
T
alleles at a gene locus
23.1 Mendel’s Laws
• The Law of Segregation
– Each individual has two factors (alleles) for
each trait
– The factors segregate (separate) during the
formation of gametes
– Each gamete contains only one factor from
each pair of factors
– Fertilization gives each new individual two
factors for each trait
23.1 Mendel’s Laws
• The Inheritance of a Single Trait
– Phenotype: Individual’s actual appearance
– Genotype: Alleles the chromosomes carry
that are responsible for a given trait
• Two alleles for a trait, one on each chromosome
• A capital letter symbolizes a dominant allele (W)
• A lowercase letter symbolizes a recessive allele
(w)
• Dominant refers to the allele that will mask the
expression of the alternate (recessive) allele
23.1 Mendel’s Laws 23.1 Mendel’s Laws
• Gamete Formation
– During meiosis, homologous chromosomes separate
so there is only one member of each pair in a gamete
– There is one allele for each trait, such as hairline, in
each gamete
– If parental genotype is Ww, then gametes from this
individual will contain either a W or a w
23.1 Mendel’s Laws
• One-Trait Cross
– A homozygous
man with a
widow’s peak
reproduces with a
woman with a
straight hairline
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gametes
meiosis
Offspring
!Parents
W w
widow’s peak
WW
Ww
widow’s peak
straight hairline
ww
23.1 Mendel’s Laws
• One-Trait Cross
– Two individuals who
are both Ww
– A Punnett Square is
useful to solve this
problem
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3
Straight hairline
Phenotypic Ratio
Straight hairline
Key
oocytes
sp
erm
Parents
!Ww Ww
WW Ww
Ww ww
wW
W
w
Offspring
Widow’s peak
Widow’s peak
W = Widow’s peak
w = Straight hairline
1
! "
"!
23.1 Mendel’s Laws
• One-Trait Crosses and Probability– The chance of two or more independent events
occurring together is the product of their chance of
occurring separately
– In the cross Ww X Ww, what is the chance of
obtaining either a W or a w from a parent?
• Chance of W = ! and the chance of w = !
– Probability of having these genotypes is as follows
1.Chance of WW = ! X ! = "
2.Chance of Ww = ! X ! = "
3.Chance of wW = ! X ! = "
4.Chance of ww = ! X ! = "
23.1 Mendel’s Laws
• The One-Trait Testcross
– Breeders of plants and animals may do a test
cross to determine the likely genotype of an
individual with the dominant phenotype
• Cross with a recessive individual (has a known
genotype)
• If there are any offspring produced with the
recessive phenotype, then the dominant parent
must be heterozygous
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a.
All
Phenotypic Ratio
Keyoocytes
sp
erm
Parents
!WW ww
Ww Ww
Ww Ww
ww
W
W
Offspring
W = Widow’s peakw = Straight hairline
Widow’s peak
Straight hairline
Widow’s peak
"!
! "
b.
1
Straight hairline
Phenotypic Ratio
Straight hairline
oocytes
sp
erm
Parents
Ww ww
Ww Ww
wwww
ww
W
w
Offspring
!
W = Widow’s peakw = Straight hairline
Widow’s peak
Widow’s peak
1
Key
"!
! "
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23.1 Mendel’s Laws
• The Inheritance of Two Traits
– The Law of Independent Assortment
• Each pair of factors (alleles) assorts
independently (without regard to how the
others separate)
• All possible combinations of factors can
occur in the gametes
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MEIOSIS I
MEIOSIS II
either or
one pair
one pair
S
S
S
W
W
W
s
s
s
w w
w
W Ww w
W W
W W W W
sWSwswSW
w w w w
WW w ww w
s s
s ss
s s s s
s
S S
S
S S S S
SS S
Allele Key
W = Widow’s peakw = Straight hairline
S = Short fingerss = Long fingers
Cell has
two pairs of
homologues.
The Inheritance of Two Traits
23.1 Mendel’s Laws
• Two-Trait Crosses (Dihybrid Cross)
• WwSs (X) WwSs
–Phenotypic Ratio:
• 9 widow’s peak, short fingers
• 3 widow’s peak, long fingers
• 3 straight hairline, short fingers
• 1 straight hairline, long fingers
• 9:3:3:1 phenotypic ratio is always expectedfor a dihybrid cross when simpledominance is present
Two-Trait Crosses (Dihybrid Cross)
!
wsWS
wwssWWSS
WwSs
P generation
P gametes
F1 generation
F1 gametes
F2 generation
Allele Key Phenotypic Ratio
9
3
Straight hairline, short fingers
Straight hairline, long fingers
Widow’s peak, short fingers
Widow’s peak, long fingers
31
W = Widow’s peak
w = Straight hairline
S = Short fingers
s = Long fingers
"!
WwSs
WWSS
Offspring
oocytes
sp
erm
WS Ws
WS
Ws
ws
wS
wS ws
WWSs WwSS WwSs
WWSs Wwss
WwSS wwSS wwSs
WwSs Wwss wwSs
WWss
wwSs
WwSS
"
!
23.1 Mendel’s Laws
• Two-Trait Crosses and Probability– Probability Laws
• Probability of widow’s peak = #
• Probability of short fingers= #
• Probability of straight hairline= "
• Probability of long fingers= "
– Using the Product Rule
• Probability of widow’s peak and short fingers = # X # = 9/16
• Probability of widow’s peak and long fingers = # X " = 3/16
• Probability of straight hairline and short fingers = " X # = 3/16
• Probability of straight hairline and long fingers = " X " = 1/16
23.2 Pedigree Analysis
and Genetic Disorders
• Pedigree
– A chart of family’s history with regard to a
particular genetic trait
• Pedigree for an autosomal disorder
– Decide if inherited condition due to autosomal
dominant or an autosomal recessive allele by
studying a pedigree
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Pattern I
Key
= affected
= unaffected
23.2 Pedigree Analysis
and Genetic Disorders
• In this pattern, the child is affected, but neither parent is
• This can happen only if the disorder is recessive and the
parents are heterozygotes
• Notice that the parents are carriers because they are
unaffected but are capable of having a child with the
genetic disorder
Autosomal Recessive PedigreeCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Heterozygotes (Aa) have an unaffected phenotype.
Both males and females are affected with equal frequency.
•
•
•
•
•
•
aa aa
Aa Aa
Aa
A? A?
A?
Aa
*
aa A?
A?A?
Autosomal recessive disorders
Key
aa = affected
Aa = carrier (unaffected)
AA = unaffected
A? =
I
II
III
IV
Relatives
unaffected
(one allele unknown)Most affected children have unaffected
parents.
Two affected parents will always have affected children.
Affected individuals with homozygous unaffected mates will have
unaffected children.
Close relatives who reproduce are more likely to have
affected children.
"!
23.2 Pedigree Analysis
and Genetic DisordersCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Pattern II
Key
= affected
= unaffected
• In this pattern, the child is unaffected, but the parents are
both affected
• This can happen if the condition is autosomal dominant
and the parents are heterozygotes
• This pedigree also illustrates that when both parents are
unaffected, all their children are unaffected
– Neither parent has a dominant gene that passes the condition on
Autosomal Dominant PedigreeCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Aa
aa aa aa
aaaaaaaaAa
aa
Aa
Aa A?
Aa
Autosomal dominant disorders
*
I
II
III
•
• Heterozygotes (Aa) are affected.
•
•
• Both males and females are affected with equal frequency.
Key
= affected
Aa = affected
A?
(one allele unknown)
= affected
AA
aa = unaffectedAffected children will usually have
an affected parent.
Two affected parents can produce an unaffected child.
Two unaffected parents will not have affected children.
" !
23.2 Pedigree Analysis
and Genetic Disorders
• Autosomal Recessive Disorders
– Cystic Fibrosis (CF)
• Most common lethal genetic disorder among Caucasians in
the United States
• Chloride ions fail to pass through a plasma membrane
channel protein in cells
• Causes abnormally thick mucus in bronchial tubes and
pancreatic ducts
– Sickle cell disease
• Red blood cells are sickle shaped due to abnormal
hemoglobin
• Differs from normal hemoglobin by one amino acid
23.3 Beyond Simple Inheritance
Patterns
• Incomplete Dominance
– Occurs when the heterozygote is intermediate
between the two homozygotes
• Codominance
– Occurs when alleles are equally expressed in a
heterozygote
– Blood type AB is an example
• Red blood cells have both Type A and Type B surface
antigens
Offspring
oocytes
sp
erm
!Straight hair
Curly hair
Key
1
2
1
Phenotypic Ratio
A person with naturally curly
hair (H2H2)
A person with straight hair
(H1H1)
Heterozygous
Parents (H1 H2)
H1 H2 H1 H2
H1 H1
H1 H2 H2 H2
H1 H2H1
H2
H1 H2
Wavy hair
"!
"!
Incomplete Dominance 23.3 Beyond Simple Inheritance
Patterns
• Multiple Allele Inheritance
– A trait is controlled by multiple alleles, the
gene exists in several allelic forms
• Each person has only two of the possible alleles
23.3 Beyond Simple Inheritance
Patterns
• Multiple Allele Inheritance
– ABO Blood Types
• IA = A antigens on red blood cells
• IB = B antigens on red blood cells
• i = has neither A nor B antigens on red blood cells
• Both IA and IB are dominant over i, IA and IB are codominant
Phenotype Genotype
A IAIA or IAi
B IBIB or IBi
AB IAIB
O ii
Inheritance of Blood Types
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Key
1
1
1
1
Phenotypic Ratio
Offspring
oocytes
sp
erm
Parents
!
IAi ii
iIA
IB
i
Blood type A
Blood type B
Blood type AB
Blood type O
IBi IAi
IAIB IBi
"!
"!
23.3 Beyond Simple Inheritance
Patterns
• Polygenic Inheritance
– Occurs when a trait is governed by two or
more genes (sets of alleles)
– Dominant alleles have a quantitative effect on
the phenotype, and these effects are additive
– Skin color
Polygenic Inheritance
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Nu
mb
er
of
Peo
ple
Skin Color
23.4 Environmental Influences
• Environmental factors can influence the
expression of genetic traits
– In the case of height, differences in nutrition are one
of the factors that bring about a bell-shaped curve
Coat Color in Himalayan Rabbits
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© Jane Burton/Bruce Coleman, Inc.
Temperature can also affect the phenotypes of plants and animals