Transcript Slide 1

MENDEL’S LAWS
Copyright © 2009 Pearson Education, Inc.
Intro to Genetics
 Heredity is the transmission of traits from one
generation to the next.
 Genetics is the scientific study of heredity.
 Gregor Mendel
– began the field of genetics in the 1860s,
– deduced the principles of genetics by breeding garden
peas, and
– relied upon a background of mathematics, physics, and
chemistry.
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Intro to Genetics
 Gregor Mendel discovered principles of genetics in
experiments with the garden pea
– Mendel showed that parents pass heritable factors to
offspring (heritable factors are now called genes)
– Advantages of using pea plants
– Controlled matings
– Self-fertilization or cross-fertilization
– Observable characteristics with two distinct forms
– True-breeding strains
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Intro to Genetics
 True-breeding varieties result when self-fertilization
produces offspring all identical to the parent.
 The offspring of two different varieties are hybrids.
 The cross-fertilization is a hybridization, or genetic
cross.
 True-breeding parental plants are the P generation.
 Hybrid offspring are the F1 generation.
 A cross of F1 plants produces an F2 generation.
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Figure 9.2D
Traits
Character
Dominant
Recessive
Flower color
Purple
White
Axial
Terminal
Yellow
Green
Round
Wrinkled
Inflated
Constricted
Green
Yellow
Tall
Dwarf
Flower position
Seed color
Seed shape
Pod shape
Pod color
Stem length
Mendel’s law of segregation describes the
inheritance of a single character
 Example of a monohybrid cross
– Parental generation: purple flowers  white flowers
– F1 generation: all plants with purple flowers
– F2 generation: 3/4 of plants with purple flowers
1/4 of plants with white flowers
 Mendel needed to explain
– Why one trait seemed to disappear in the F1
generation
– Why that trait reappeared in one quarter of the F2
offspring
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Figure 9.3B_s3
The Explanation
P generation
Genetic makeup (alleles)
White flowers
Purple flowers
PP
pp
Gametes All P
All p
F1 generation
(hybrids)
All Pp
Gametes
1
2
P
Alleles
segregate
1
2
p
Fertilization
Sperm from F1 plant
F2 generation
P
Phenotypic ratio
3 purple : 1 white
P
Eggs
from F1
plant
Genotypic ratio
p
1 PP : 2 Pp : 1 pp
p
PP
Pp
Pp
pp
Mendel’s law of segregation describes the
inheritance of a single character
 Genes are found in alternative versions called
alleles
 a genotype is the listing of alleles an individual
carries for a specific gene
 For each characteristic, an organism inherits two
alleles, one from each parent; the alleles can be
the same or different
– A homozygous genotype has identical alleles
– A heterozygous genotype has two different alleles
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Gene loci
Genotype:
Dominant
allele
P
a
B
P
a
b
Recessive
allele
Bb
PP
aa
Homozygous
Heterozygous
Homozygous
for the
for the
dominant allele recessive allele
Mendel’s law of segregation describes the
inheritance of a single character
 If heterozygous, the dominant allele
determines the organism’s appearance, and the
recessive allele has no noticeable effect
– The phenotype is the appearance or expression of a trait
– The same phenotype may be determined by more than
one genotype
 Law of segregation: Allele pairs separate
(segregate) from each other during the
production of gametes so that a sperm or
egg carries only one allele for each gene
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The law of independent assortment is revealed
by tracking two characters at once
 Example of a dihybrid cross
– Parental generation: round yellow seeds  wrinkled green
seeds
– F1 generation: all plants with round yellow seeds
– F2 generation: 9/16 of
3/16 of
3/16 of
1/16 of
plants
plants
plants
plants
with
with
with
with
round yellow seeds
round green seeds
wrinkled yellow seeds
wrinkled green seeds
 Mendel needed to explain
– Why nonparental combinations were observed
– Why a 9:3:3:1 ratio was observed among the F2 offspring
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The law of independent assortment is revealed
by tracking two characters at once
 Law of independent assortment
– Each pair of alleles segregates independently of the
other pairs of alleles during gamete formation
– For genotype RrYy, four gamete types are possible:
RY, Ry, rY, and ry
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Hypothesis: Independent assortment
Hypothesis: Dependent assortment
P
generation
rryy
RRYY
ry
Gametes RY
F1
generation
rryy
RRYY
ry
Gametes RY
RrYy
RrYy
Sperm
Sperm
1
–
2
F2
generation
1
–
2
RY
1
–
2
1
–
4
ry
1
–
4
RY
Eggs
1
–
2
RY
1
–
4
ry
Hypothesized
(not actually seen)
1
–
4
rY
1
–
4
Ry
1
–
4
ry
RY
RRYY
RrYY
RRYy
RrYy
RrYY
rrYY
RrYy
rrYy
rY
Eggs
1
–
4
1
–
4
9
––
16
Ry
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
ry
Actual results
(support hypothesis)
3
––
16
3
––
16
1
––
16
Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled
Chromosome separation during meiosis accounts
for Mendel’s laws
 Mendel’s Laws correlate with chromosome
separation in meiosis
– The law of segregation depends on separation of
homologous chromosomes in anaphase I
– The law of independent assortment depends on
alternative orientations of chromosomes in
metaphase I
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F1 generation
All round yellow seeds
(RrYy)
R
r
y
Y
r
R
y
Y
R
Y
y
Y
y
R
R
Y
y
Anaphase I
of meiosis
r
Y
R
r
R
Y
Metaphase I
of meiosis
(alternative
arrangements)
r
Metaphase II
of meiosis
r
Y
y
r
R
Y
y
y
Y
Y
r
r
r
1
– ry
4
1
– rY
4
Fertilization among the F1 plants
F2 generation
R
Gametes
y
1
– RY
4
r
9
:3
:3
:1
y
y
R
R
1
–
4
Ry
Geneticists use the testcross to determine
unknown genotypes
 Testcross
– Mating between an individual of unknown genotype
and a homozygous recessive individual
– Will show whether the unknown genotype includes a
recessive allele
– Used by Mendel to confirm true-breeding genotypes
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Testcross:
B_
Genotypes
bb
Two possibilities for the black dog:
BB
B
Gametes
b
Offspring
Bb
or
Bb
All black
b
B
b
Bb
bb
1 black : 1 chocolate
Mendel’s laws reflect the rules of probability
 The probability of a specific event is the number
of ways that event can occur out of the total
possible outcomes.
 Rule of multiplication
– Multiply the probabilities of events that must occur
together
 Rule of addition
– Add probabilities of events that can happen in
alternate ways
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F1 genotypes
Bb male
Formation of sperm
Bb female
Formation of eggs
1
–
2
1
–
2
1
–
2
B
B
B
b
B
B
1
–
4
1
–
4
1
–
2
b
b
B
1
–
4
F2 genotypes
b
b
b
1
–
4
VARIATIONS ON MENDEL’S
LAWS
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Incomplete dominance results in intermediate
phenotypes
 Incomplete dominance
– Neither allele is dominant over the other
– Expression of both alleles is observed as an intermediate
phenotype in the heterozygous individual
 Codominance
– Neither allele is dominant over the other
– Expression of both alleles is observed as a distinct phenotype
in the heterozygous individual
– Observed for type AB blood
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P generation
Red
RR
White
rr
r
R
Gametes
F1 generation
Incomplete
Dominance
in snapdragon flower
color
Pink
Rr
Gametes
1
–
2
R
1
–
2
r
Sperm
1
–
2
F2 generation
R
1
–
2
r
1
–
2
R
RR
rR
1
–
2
r
Rr
rr
Eggs
Blood
Group
(Phenotype) Genotypes
Red Blood Cells
O
ii
A
IAIA
or
IAi
Carbohydrate A
B
IBIB
or
IBi
Carbohydrate B
CoDominance of AB Blood Type
AB
IAIB
Many genes have more than two alleles in the
population
 Multiple alleles
– More than two alleles are found in the population
– A diploid individual can carry any two of these alleles
– The ABO blood group has three alleles, leading to four
phenotypes: type A, type B, type AB, and type O
blood
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Blood
Group
(Phenotype) Genotypes
Red Blood Cells
Antibodies
Present in
Blood
Anti-A
Anti-B
O
ii
A
I AI A
or
I Ai
Carbohydrate A
Anti-B
B
IBIB
or
IBi
Carbohydrate B
Anti-A
AB
IAIB
—
Reaction When Blood from Groups Below Is Mixed
with Antibodies from Groups at Left
O
A
B
AB
A single character may be influenced by many
genes
 Polygenic inheritance
– Many genes influence one trait
– Skin color is affected by at least three genes
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Figure 9.14
P generation

aabbcc
AABBCC
(very light) (very dark)
F1 generation

AaBbCc AaBbCc
Sperm
1
8
F2 generation
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
Fraction of population
Eggs
1
8
1
8
1
8
1
8
1
8
1
64
6
64
15
64
20
64
15
64
6
64
1
64
Skin color
Fraction of population
20
––
64
15
––
64
6
––
64
1
––
64
Skin color
SEX CHROMOSOMES AND
SEX-LINKED GENES
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9.21 Sex-linked genes exhibit a unique pattern of
inheritance
 Sex-linked genes are located on either of the
sex chromosomes
– Reciprocal crosses show different results
– White-eyed female  red-eyed male
and white-eyed males
red-eyed females
– Red-eyed female  white-eyed male
and red-eyed males
red-eyed females
– X-linked genes are passed from mother to son and
mother to daughter
– X-linked genes are passed from father to daughter
– Y-linked genes are passed from father to son
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Female
Male
Female
Male
XR Xr
Xr Y
Xr Y
XR XR
Sperm
Xr
Y
Eggs XR
XR Xr
XR Y
Sperm
XR
Xr
Y
XR XR
XR Y
Xr Xr
Xr Y
Eggs
R = red-eye allele
r = white-eye allele
Xr
Female
Male
XR Xr
XR Y
Sperm
XR
Y
XR
XR XR
XR Y
Xr
Xr XR
Xr Y
Eggs
Pedigree Analysis
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9.8 CONNECTION: Genetic traits in humans can
be tracked through family pedigrees
 A pedigree
– Shows the inheritance of a trait in a family through
multiple generations
– Demonstrates dominant or recessive inheritance
– Can also be used to deduce genotypes of family
members
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Dominant Traits
Recessive Traits
Freckles
No freckles
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe
Figure 9.8B
First generation
(grandparents)
Second generation
(parents, aunts,
and uncles)FF
or
Ff
Third generation
(two sisters)
Female Male
Attached
Free
Ff
Ff
ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
9.9 CONNECTION: Many inherited disorders in
humans are controlled by a single gene
 Inherited human disorders show
– Recessive inheritance
– Two recessive alleles are needed to show disease
– Heterozygous parents are carriers of the disease-causing
allele
– Probability of inheritance increases with inbreeding,
mating between close relatives
– Dominant inheritance
– One dominant allele is needed to show disease
– Dominant lethal alleles are usually eliminated from the
population
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Sex-linked disorders affect mostly males
 Males express X-linked disorders such as the
following when recessive alleles are present in
one copy
– Hemophilia
– Colorblindness
– Duchenne muscular dystrophy
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Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Gene Linkage
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9.17 Genes on the same chromosome tend to be
inherited together
 Linked Genes
– Are located close together on the same chromosome
– Tend to be inherited together
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Figure 9.18C
The Explanation
The Experiment
Gray body,
long wings
(wild type)
Black body,
vestigial wings
GgLl
Female
Male
Female
gl
gl
gl
ggll
Male
Crossing over
ggll
GgLl
GL
GL
Gl
g l
gl
gL
Eggs
Sperm
Offspring
Gray long
Black vestigial Gray vestigial Black long
Offspring
GL
g l
G l
g L
g l
g l
g l
g l
Parental
965
944
Parental
phenotypes
206
185
Recombinant
phenotypes
Recombination frequency  391 recombinants
2,300 total offspring
 0.17 or 17%
Recombinant
9.18 Crossing over produces new combinations
of alleles
 Linked alleles can be separated by crossing over
– Recombinant chromosomes are formed
– Thomas Hunt Morgan demonstrated this in early
experiments
– Geneticists measure genetic distance by
recombination frequency
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Figure 9.18A
p L
p l
PL
Parental gametes
pl
p L
Tetrad
(pair of
homologous
chromosomes)
P l
Crossing over
Recombinant gametes
9.19 Geneticists use crossover data to map genes
 Genetic maps
– Show the order of genes on chromosomes
– Arrange genes into linkage groups representing
individual chromosomes
Copyright © 2009 Pearson Education, Inc.
Figure 9.19A
Section of chromosome carrying linked genes
g
c
l
17%
9%
9.5%
Recombination
frequencies