Transcript File

Mendelian
Genetics
In honor of Mendel’s 189th birthday Google used this as their
header on July 20, 2011
1
Gregor Mendel
(1822-1884)
Responsible
for the Laws
governing
Inheritance of
Traits
2
Gregor Johann Mendel
Austrian monk
Studied the
inheritance of
traits in pea plants
Developed the laws
of inheritance
Mendel's work was
not recognized until
the turn of the
20th century
3
Gregor Johann Mendel
Between 1856 and
1863, Mendel
cultivated and
tested some 28,000
pea plants
He found that the
plants' offspring
retained traits of
the parents
Called the “Father
of Genetics"
4
Mendel’s Pea Plant
Experiments
5
Site of
Gregor
Mendel’s
experimental
garden in the
Czech
Republic
6
Why peas, Pisum sativum?
Can be grown in a
small area
Produce lots of
offspring
Produce pure plants
when allowed to
self-pollinate
several generations
Can be artificially
cross-pollinated
7
Reproduction in Flowering Plants
Pollen contains sperm
Produced by the
stamen
Ovary contains eggs
Found inside the
flower
Pollen carries sperm to the
eggs for fertilization
Self-fertilization can
occur in the same flower
Cross-fertilization can
occur between flowers
8
Mendel’s Experimental
Methods
Mendel hand-pollinated
flowers using a
paintbrush
He could snip the
stamens to prevent
self-pollination
He traced traits
through the several
generations
9
How Mendel Began
Mendel
produced
pure
strains by
allowing the
plants to
selfpollinate
for several
generations
10
11
Crossing pea plants
1
Removed stamens
from purple flower
2 Transferred sperm-
bearing pollen from
stamens of white
flower to eggbearing carpel of
purple flower
By crossing (mating) two true-breeding
varieties of an organism, scientists can study patterns of
inheritance. In this example, Mendel crossed pea plants
that varied in flower color.
APPLICATION
TECHNIQUE
Parental
generation
(P)
3 Pollinated carpel
Stamens
Carpel (male)
(female)
matured into pod
4 Planted seeds
from pod
RESULTS
When pollen from a white flower fertilizes
eggs of a purple flower, the first-generation hybrids all
have purple flowers. The result is the same for the
reciprocal cross, the transfer of pollen from purple flowers
to white flowers.
Figure 14.2
5 Examined
First
generation
offspring
(F1)
offspring:
all purple
flowers
Eight Pea Plant Traits
Seed shape --- Round (R) or Wrinkled (r)
Seed Color ---- Yellow (Y) or Green (y)
Pod Shape --- Smooth (S) or wrinkled (s)
Pod Color --- Green (G) or Yellow (g)
Seed Coat Color ---Gray (G) or White (g)
Flower position---Axial (A) or Terminal (a)
Plant Height --- Tall (T) or Short (t)
Flower color --- Purple (P) or white (p)
13
14
15
What Do the Peas Look Like?
16
Punnett Square
Used to help
solve genetics
problems
17
18
How to make a Punnett Square: Rr x rr
19
20
Genetic Practice
Problems
21
Breed the P1 generation
tall (TT) x dwarf (tt) pea plants
T
T
t
t
22
Solution:
tall (TT) vs. dwarf (tt) pea plants
T
T
t
Tt
Tt
produces the
F1 generation
t
Tt
Tt
All Tt = tall
(heterozygous tall)
23
Breed the F1 generation
tall (Tt) vs. tall (Tt) pea plants
T
t
T
t
24
Solution:
tall (Tt) x tall (Tt) pea plants
T
t
T
TT
Tt
t
Tt
tt
produces the
F2 generation
1/4 (25%) = TT
1/2 (50%) = Tt
1/4 (25%) = tt
1:2:1 genotype
3:1 phenotype
25
Genetic Terminology
 Trait - any characteristic that
can be passed from parent to
offspring
 Heredity - passing of traits
from parent to offspring
 Genetics - study of heredity
26
Following the Generations
Cross 2
Pure
Plants
TT x tt
Results
in all
Hybrids
Tt
Cross 2 Hybrids
get
3 Tall & 1 Short
TT, Tt, tt
27
28
Did the observed ratio match
the theoretical ratio?
The theoretical or expected ratio of
plants producing round or wrinkled seeds
is 3 round :1 wrinkled
Mendel’s observed ratio was 2.96:1
The discrepancy is due to statistical
error
The larger the sample the more nearly
the results approximate to the
theoretical ratio
29
Generation “Gap”
Parental P1 Generation = the parental
generation in a breeding experiment.
F1 generation = the first-generation
offspring in a breeding experiment. (1st
filial generation)
From breeding individuals from the P1
generation
F2 generation = the second-generation
offspring in a breeding experiment.
(2nd filial generation)
From breeding individuals from the F1
generation
30
Particulate Inheritance
Mendel stated that
physical traits are
inherited as “particles”
Mendel did not know
that the “particles”
were actually
Chromosomes & DNA
31
Designer “Genes”
 Alleles - two forms of a gene
(dominant & recessive)
 Dominant - stronger of two genes
expressed in the hybrid;
represented by a capital letter (R)
 Recessive - gene that shows up less
often in a cross; represented by a
lowercase letter (r)
32
More Terminology
 Genotype - gene combination
for a trait (e.g. RR, Rr, rr)
 Phenotype - the physical
feature resulting from a
genotype (e.g. red, white)
33
Genotype & Phenotype in Flowers
Genotype of alleles:
R = red flower
r = yellow flower
All genes occur in pairs, so 2
alleles affect a characteristic
Possible combinations are:
Genotypes
RR
Rr
rr
Phenotypes
RED
RED
YELLOW
34
Genotypes
 Homozygous genotype - gene
combination involving 2 dominant
or 2 recessive genes (e.g. RR or
rr); also called purebred
 Heterozygous genotype - gene
combination of one dominant &
one recessive allele
(e.g. Rr);
also called hybrid
35
Phenotype vs. Genotype
ure 14.6
Phenotype
Purple
3
Purple
Genotype
PP
(homozygous)
1
Pp
(heterozygous)
2
Pp
(heterozygous)
Purple
1
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
1
36
Genes and Environment
Determine Characteristics
37
Types of Genetic Crosses
 Monohybrid cross - cross
involving a single trait
e.g. flower color
 Dihybrid cross - cross involving
two traits
e.g. flower color & plant height
38
Monohybrid
Crosses
39
P1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Wrinkled seeds
RR
x
rr
r
r
R
Rr
Rr
R
Rr
Rr
Genotype: Rr
Phenotype: Round
Genotypic
Ratio: All alike
Phenotypic
Ratio: All alike
40
P1 Monohybrid Cross Review
 Homozygous dominant x Homozygous
recessive
 Offspring all Heterozygous
(hybrids)
 Offspring called F1 generation
 Genotypic & Phenotypic ratio is ALL
ALIKE
41
F1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Round seeds
Rr
x
Rr
R
r
R
RR
Rr
r
Rr
rr
Genotype: RR, Rr, rr
Phenotype: Round &
wrinkled
G.Ratio: 1:2:1
P.Ratio: 3:1
42
Results of Monohybrid Crosses
Inheritable factors or genes are
responsible for all heritable
characteristics
Phenotype is based on Genotype
Each trait is based on two genes,
one from the mother and the
other from the father
True-breeding individuals are
homozygous ( both alleles) are the
same
43
F1 Monohybrid Cross Review
 Heterozygous x heterozygous
 Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Homozygous Recessive rr
 Offspring called F2 generation
 Genotypic ratio is 1:2:1
 Phenotypic Ratio is 3:1
44
…And Now the Test Cross
Mendel then crossed a pure & a
hybrid from his F1 generation
This is known as an F2 or test
cross
There are two possible
testcrosses:
Homozygous dominant x Hybrid
Homozygous recessive x Hybrid
45
F2 Monohybrid Cross
st
(1 )
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Round seeds
RR
x
Rr
R
r
R
RR
Rr
R
RR
Rr
Genotype: RR, Rr
Phenotype: Round
Genotypic
Ratio: 1:1
Phenotypic
Ratio: All alike
46
F2 Monohybrid Cross (2nd)
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Wrinkled seeds x Round seeds
rr
x
Rr
R
r
r
Rr
Rr
r
rr
rr
Genotype: Rr, rr
Phenotype: Round &
Wrinkled
G. Ratio: 1:1
P.Ratio: 1:1
47
F2 Monohybrid Cross Review
 Homozygous x heterozygous(hybrid)
 Offspring:
50% Homozygous RR or rr
50% Heterozygous Rr
 Phenotypic Ratio is 1:1
 Called Test Cross because the
offspring have SAME genotype as
parents
48
Mendel’s Laws
49
Law of Dominance
If one copy of the dominant trait
is present, the dominant trait will
be expressed.
50
Law of Dominance
51
Law of Segregation
During the formation of gametes
(eggs or sperm), the two alleles
responsible for a trait separate
from each other.
Alleles for a trait are then
"recombined" at fertilization,
producing the genotype for the
traits of the offspring.
52
Applying the Law of Segregation
53
Law of Independent
Assortment
Alleles for different traits are
distributed to sex cells (&
offspring) independently of one
another.
This law can be illustrated using
dihybrid crosses.
54
Answer:
1. RrYy: 2n = 22 = 4 gametes
RY
Ry
rY ry
2. AaBbCCDd: 2n = 23 = 8 gametes
ABCD ABCd AbCD AbCd
aBCD aBCd abCD abCD
3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64
gametes
55
Dihybrid Cross
A breeding experiment that tracks
the inheritance of two traits.
Mendel’s “Law of Independent
Assortment”
a. Each pair of alleles segregates
independently during gamete formation
b. Formula: 2n (n = # of heterozygotes)
56
Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R round
r wrinkled
Y yellow
y green
RrYy
RY Ry rY ry
x
RrYy
RY Ry rY ry
All possible gamete combinations
57
Dihybrid Cross
RY
Ry
rY
ry
RY
Ry
rY
ry
58
Dihybrid Cross
RY
RY RRYY
Ry RRYy
rY RrYY
ry
RrYy
Ry
rY
ry
RRYy
RrYY
RrYy
RRyy
RrYy
Rryy
RrYy
rrYY
rrYy
Rryy
rrYy
rryy
Round/Yellow:
Round/green:
9
3
wrinkled/Yellow: 3
wrinkled/green:
1
9:3:3:1 phenotypic
ratio
59
Dihybrid Cross
Round/Yellow: 9
Round/green:
3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
60
A dihybrid cross
Illustrates the inheritance of two characters
Produces four phenotypes in the F2 generation
EXPERIMENT Two true-breeding pea plants—
one with yellow-round seeds and the other with
green-wrinkled seeds—were crossed, producing
dihybrid F1 plants. Self-pollination of the F1 dihybrids,
which are heterozygous for both characters,
produced the F2 generation. The two hypotheses
predict different phenotypic ratios. Note that yellow
color (Y) and round shape (R) are dominant.
P Generation
YYRR
yyrr
Gametes
YR

F1 Generation
yr
YyRr
Hypothesis of
independent
assortment
Hypothesis of
dependent
assortment
Sperm
Sperm
1⁄ YR
2
RESULTS
CONCLUSION The results support the hypothesis of
independent assortment. The alleles for seed color and seed
shape sort into gametes independently of each other.
1⁄
2
yr
Eggs
1⁄
2 YR
F2 Generation
YYRR YyRr
(predicted
offspring)
1 ⁄ yr
2
YyRr yyrr
3⁄
4
1⁄
4
YR
1⁄
4
Yr
1⁄
4
yR
1⁄
4
yr
Eggs
1 ⁄ YR
4
1⁄
4
Yr
1⁄
4
yR
1⁄
4
yr
YYRR YYRr YyRR YyRr
YYrr
YYrr YyRr
Yyrr
YyRR YyRr yyRR yyRr
1⁄
4
Phenotypic ratio 3:1
9⁄
16
YyRr
3⁄
16
Yyrr
yyRr
3⁄
16
yyrr
1⁄
16
Phenotypic ratio 9:3:3:1
315
108
101
32
Phenotypic ratio approximately 9:3:3:1
Figure 14.8
61
Question:
How many gametes will be produced
for the following allele arrangements?
Remember: 2n (n = # of heterozygotes)
1. RrYy
2. AaBbCCDd
3. MmNnOoPPQQRrssTtQq
62
Test Cross
A mating between an individual of unknown
genotype and a homozygous recessive
individual.
Example: bbC__ x bbcc
BB = brown eyes
Bb = brown eyes
bb = blue eyes
CC = curly hair
Cc = curly hair
cc = straight hair
bC
b___
bc
63
Test Cross
Possible results:
bc
bC
b___
C
bbCc
bbCc
or
bc
bC
b___
c
bbCc
bbcc
64
Summary of Mendel’s laws
LAW
DOMINANCE
SEGREGATION
INDEPENDENT
ASSORTMENT
PARENT
CROSS
OFFSPRING
TT x tt
tall x short
100% Tt
tall
Tt x Tt
tall x tall
75% tall
25% short
RrGg x RrGg
round & green
x
round & green
9/16 round seeds & green
pods
3/16 round seeds & yellow
pods
3/16 wrinkled seeds & green
pods
1/16 wrinkled seeds & yellow
pods
65
Beyond Dominant and Recessive Alleles
What are some exceptions to Mendel’s
principles?
Beyond Dominant and Recessive Alleles
Some alleles are neither dominant nor recessive.
Many genes exist in several different forms, and are
therefore said to have multiple alleles.
Many traits are produced by the interaction of several
genes.
Beyond Dominant and Recessive Alleles
Despite the importance of Mendel’s work, there are important exceptions to most of his
principles.
In most organisms, genetics is more complicated, because the majority of genes have
more than two alleles.
In addition, many important traits are controlled by more than one gene.
Mendel’s principles alone cannot predict traits that are controlled by multiple alleles or
multiple genes.
Incomplete Dominance
and
Codominance
69
THINK ABOUT IT
Mendel’s principles offer a set of rules with
which to predict various patterns of
inheritance.
There are exceptions to every rule, and
exceptions to the exceptions.
What happens if one allele is not
completely dominant over another? What if
a gene has several alleles?
Incomplete Dominance
A cross between two four
o’clock plants shows a
common exception to
Mendel’s principles.
The F1 generation produced
by a cross between redflowered (RR) and whiteflowered (WW) plants
consists of pink-colored
flowers (RW), as shown.
Incomplete Dominance
In this case, neither allele
is dominant. Cases in
which one allele is not
completely dominant over
another are called
incomplete dominance.
In incomplete dominance,
the heterozygous
phenotype lies somewhere
between the two
homozygous phenotypes.
Incomplete Dominance
F1 hybrids have an appearance somewhat
in between the phenotypes of the two
parental varieties.
Example: snapdragons (flower)
red (RR) x white (rr)
R
R
RR = red flower
rr = white flower
r
r
73
Incomplete Dominance
R
R
r
Rr
Rr
r
Rr
Rr
produces the
F1 generation
All Rr = pink
(heterozygous pink)
74
Incomplete Dominance
75
Codominance
Cases in which the phenotypes produced by both
alleles are clearly expressed are called
codominance.
For example, in certain varieties of chicken, the
allele for black feathers is codominant with the
allele for white feathers.
Heterozygous chickens have a color described
as “erminette,” speckled with black and white
feathers.
Codominance
Two alleles are expressed (multiple
alleles) in heterozygous individuals.
Example: blood type
1.
2.
3.
4.
type
type
type
type
A
B
AB
O
=
=
=
=
IAIA or IAi
IBIB or IBi
IAIB
ii
77
The ABO blood group in humans
Is determined by multiple alleles
Table 14.2
78
Co-dominance and Multiple Alleles
Rh Factor
79
Codominance Problem
Example: homozygous male Type B (IBIB)
x
heterozygous female Type A (IAi)
IB
IB
IA
IAIB
IAIB
i
IBi
IBi
1/2 = IAIB
1/2 = IBi
80
Another Codominance Problem
• Example: male Type O (ii)
x
female type AB (IAIB)
IA
IB
i
IAi
IBi
i
IAi
IBi
1/2 = IAi
1/2 = IBi
81
Codominance
Question:
If a boy has a blood type O and
his sister has blood type
AB,
what are the genotypes
and
phenotypes of their
parents?
boy - type O (ii)
AB (IAIB)
X
girl - type
82
Codominance
Answer:
IA
IB
i
i
IAIB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
83
Sex-linked Traits
Traits (genes) located on the sex
chromosomes
Sex chromosomes are X and Y
XX genotype for females
XY genotype for males
Many sex-linked traits carried on
X chromosome
84
Hemophilia is a sex-linked trait in humans.
The disorder results in failure of the blood
to clot.
85
Female Carriers
86
Hemophilia in the Royal Families of Europe
87
88
89
Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
90
Sex-linked Trait Problem
Example: Eye color in fruit flies
(red-eyed male) x (white-eyed female)
X RY
x
Xr X r
Remember: the Y chromosome in males
does not carry traits.
X
y
RR = red eyed
Rr = red eyed
X
rr = white eyed
Xy = male
XX = female
X-
91
Sex-linked Trait Solution:
X
X-
X
X-
X- X X-
y
X-
y
X- y
100% red eyed
carriers female
50% white eyed
male
92
Sex-linked Traits
Calico cats are all female, unless there is a
genetic mutation (XXY) male
93
Thomas Hunt Morgan discovered that
Mendel’s principles apply to animals using
fruit flies
94
Many human traits follow Mendelian patterns
of inheritance
Humans are not convenient subjects for
genetic research
However, the study of human genetics
continues to advance
Cloverleaf tongue
folding
Genetic or not?
95
Pedigree Analysis
A pedigree
Is a family tree that describes the interrelationships
of parents and children across generations. Circles
are females, squares are males. If it is filled in-the
person has the trait
96
97
98
Inheritance patterns of particular traits
Can be traced and described using pedigrees
Ww
Ww ww
ww
ww Ww
WW
or
Ww
Widow’s peak
ww
Ww
Ww
ww
First generation
(grandparents)
Second generation
(parents plus aunts
and uncles)
Ff
FF or Ff
Ff
Ff
Third
generation
(two sisters)
ww
No Widow’s peak
Attached earlobe
ff
ff
Ff
Ff
ff
FF
or
Ff
Ff
ff
Free earlobe
(a) Dominant trait (widow’s peak)
(b) Recessive trait (attached earlobe)
99
Normal male karyotype
Normal female karyotype
100
101
102
Turner Syndrome XO female
103
Klinefleter’s Syndrome XXY male
Rate of occurrence:
About 1 in 500 to
1 in 1000 males
104
105
106
Recessively inherited disorders
Show up only in individuals homozygous
for the allele
Carriers
Are heterozygous individuals who carry
the recessive allele but are
phenotypically normal
107
Cystic Fibrosis
Symptoms of cystic fibrosis include
Mucus buildup in the some internal organs
Abnormal absorption of nutrients in the small intestine
108
Sickle-Cell Disease
Sickle-cell disease
Affects one out of 400 African-Americans
Is caused by the substitution of a single amino acid in the hemoglobin protein in
red blood cells
Carriers (hybrids) are resistant to malaria
Symptoms include
Physical weakness, pain, organ damage, and even paralysis
109
Dominantly Inherited Disorders
Some human disorders
Are due to dominant alleles-polydactyly (extra digits) and
achondroplasia A form of dwarfism that is lethal when
homozygous for the dominant allele
Figure 14.15
110
Dominantly Inherited Disorders
Huntington’s disease
Is a degenerative disease of the nervous system
Has no obvious phenotypic effects until about 35 to 40
years of age.
The singer Arlo Guthrie died of Huntington’s.
Figure 14.16
111
Fetal testing
(b) Chorionic villus sampling (CVS)
(a) Amniocentesis
Amniotic
fluid
withdrawn
A sample of chorionic villus
tissue can be taken as early
as the 8th to 10th week of
pregnancy.
A sample of
amniotic fluid can
be taken starting at
the 14th to 16th
week of pregnancy.
Fetus
Fetus
Suction tube
Inserted through
cervix
Centrifugation
Placenta
Placenta
Uterus
Chorionic viIIi
Cervix
Fluid
Fetal
cells
Fetal
cells
Biochemical tests can be
Performed immediately on
the amniotic fluid or later
on the cultured cells.
Fetal cells must be cultured
for several weeks to obtain
sufficient numbers for
karyotyping.
Biochemical
tests
Several
weeks
Several
hours
Karyotyping
Figure 14.17 A, B
Karyotyping and biochemical
tests can be performed on
the fetal cells immediately,
providing results within a day
or so.
Newborn Screening
Some genetic disorders can be detected at
birth by simple tests that are now
routinely performed in most hospitals in
the United States