Download self-fertilize

Genetics, the oldest branch of Biology
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Genetics = Information Flow
Transmission Genetics =
information flow between
generations
Molecular Genetics =
information flow within
cells/organisms
DNA RNA  Protein
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Data of Goss (1824)
pea plant from
green seed
X
pea plant from
yellow seed
All seeds yellow – grow and
self fertilize
Some pods with all Many pods with both
yellow and green seeds
green seeds
Self fertilization of
plants grown from green
All progeny plants
Have pods with
green seeds only
Some pods with all
yellow seeds –
grow into plants and
self fertilize
Some pods with all seeds
yellow, some with green
and yellow seeds
Data of Mendel (1866)
pea plant from
green seed
X
pea plant from
yellow seed
First filial (F1) All seeds yellow Grow into plants and self fertilize
generation
second filial(F2) Count # of green and yellow seeds:
generation
-8023 total seeds
-6022 yellow
-2001 green – grown into plants: self fertilization yields
all green seeds
Take 519 yellow seeds – grown into plants: self fertilization
Of these 519 plants, 166 bred true (all yellow seeds), 353 did not
(mixed yellow and green seeds)
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Mendel’s model
True breeding yellow
AA
True breeding green
aa
egg cells
fertilize
A
F1
x
Aa (yellow seeds) – grow into plants and self fertilize
A
F2
pollen cells
a
A
AA
a
aA
(eggs)
a (pollen)
3:1 yellow:green
Aa __________________
¼ true breeding yellow
aa ½ “impure” yellow
¼ true breeding green
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Mendel’s First Law
Each trait is governed by 2 particles*, one inherited from each
parent. These two particles do not influence each other
in any way within an individual, but separate, uncontaminated
in any way, into gametes at the time of reproductive cell
Formation. (an unstated corollary is that any pollen cell can
fertilize any egg cell = random fertilization).
Testing the law:
- the test cross (Aa x aa) predicts new ratios
- other traits tested
*Introduce modern terms:
dominant, recessive, alleles, phenotype, genotype, heterozygote,
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homozygote
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Results of all Mendel's crosses in which parents differed for one character
Parental phenotype
F1
F2
F2 ratio
1 . Round X wrinkled seeds
All round
5474 round; 1850 wrinkled
2.96:1
2. Yellow X green seeds
All yellow
6022 yellow; 2001 green
3.01:1
3. Purple X white petals
All purple
705 purple; 224 white
3.15:1
4. Inflated X pinched pods
All inflated
882 inflated; 299 pinched
2.95:1
5. Green X yellow pods
All green
428 green; 152 yellow
2.82:1
6. Axial X terminal flowers
All axial
651 axial; 207 terminal
3.14: 1
7. Long X short stems
All long
787 long; 277 short
2.84: 1
What happens if two character traits are followed simultaneously?
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Fig. 13.16
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Mendel’s Second Law
Second Law=The Law of Independent Assortment:
During the formation of gametes, the segregation
of alleles at one locus is independent of that of the
segregation of alleles at any other.
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Genes’ eye view of meiosis and mitosis
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Mendel’s First Law
Each trait is governed by 2 particles*, one inherited from each
parent. These two particles do not influence each other
in any way within an individual, but separate, uncontaminated
in any way, into gametes at the time of reproductive cell
Formation. (an unstated corollary is that any pollen cell can
fertilize any egg cell = random fertilization).
Mendel’s Second Law
The Law of Independent Assortment: During the formation of
gametes, the segregation of alleles at one locus is independent
of that of the segregation of alleles at any other.
A Gene's (allele) Eye View of Mitosis and Meiosis
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Figure 10.5 Meiosis Accounts
for the Segregation of Alleles
(Part 1)
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Figure 10.5 Meiosis Accounts
for the Segregation of Alleles
(Part 2)
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Figure 10.8 Meiosis Accounts
for Independent Assortment of
Alleles
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A
a
A
A
a
a
mitotic metaphase
anaphase, telophase,
cytokinesis
A
a
A
a
B
A
b
a
genotype: Aa; Bb
replication
A
A
B
B
a
a
b
b
Meiosis I anaphase,
telophase, cytokinesis
Meiosis I metaphase
Meiosis I product cells
A
A
B
B
a
a
b
b
Meiosis I product cells
A
A
B
B
Meiosis II metaphase
a
a
b
b
Meiosis II metaphase
Meiosis II anaphase,
telophase, cytokinesis
Meiosis II product cells
A
AB
B
A
AB
B
a
ab
b
a
ab
b
Meiosis I product cells
A
A
b
b
Meiosis II metaphase
a
a
B
B
Meiosis II metaphase
Meiosis II anaphase,
telophase, cytokinesis
Meiosis II products cells
A
Ab
b
A
Ab
b
a
aB
B
a
aB
B
Eye Color Is a Sex-Linked Trait in Drosophila
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Probability – Predicting Results
Rule of addition: the probability of 2
mutually exclusive events occurring
simultaneously is the sum of their
individual probabilities.
When crossing Pp x Pp, the probability of
producing Pp offspring is
probability of obtaining Pp (1/4), PLUS
probability of obtaining pP (1/4)
¼ + ¼ = ½
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Probability – Predicting Results
Rule of multiplication: the probability of 2
independent events occurring
simultaneously is the PRODUCT of their
individual probabilities.
When crossing Rr Yy x RrYy, the probability
of obtaining rr yy offspring is:
probability of obtaiing rr = ¼
probability of obtaining yy = ¼
probability of rr yy = ¼ x ¼ = 1/16
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Testcross
Testcross: a cross used to determine the
genotype of an individual with dominant
phenotype
-cross the individual with unknown genotype
(e.g. P_) with a homozygous recessive (pp)
-the phenotypic ratios among offspring are
different, depending on the genotype of the
unknown parent
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Genes
Phenotypes
Genes
pleiotropy
polygenic inheritance
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Figure 10.12 Inheritance of
“continuous” variation: multiple alleles of one gene
Coat Color in Rabbits
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Gene Interaction (alleles of same gene)
- dominance
- incomplete dominance
- co-dominance
- lethal alleles
Gene Interaction (alleles of different genes):
- in different pathways
(Drosophila eye pigmentation)
- in same pathway
- recessive epistasis
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Fig. 13.18
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codominance
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Fig. 13.20
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fly heads
+/+
wild-type
(+/+)
x
Antp/+
½ “Antp” (Antp/+)
½ “+”
(+/+)
Antennapedia
mutant
(Antp/+)
Antp/+ x Antp/+
2/3 “Antp” (Antp/+)
1/3 “+”
(+/+)
? ¼ +/+ (“+”)
½ Antp/+ (“Antp”)
¼ Antp/Antp (lethal)
Fig. 13.19
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Extensions to Mendel
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Eye Color Is a Sex-Linked Trait in Drosophila
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white-eyed, normal-winged female
w
m+
w
m+
x
red-eyed, miniature winged male (wild type)
w+
m
white-eyed, normal-winged males
w
m+
wild type females
w
m+
x
w+
m
for male progeny, EXPECT:
½ white-eyed, normal-winged
w
m+
½ red-eyed, miniature winged
w+
m
64% of males fell into above classes, but 36% were either wild type 46
Or doubly mutant !!!!!!!
genetic recombination = chromosomal crossing over
36% of chromosomes in meiosis I:
white-eyed, normal-winged males
wild type females
w
w
m+
m+
x
w+
m
36% of males are either doubly mutant or wild type :
w+
m+
w
m
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Chiasmata visible in
Locusta migratoria
spermatogenesis
A synaptonemal complex
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Genetic Mapping
Mapping genes in humans involves
determining the recombination frequency
between a gene and an anonymous
marker
Anonymous markers such as single
nucleotide polymorphisms (SNPs) can
be detected by molecular techniques.
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
New Genes Identified on the Human Y Chromosome
Testis Determining Factor (SRY)
Channel Flipping (FLP)
Catching and Throwing (BLZ-1)
Self Confidence (BLZ-2) - (note: unlinked to ability)
Preadolescent fascination with Arachinida and Reptilia (MOM-4U)
Addiction to Death and Destruction Films (T2)
Sitting on John Reading (SIT)
Selective Hearing Loss (HUH?)
Lack of Recall for Important Dates (OOPS)
Inability to Express Affection Over the Phone (ME-2)
Spitting (P2E)

• effects of
recombination
on chromosomes
within a family
• grandson inherits
chromosome regions
from all four of his
grandparents’
chromosomes
• siblings inherit different chromosome
regions from their parents
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Early Ideas of Heredity
Before the 20th century, 2 concepts were the
basis for ideas about heredity:
-heredity occurs within species
-traits are transmitted directly from parent
to offspring
This led to the belief that inheritance is a
matter of blending traits from the parents.
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Early Ideas of Heredity
Botanists in the 18th and 19th centuries
produced hybrid plants.
When the hybrids were crossed with each
other, some of the offspring resembled the
original strains, rather than the hybrid
strains.
This evidence contradicted the idea that
traits are directly passed from parent to
offspring.
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Early Ideas of Heredity
Gregor Mendel
-chose to study 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 crossfertilized
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Early Ideas of Heredity
Mendel’s experimental method:
1. produce true-breeding strains for each
trait he was studying
2. cross-fertilize true-breeding strains having
alternate forms of a trait
-perform reciprocal crosses as well
3. allow the hybrid offspring to self-fertilize
and count the number of offspring showing
each form of the trait
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Monohybrid Crosses
Monohybrid cross: a cross to study only 2
variations of a single trait
Mendel produced true-breeding pea strains
for 7 different traits
-each trait had 2 alternate forms (variations)
-Mendel cross-fertilized the 2 true-breeding
strains for each trait
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Monohybrid Crosses
F1 generation (1st filial generation):
offspring produced by crossing 2 truebreeding strains
For every trait Mendel studied, all F1 plants
resembled only 1 parent
-no plants with characteristics intermediate
between the 2 parents were produced
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Monohybrid Crosses
F1 generation: offspring resulting from a
cross of true-breeding parents
F2 generation: offspring resulting from the
self-fertilization of F1 plants
dominant: the form of each trait expressed
in the F1 plants
recessive: the form of the trait not seen in
the F1 plants
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Monohybrid Crosses
F2 plants exhibited both forms of the trait in a
very specific pattern:
¾ plants with the dominant form
¼ plant 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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Monohybrid Crosses
gene: information for a trait passed from
parent to offspring
alleles: alternate forms of a gene
homozygous: having 2 of the same allele
heterozygous: having 2 different alleles
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Monohybrid Crosses
genotype: total set of alleles of an individual
PP = homozygous dominant
Pp = heterozygous
pp = homozygous recessive
phenotype: outward appearance of an
individual
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Monohybrid Crosses
Principle of Segregation
Two alleles for a gene segregate during
gamete formation and are rejoined at
random, one from each parent, during
fertilization.
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Monohybrid Crosses
Some human traits are controlled by a single
gene.
-some of these exhibit dominant
inheritance
-some of these exhibit recessive
inheritance
Pedigree analysis is used to track
inheritance patterns in families.
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Dihybrid Crosses
Dihybrid cross: examination of 2 separate
traits in a single cross
-for example: RR YY x rryy
The F1 generation of a dihybrid cross (RrYy)
shows only the dominant phenotypes for
each trait.
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Dihybrid Crosses
The F2 generation is produced by crossing
members of the F1 generation with each
other or allowing self-fertilization of the F1.
-for example RrYy x RrYy
The F2 generation shows all four possible
phenotypes in a set ratio:
9:3:3:1
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Dihybrid Crosses
Principle of Independent Assortment
In a dihybrid cross, the alleles of each gene
assort independently.
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Extensions to Mendel
Mendel’s model of inheritance assumes
that:
-each trait is controlled by a single gene
-each gene has only 2 alleles
-there is a clear dominant-recessive
relationship between the alleles
Most genes do not meet these criteria.
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Extensions to Mendel
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
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Extensions to Mendel
Pleiotropy refers to an allele which has
more than one effect on the phenotype.
This can be seen in human diseases such
as cystic fibrosis or sickle cell anemia.
In these diseases, multiple symptoms can
be traced back to one defective allele.
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Extensions to Mendel
Incomplete dominance: the heterozygote
is intermediate in phenotype between the
2 homozygotes.
Codominance: the heterozygote shows
some aspect of the phenotypes of both
homozygotes.
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Extensions to Mendel
The human ABO blood group system
demonstrates:
-multiple alleles: there are 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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Extensions to Mendel
The expression of some genes can be
influenced by the environment.
for example: coat color in Himalayan rabbits
and Siamese cats
-an allele produces an enzyme that allows
pigment production only at temperatures
below 30oC
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Extensions to Mendel
The products of some genes interact with
each other and influence the phenotype of
the individual.
Epistasis: one gene can interfere with the
expression of another gene
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