Transcript Meiosis and Variation

E. The Power of Independent Assortment
1. If you can assume that the genes assort independently, then you
can calculate ‘single gene’ outcomes and multiply results together…
2. You can easily address more difficult multigene problems:
(female) AaBbCcdd
x AABbccDD (male)
- how many types of gametes can each parent produce?
- What is the probability of an offspring expressing ABCD?
- How many genotypes are possible in the offspring? 2 x 3 x 2 x 1= 12
- how many phenotypes are possible in the offspring? 1 x 2 x 2 x 1 = 4
At A:
At B:
A
A
A
AA
AA
a
Aa
Aa
At C:
B
b
B
BB
Bb
C
Cc
b
Bb
bb
c
cc
At D:
c
D
d
Dd
E. The Power of Independent Assortment
1. If you can assume that the genes assort independently, then you
can calculate ‘single gene’ outcomes and multiply results together…
2. You can easily address more difficult multigene problems.
As you can see, IA produces lots of variation, because of the multiplicative
effect of combining genes from different loci together in gametes, and then
combining them together during fertilization… we’ll look at this again;
especially with respect to Darwin’s 3rd dilemma.
F. WHY do these patterns occur?
Meiosis: Gamete Formation
a. Overview:
In sexually reproducing species, gametes carry
exactly ½ the genetic information as the parent; so
that the fusion of gametes reconstitutes the correct
genetic complement.
1n = 2
2n = 4
Fertilization (fusion)
F. WHY do these patterns occur?
Meiosis: Gamete Formation
Diploid
Haploid
a. Overview:
In sexually reproducing species, gametes carry
exactly ½ the genetic information as the parent; so
that the fusion of gametes reconstitutes the correct
genetic complement.
MEIOSIS
The divisional process that produces these special
cells is called meiosis.
Thus, meiosis ONLY occurs in reproductive tissue 2n = 4
(ovary, testis), and only produces gametes.
1n = 2
All other cells in multicellular organisms are
produced by mitosis.
Fertilization (fusion)
F. WHY do these patterns occur?
Meiosis: Gamete Formation
Diploid
Haploid
a. Overview:
In sexually reproducing species, gametes carry
exactly ½ the genetic information as the parent; so
that the fusion of gametes reconstitutes the correct
genetic complement.
MEIOSIS
The divisional process that produces these special
cells is called meiosis.
Thus, meiosis ONLY occurs in reproductive tissue 2n = 4
(ovary, testis), and only produces gametes.
1n = 2
All other cells in multicellular organisms are
produced by mitosis.
MEIOSIS has two divisional cycles, “reduction” and
“division”
Fertilization (fusion)
F. WHY do these patterns occur?
Meiosis: Gamete Formation
a. Overview:
b. Meiosis I: “The Reduction Cycle”
- Prophase I:
condensation in pairs
possible crossing over
F. WHY do these patterns occur?
Meiosis: Gamete Formation
a. Overview:
b. Meiosis I: “The Reduction Cycle”
- Prophase I:
condensation in pairs
possible crossing over
- Metaphase I:
Homologs align In PAIRS
F. WHY do these patterns occur?
Meiosis: Gamete Formation
a. Overview:
b. Meiosis I: “The Reduction Cycle”
- Prophase I:
condensation in pairs
possible crossing over
- Metaphase I:
Homologs align In PAIRS
- Anaphase I:
Replicated chromosomes move
to opposite poles
F. WHY do these patterns occur?
Meiosis: Gamete Formation
a. Overview:
b. Meiosis I: “The Reduction Cycle”
- Prophase I:
condensation in pairs
possible crossing over
- Metaphase I:
Homologs align In PAIRS
- Anaphase I:
Replicated chromosomes move
to opposite poles
- Telophase – Prophase II transition
F. WHY do these patterns occur?
Meiosis: Gamete Formation
a. Overview:
b. Meiosis I: “The Reduction Cycle”
- Prophase I:
condensation in pairs
possible crossing over
- Metaphase I:
Homologs align In PAIRS
- Anaphase I:
Replicated chromosomes move
to opposite poles
- Telophase – Prophase II transition
PLOIDY REDUCED 2n parent cell 1n daughter cells
F. WHY do these patterns occur?
Meiosis: Gamete Formation
a. Overview:
b. Meiosis I: “The Reduction Cycle”
c. Meiosis II: “The Division Cycle”
Like mitosis, but a haploid cell… chromsomes line
up in single file in Metaphase II, and sister
chromatids (of replicated chromosomes) separate
and move to opposite poles.
F. WHY do these patterns occur?
Meiosis: Gamete Formation
a. Overview:
b. Meiosis I: “The Reduction Cycle”
c. Meiosis II: “The Division Cycle”
d. Variations in the Process:
In spermatogenesis: Karyokinesis is equal and
cytokinesis is equal, resulting in 4 equal-sized sperm.
F. WHY do these pattern occur?
Meiosis: Gamete Formation
a. Overview:
b. Meiosis I: “The Reduction Cycle”
c. Meiosis II: “The Division Cycle”
d. Variations in the Process:
In oogenesis, karyokinesis is equal (dividing
the genetic information exactly in half), but
cytokinesis is unequal, with one daughter cell
getting the majority of the cytoplasm and
organelles. The smaller may/may not be able
to divide. These are ‘polar bodies’
III. Uniting Genetics and Cell Biology
A. The Chromosomal Theory – Sutton and Boveri
Theodor Boveri
Walter Sutton
III. Uniting Genetics and Cell Biology
A. The Chromosomal Theory – Sutton and Boveri
Saw homologous chromosomes separating (segregating). If they
carried genes, this would explain Mendel’s first law.
A
a
Theodor Boveri
Walter Sutton
III. Uniting Genetics and Cell Biology
A. The Chromosomal Theory – Sutton and Boveri
And if the way one pair of homologs separated had no effect on
how others separated, then the mvmnt of chromosomes would
explain Mendel’s second law, also!
A
a
AB
A
ab
B
b
Theodor Boveri
a
Ab
aB
b
B
Walter Sutton
III. Uniting Genetics and Cell Biology
A. The Chromosomal Theory – Sutton and Boveri
This was a major achievement in science. The patterns of
heredity had been associated with physical entities in biological
cells. The movement of chromosomes correlated with Mendel’s
patterns. Scientist now went about testing if this relationship
was causal – and they found it was.
Theodor Boveri
Walter Sutton
III. Uniting Genetics and Cell Biology
A. The Chromosomal Theory – Sutton and Boveri
B. Solving Darwin’s Dilemma – The Source of Variation
III. Uniting Genetics and Cell Biology
A. The Chromosomal Theory – Sutton and Boveri
B. Solving Darwin’s Dilemma – The Source of Variation
Independent Assortment produces an amazing amount of
genetic variation.
Consider an organism, 2n = 4, with two pairs of homologs. They
can make 4 different gametes (long Blue, Short Red) (Long Blue,
Short Blue), (Long Red, Short Red), (Long Red, Short blue).
Gametes carry thousands of genes, so homologous
chromosomes will not be identical over their entire length, even
though they may be homozygous at particular loci.
Well, the number of gametes can be calculated as 2n
or
III. Uniting Genetics and Cell Biology
A. The Chromosomal Theory – Sutton and Boveri
B. Solving Darwin’s Dilemma – The Source of Variation
Independent Assortment produces an amazing amount of
genetic variation.
Consider an organism with 2n = 6 (AaBbCc) ….
There are 2n = 8 different gamete types.
ABC
Abc
aBC
AbC
abc
abC
Abc
aBc
III. Uniting Genetics and Cell Biology
A. The Chromosomal Theory – Sutton and Boveri
B. Solving Darwin’s Dilemma – The Source of Variation
Independent Assortment produces an amazing amount of
genetic variation.
Consider an organism with 2n = 6 (AaBbCc) ….
There are 2n = 8 different gamete types.
And humans, with 2n = 46?
ABC
Abc
aBC
AbC
abc
abC
Abc
aBc
III. Uniting Genetics and Cell Biology
A. The Chromosomal Theory – Sutton and Boveri
B. Solving Darwin’s Dilemma – The Source of Variation
Independent Assortment produces an amazing amount of
genetic variation.
Consider an organism with 2n = 6 (AaBbCc) ….
There are 2n = 8 different gamete types.
And humans, with 2n = 46?
223 = ~ 8 million different types of gametes.
And each can fertilize ONE of the ~ 8 million types of gametes of
the mate… for a total 246 = ~70 trillion different chromosomal
combinations possible in the offspring.
ABC
Abc
aBC
AbC
abc
abC
Abc
aBc
III. Uniting Genetics and Cell Biology
A. The Chromosomal Theory – Sutton and Boveri
B. Solving Darwin’s Dilemma – The Source of Variation
Independent Assortment produces an amazing amount of
genetic variation.
And each can fertilize ONE of the ~ 8 million types of gametes of
the mate… for a total 246 = 70 trillion different chromosomal
combinations possible in the offspring.
YOU are 1 of the 70 trillion combinations your own parents
could have made. IA creates a huge amount of genetic
variation, and that doesn’t include crossing over!!!!
III. Uniting Genetics and Cell Biology
A. The Chromosomal Theory – Sutton and Boveri
B. Solving Darwin’s Dilemma – The Source of Variation
C. Modification to Evolutionary Theory
Darwin’s Model
Sources of Variation
????????????????
(use and disuse??)
Causes of Change
 VARIATION 
NATURAL SELECTION
III. Uniting Genetics and Cell Biology
A. Early Studies
B. Divisional Processes
C. The Chromosomal Theory – Sutton and Boveri
D. Solving Darwin’s Dilemma – The Source of Variation
E. Modification to Evolutionary Theory
Darwin’s Model
Sources of Variation
Independent Assortment
Causes of Change
 VARIATION 
NATURAL SELECTION
IV. Modifications to Mendelian Patterns
IV. Modifications to Mendelian Patterns
- Overview:
Mendels conclusions regarding dominance, equal
segregation, independent assortment, and independent effects
hold for many genes and traits. But they are not true of ALL
traits.
IV. Modifications to Mendelian Patterns
- Overview:
Mendels conclusions regarding dominance, equal
segregation, independent assortment, and independent effects
hold for many genes and traits. But they are not true of ALL
traits.
Here, we will consider how the effect of a gene is
influenced at three levels:
IV. Modifications to Mendelian Patterns
- Overview:
Mendels conclusions regarding dominance, equal
segregation, independent assortment, and independent effects
hold for many genes and traits. But they are not true of ALL
traits.
Here, we will consider how the effect of a gene is
influenced at three levels:
- Intralocular (effects of other alleles at this locus)
IV. Modifications to Mendelian Patterns
- Overview:
Mendels conclusions regarding dominance, equal
segregation, independent assortment, and independent effects
hold for many genes and traits. But they are not true of ALL
traits.
Here, we will consider how the effect of a gene is
influenced at three levels:
- Intralocular (effects of other alleles at this locus)
- Interlocular (effects of other genes at other loci)
IV. Modifications to Mendelian Patterns
- Overview:
Mendels conclusions regarding dominance, equal
segregation, independent assortment, and independent effects
hold for many genes and traits. But they are not true of ALL
traits.
Here, we will consider how the effect of a gene is
influenced at three levels:
- Intralocular (effects of other alleles at this locus)
- Interlocular (effects of other genes at other loci)
- Environmental (the effect of the environment on determining
the effect of a gene on the phenotype)
IV. Modifications to Mendelian Patterns
- Overview:
Mendels conclusions regarding dominance, equal
segregation, independent assortment, and independent effects
hold for many genes and traits. But they are not true of ALL
traits.
Here, we will consider how the effect of a gene is
influenced at three levels:
- Intralocular (effects of other alleles at this locus)
- Interlocular (effects of other genes at other loci)
- Environmental (the effect of the environment on determining
the effect of a gene on the phenotype)
And finally, we will examine the VALUE of an allele – are there
“good genes” and “bad genes”?
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
A
a
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
1. Complete Dominance:
- The presence of one allele is enough
to cause the complete expression of a given
phenotype.
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
- The heterozygote expresses a
phenotype between or intermediate to the
phenotypes of the homozygotes.
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
- Both alleles are expressed completely;
the heterozygote does not have an intermediate
phenotype, it has BOTH phenotypes.
ABO Blood Type:
A = ‘A’ surface antigens
Genotypic and phenotypic
ratios of F1 x F1 crosses are
both 1:2:1
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
- Both alleles are expressed completely;
the heterozygote does not have an intermediate
phenotype, it has BOTH phenotypes.
ABO Blood Type:
A = ‘A’ surface antigens
B = ‘B’ surface antigens
Genotypic and phenotypic
ratios of F1 x F1 crosses are
both 1:2:1
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
- Both alleles are expressed completely;
the heterozygote does not have an intermediate
phenotype, it has BOTH phenotypes.
ABO Blood Type:
A = ‘A’ surface antigens
B = ‘B’ surface antigens
O = no surface antigen
from this locus
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
- Both alleles are expressed completely;
the heterozygote does not have an intermediate
phenotype, it has BOTH phenotypes.
AB Phenotype
ABO Blood Type:
A = ‘A’ surface antigens
B = ‘B’ surface antigens
O = no surface antigen from this
locus
Phenotype
Genotypes
A
AA, AO
B
BB, BO
O
O
AB
codominance
AB
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Multiple Alleles:
- While not really specifying an
‘interaction’, it does raise a complication of looking
at a single trait.
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Multiple Alleles:
- While not really specifying an
‘interaction’, it does raise a complication of looking
at a single trait.
- You might presume that a ‘single-gene’
trait could only have a maximum of three
phenotypes (AA, Aa, aa). But with many alleles
possible for a gene (as for A,B,O), there are many
diploid combinations and effects that are possible
(as in the 4 phenotypes for the A,B,O system).
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance – the
heterozygote expresses a phenotype MORE
extreme than either homozygote – see class
notes.
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
- Summary and Implications:
populations can harbor
extraordinary genetic variation at each locus,
and these alleles can interact in myriad ways
to produce complex and variable phenotypes.
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
- Summary and Implications:
populations can harbor
extraordinary genetic variation at each locus,
and these alleles can interact in myriad ways
to produce complex and variable phenotypes.
-Consider this cross: AaBbCcDd x AABbCcDD
Assume:
The genes assort independently
A and a are codominant
B is incompletely dominant to b
C is incompletely dominant to c
D is completely dominant to d
How many phenotypes are possible in the
offspring?
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
- Summary and Implications:
populations can harbor
extraordinary genetic variation at each locus,
and these alleles can interact in myriad ways
to produce complex and variable phenotypes.
A
B
2 x
3
C
x
3
D
x
1 = 18
-Consider this cross: AaBbCcDd x AABbCcDD
If they had all exhibited complete
dominance, there would have
been only:
Assume:
1 x
The genes assort independently
A and a are codominant
B is incompletely dominant to b
C is incompletely dominant to c
D is completely dominant to d
How many phenotypes are possible in the
offspring?
2
x
2
x
1 =4
So the variety of allelic interactions
that are possible increases
phenotypic variation
multiplicatively. In a population
with many alleles at each locus,
there is an nearly limitless
amount of phenotypic variability.
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
B. Interlocular Interactions:
The genotype at one locus can
influence how the genes at other loci are
expressed.
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
B. Interlocular Interactions:
The genotype at one locus can
influence how the genes at other loci are
expressed.
1. Epistasis: one gene curtails the
expression at another locus; the phenotype in
the A,B,O blood group system can be affected
by the genotype at the fucosyl transferase
locus.
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
B. Interlocular Interactions:
The genotype at one locus can
influence how the genes at other loci are
expressed.
1. Epistasis: one gene curtails the
expression at another locus; the phenotype in
the A,B,O blood group system can be affected
by the genotype at the fucosyl transferase
locus.
This locus makes the ‘H substance’ to which
the sugar groups are added to make the A and
B surface antigens.
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
B. Interlocular Interactions:
The genotype at one locus can
influence how the genes at other loci are
expressed.
1. Epistasis: one gene curtails the
expression at another locus; the phenotype in
the A,B,O blood group system can be affected
by the genotype at the fucosyl transferase
locus.
This locus makes the ‘H substance’ to which
the sugar groups are added to make the A and
B surface antigens.
A non-function ‘h’ gene makes a nonfunctional foundation and sugar groups can’t
be added – resulting in O blood regardless of
the genotype at the A,B,O locus
Genotype
at H
Genotype
at A,B,O
Phenotype
H-
A-
A
H-
B-
B
H-
OO
O
H-
AB
AB
hh
A-
O
hh
B-
O
hh
OO
O
hh
AB
O
IV. Modifications to Mendelian Patterns
So, what are the phenotypic ratios
from this cross:
A. Intralocular Interactions
HhAO x
B. Interlocular Interactions:
The genotype at one locus can
influence how the genes at other loci are
expressed.
1. Epistasis: one gene curtails the
expression at another locus; the phenotype in
the A,B,O blood group system can be affected
by the genotype at the fucosyl transferase
locus.
This locus makes the ‘H substance’ to which
the sugar groups are added to make the A and
B surface antigens.
A non-function ‘h’ gene makes a nonfunctional foundation and sugar groups can’t
be added – resulting in O blood regardless of
the genotype at the A,B,O locus
HhBO?
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
B. Interlocular Interactions:
The genotype at one locus can
influence how the genes at other loci are
expressed.
1. Epistasis:
-example #2: in a enzymatic process, all
enzymes may be needed to produce a
given phenotype. Absence of either may
produce the same alternative ‘null’.
Process:
enzyme 1
Precursor 1
enzyme 2
precursor2
product
(pigment)
IV. Modifications to Mendelian Patterns Process:
enzyme 1
enzyme 2
A. Intralocular Interactions
B. Interlocular Interactions:
The genotype at one locus can
influence how the genes at other loci are
expressed.
1. Epistasis:
Precursor 1
precursor2
product
(pigment)
Strain 1:
enzyme 1
Precursor 1
enzyme 2
precursor2
no product
(white)
-example #2: in a enzymatic process, all
enzymes may be needed to produce a
given phenotype. Absence of either may Strain 2:
produce the same alternative ‘null’.
enzyme 1
enzyme 2
For example, two strains of white flowers
may be white for different reasons; each
lacking a different necessary enzyme to
Precursor 1
precursor2
no product
make color.
(white)
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
B. Interlocular Interactions:
The genotype at one locus can
influence how the genes at other loci are
expressed.
1. Epistasis:
-example #2: in a enzymatic process, all
enzymes may be needed to produce a
given phenotype. Absence of either may
produce the same alternative ‘null’.
For example, two strains of white flowers
may be white for different reasons; each
lacking a different necessary enzyme to
make color.
So there must be a dominant gene at
both loci to produce color.
Genotype
Phenotype
aaB-
white
aabb
white
A-bb
white
A-B-
pigment
So, what’s the phenotypic
ratio from a cross:
AaBb x AaBb ?
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
B. Interlocular Interactions:
The genotype at one locus can
influence how the genes at other loci are
expressed.
1. Epistasis:
-example #3: Novel Phenotypes.
Comb shape in chickens is governed by 2
interacting genes that independently
produce “Rose” or “Pea” combs, but
together produce something completely
different (walnut).
Genotype
Phenotype
rrpp
single
R-pp
rose
rrP-
pea
A-B-
Walnut
IV. Modifications to Mendelian Patterns
A. Intralocular Interactions
B. Interlocular Interactions:
The genotype at one locus can
influence how the genes at other loci are
expressed.
1. Epistasis:
2. Polygenic Traits:
There may be several genes that
essentially produce the same protein
product; and the phenotype is the
ADDITIVE sum of these multiple genes.
Creates continuously variable traits.