Transcript B. Intralocular Interactions

II. Meiosis and the Chromosomal Theory
A. Overview
B. Costs and Benefits of Asexual and Sexual Reproduction
C. Mixing Genomes
D. Meiosis
1. Overview
REDUCTION
DIVISION
1n
1n
1n
2n
1n
1n
1n
II. Meiosis and the Chromosomal Theory
A. Overview
B. Costs and Benefits of Asexual and Sexual Reproduction
C. Mixing Genomes
D. Meiosis
1. Overview
2. Meiosis I (Reduction)
There are four replicated
chromosomes in the
initial cell. Each
chromosomes pairs with
its homolog (that
influences the same
suite of traits), and pairs
align on the metaphase
plate. Pairs are
separated in Anaphase I,
and two cells, each with
only two chromosomes,
are produced.
REDUCTION
II. Meiosis and the Chromosomal Theory
A. Overview
B. Costs and Benefits of Asexual and Sexual Reproduction
C. Mixing Genomes
D. Meiosis
1. Overview
2. Meiosis I (Reduction)
3. Transition
4. Meiosis II (Division)
Each cell with two
chromosomes divides;
sister chromatids are
separated. There is no
change in ploidy in this
cycle; haploid cells divide
to produce haploid cells.
DIVISION
5. Modifications in anisogamous and oogamous species
II. Meiosis and the Chromosomal Theory
A. Overview
B. Costs and Benefits of Asexual and Sexual Reproduction
C. Mixing Genomes
D. Meiosis
E. Sexual Reproduction and Variation
1. Meiosis and Mendelian Heredity: The chromosomal theory of inheritance
D. Meiosis
E. Sexual Reproduction and Variation
1. Meiosis and Mendelian Heredity: The chromosomal theory
Saw homologous chromosomes separating (segregating). If they
carried genes, this would explain Mendel’s first law.
A
a
Theodor Boveri
Walter Sutton
D. Meiosis
E. Sexual Reproduction and Variation
1. Meiosis and Mendelian Heredity: The chromosomal theory
And if the way one pair of homologs separated had no effect on how
others separated, then the movement of chromosomes would
explain Mendel’s second law, also!
They proposed that chromosomes carry the heredity information.
A
a
A
Theodor Boveri
a
OR
AB
ab
B
b
Ab
aB
b
B
Walter Sutton
D. Meiosis
E. Sexual Reproduction and Variation
1. Meiosis and Mendelian Heredity: The chromosomal theory
2. Solving Darwin’s Dilemma
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
D. Meiosis
E. Sexual Reproduction and Variation
1. Meiosis and Mendelian Heredity: The chromosomal theory
2. Solving Darwin’s Dilemma
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
D. Meiosis
E. Sexual Reproduction and Variation
1. Meiosis and Mendelian Heredity: The chromosomal theory
2. Solving Darwin’s Dilemma
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?
D. Meiosis
E. Sexual Reproduction and Variation
1. Meiosis and Mendelian Heredity: The chromosomal theory
2. Solving Darwin’s Dilemma
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 of a single pair of mating
humans.
D. III. Meiosis
E. Sexual Reproduction and Variation
1. Meiosis and Mendelian Heredity: The chromosomal
theory
2. Solving Darwin’s Dilemma
3. Model of Evolution – circa 1905
Sources of Variation
Independent Assortment
Causes of Change
 VARIATION 
NATURAL SELECTION
Heredity, Gene Regulation, and Development
I. Mendel's Contributions
II. Meiosis and the Chromosomal Theory
III. Allelic, Genic, and Environmental Interactions
Heredity, Gene Regulation, and Development
I. Mendel's Contributions
II. Meiosis and the Chromosomal Theory
III. Allelic, Genic, and Environmental Interactions
A. Overview:
Environment
The effect of a gene is influenced at three levels:
- Intralocular (effects of other alleles at this locus)
A
a
- Interlocular (effects of other genes at other loci)
- Environmental (the effect of the environment on
determining the effect of a gene on the phenotype)
PHENOTYPE
III. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
A
a
III. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
- The presence of one allele is enough
to cause the complete expression of a given
phenotype.
III. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
- The heterozygote expresses a phenotype
between or intermediate to the phenotypes of the
homozygotes.
III. Allelic, Genic, and Environmental Interactions
A. Overview:
B. 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
OO
AB
codominance
AB
TT = tall (grows best in warm conditions)
tt = short (grows best in cool conditions)
Tt = Very Tall (has both alleles and so grows
optimally in cool and warm conditions)
Enzyme Activity
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
– the heterozygote expresses a
phenotype MORE EXTREME than either
homozygote
“T”
TEMP
“t”
Enzyme Activity
III. Allelic, Genic, and Environmental
Interactions
A. Overview:
B. Intralocular Interactions
TEMP
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.
III. Allelic, Genic, and Environmental
Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions
The genotype at one locus can influence how
the genes at other loci are expressed.
III. Allelic, Genic, and Environmental
Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
There may be several genes that produce
the same protein product; and the
phenotype is the ADDITIVE sum of these
multiple genes.
Creates continuously variable traits.
So here, both genes A and B produce the
same pigment. The double homozygote
AABB produces 4 ‘doses’ of pigment and
is very dark. It also means that there are
more ‘intermediate gradations’ that are
possible.
III. Allelic, Genic, and Environmental
Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
This is where one gene affects how
another gene is expressed; the genes
don’t just add together to affect the
phenotype, they interact.
-example: 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)
III. Allelic, Genic, and Environmental
Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
Process:
enzyme 1
Precursor 1
enzyme 2
precursor2
product
(pigment)
Strain 1:
enzyme 1
enzyme 2
-example: in a enzymatic process, all
Precursor 1
precursor2
no product
enzymes may be needed to produce a
(white)
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 Strain 2:
lacking a different necessary enzyme to
enzyme 1
enzyme 2
make color.
Precursor 1
precursor2
no product
(white)
III. Allelic, Genic, and Environmental
Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
-example: 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 ?
III. Allelic, Genic, and Environmental
Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
-example: 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 ?
9/16 pigment (A-B-), 7/16 white
III. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions
D. Environmental Effects:
The environment can influence whether and how an allele is expressed ,and the
effect it has.
D. Environmental Effects:
1. TEMPERATURE
- Siamese cats and Himalayan rabbits – dark feet and
ears, where temps are slightly cooler. Their pigment enzymes
function at cool temps.
- Arctic fox, hares – their pigment genes function at high
temps and are responsible for a change in coat color in spring and
fall, and a change back to white in fall and winter.
D. Environmental Effects:
1. TEMPERATURE
2. TOXINS
- people have genetically different sensitivities to different toxins.
Certain genes are associated with higher rates of certain types of
cancer, for example. However, they are not ‘deterministic’… their
effects must be activated by some environmental variable.
PKU = phenylketonuria… genetic inability to convert phenylalanine
to tyrosine. Phenylalanine can build up and is toxic to nerve cells.
Single gene recessive disorder.
But if a homozygote recessive person eats a diet low in
phenylalanine, no negative consequences develop. So, the genetic
predisposition to express the disorder is influenced by the
environment.
Nutri-sweet contains phenylalanine, which is dangerous for PKU
homozygotes to consume. So, the Food and Drug Administration
requires that products containing nutri-sweet be labeled as such.
III. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions
D. Environmental Interactions
E. The “Value” of an Allele
1. There are obvious cases where genes are bad – lethal alleles
2. But there are also ‘conditional lethals’ that are only lethal under certain
conditions – like temperature-sensitive lethals.
3. And for most genes, the relative value of one allele over another is
determined by the relative effects of those genes in a particular environment. And
these relative effects may be different in different environments.
III. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions
D. Environmental Interactions
E. The “Value” of an Allele
Survivorship in U.S., sickle-cell anemia
(incomplete dominance, one gene ‘bad’,
two ‘worse’)
SS
Ss
ss
III. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions
D. Environmental Interactions
E. The “Value” of an Allele
Survivorship in U.S., sickle-cell anemia
(incomplete dominance, one gene ‘bad’,
two ‘worse’)
SS
Ss
ss
Survivorship in tropical Africa
(one gene ‘good’, two ‘bad’)
SS
Ss
ss
Malaria is still a primary cause of death
III. Allelic, Genic, and Environmental Interactions in tropical Africa. The malarial parasite
can’t complete development in red
A. Overview:
blood cells with sickle cell hemoglobin…
B. Intralocular Interactions
so one SC gene confers a resistance to
C. Interlocular Interactions
malaria without the totally debilitating
D. Environmental Interactions
effects of sickle cell.
E. The “Value” of an Allele
Survivorship in U.S., sickle-cell anemia
(incomplete dominance, one gene ‘bad’,
two ‘worse’)
Survivorship in tropical Africa
(one gene ‘good’, two ‘bad’)
Survival in U. S.
SS
Ss
ss
Survival in Tropics
SS
Ss
ss
III. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions
D. Environmental Interactions
E. The “Value” of an Allele
Survivorship in U.S., sickle-cell anemia
(incomplete dominance, one gene ‘bad’,
two ‘worse’)
SS
Ss
ss
As Darwin realized, selection will favor
different organisms in different
environments, causing populations to
become genetically different over time.
Survivorship in tropical Africa
(one gene ‘good’, two ‘bad’)
SS
Ss
ss