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Heredity, Gene Regulation, and Development
I. Mendel's Contributions
II. Meiosis and the Chromosomal Theory
III. Allelic, Genic, and Environmental Interactions
IV. Sex Determination and Sex Linkage
V. Linkage
VI. Mutation
A. Overview
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VI. Mutation
A. Overview
A change in the genome
Occurs at four scales of genetic organization:
1: Change in the number of sets of chromosomes ( change in ‘ploidy’)
2: Change in the number of chromosomes in a set (‘aneuploidy’)
3: Change in the number and arrangement of genes on a chromosome
4: Change in the nitrogenous base sequence within a gene
VI. Mutation
A. Overview
B. Changes in Ploidy
- These are the most dramatic changes, adding a whole SET of chromosomes
Triploidy occurs in 2-3% of all
human pregnancies, but almost
always results in spontaneous
abortion of the embryo.
Some triploid babies are born
alive, but die shortly after.
Syndactyly (fused fingers),
cardiac, digestive tract, and
genital abnormalities occur.
VI. Mutation
A. Overview
B. Changes in Ploidy
- These are the most dramatic changes, adding a whole SET of chromosomes
1. Mechanism #1: Complete failure of Meiosis
- if meiosis fails, reduction does not occur and a diploid gamete is produced. This can occur
because of failure of homologs OR sister chromatids to separate in Meiosis I or II, respectively.
Failure of Meiosis I
2n = 4
Gametes:
2n = 4
VI. Mutation
A. Overview
B. Changes in Ploidy
- These are the most dramatic changes, adding a whole SET of chromosomes
1. Mechanism #1: Complete failure of Meiosis
- if meiosis fails, reduction does not occur and a diploid gamete is produced. This can occur
because of failure of homologs OR sister chromatids to separate in Meiosis I or II, respectively.
Failure of Meiosis II
2n = 4
Normal gamete formation is on the bottom, with 1n=2 gametes. The error occurred
up top, with both sister chromatids of both chromosomes going to one pole,
creating a gametes that is 2n = 4.
VI. Mutation
A. Overview
B. Changes in Ploidy
- These are the most dramatic changes, adding a whole SET of chromosomes
1. Mechanism #1: Complete failure of Meiosis
- if meiosis fails, reduction does not occur and a diploid gamete is produced. This can occur
because of failure of homologs OR sister chromatids to separate in Meiosis I or II, respectively.
- this results in a single diploid gamete, which will probably fertilize a normal haploid gamete,
resulting in a triploid offspring.
-
negative consequences of Triploidy:
1) quantitative changes in protein production and regulation.
2) can’t reproduce sexually; can’t produce gametes if you are 3n.
VI. Mutation
A. Overview
B. Changes in Ploidy
- These are the most dramatic changes, adding a whole SET of chromosomes
1. Mechanism #1: Complete failure of Meiosis
- negative consequences of Triploidy:
1) quantitative changes in protein production and regulation.
2) can’t reproduce sexually; can’t produce gametes if you are 3n.
3) but, some organisms can survive, and reproduce parthenogenetically (mitosis)
Like this Blue-spotted Salamander A. laterale,
which has a triploid sister species, A. tremblayi
A. tremblayi is a species that consists of
3n females that reproduce clonally –
laying 3n eggs that divide without
fertilization.
VI. Mutation
A. Overview
B. Changes in Ploidy
- These are the most dramatic changes, adding a whole SET of chromosomes
1. Mechanism #1: Complete failure of Meiosis
2. Mechanism #2: Failure of Mitosis in Gamete-producing Tissue
2n
1) Consider a bud cell in
the flower bud of a plant.
2n
1) Consider a bud cell in
the flower bud of a plant.
4n
2) It replicates it’s DNA
but fails to divide... Now
it is a tetraploid bud cell.
2n
1) Consider a bud cell in
the flower bud of a plant.
3) A tetraploid flower develops
from this tetraploid cell; eventually
producing 2n SPERM and 2n EGG
4n
2) It replicates it’s DNA
but fails to divide... Now
it is a tetraploid bud cell.
2n
1) Consider a bud cell in
the flower bud of a plant.
4n
2) It replicates it’s DNA
but fails to divide... Now
it is a tetraploid bud cell.
3) A tetraploid flower develops
from this tetraploid cell; eventually
producing 2n SPERM and 2n EGG
4) If it is self-compatible, it can mate
with itself, producing 4n zygotes
that develop into a new 4n species.
Why is it a new species?
How do we define ‘species’?
“A group of organisms that reproduce with one another and are
reproductively isolated from other such groups”
(E. Mayr – ‘biological species concept’)
How do we define ‘species’?
Here, the tetraploid population is even reproductively isolated from its
own parent species…So speciation can be an instantaneous genetic event…
2n
4n
4n
1n
2n
2n
3n
Zygote
1n
2n
Gametes
Triploid is a dead-end…
so species are separate
Zygote
Gametes
VI. Mutation
A. Overview
B. Changes in Ploidy
- These are the most dramatic changes, adding a whole SET of chromosomes
1. Mechanism #1: Complete failure of Meiosis
2. Mechanism #2: Complete failure of Mitosis
3. The Frequency of Polyploidy
For reasons we just saw, we might expect polyploidy to occur more frequently in
hermaphroditic species, because the chances of ‘jumping’ the triploidy barrier to
reproductive tetraploidy are more likely. Over 50% of all flowering plants are
polyploid species; many having arisen by this duplication of chromosome number
within a lineage.
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘aneuploidy’ (changes in chromosome number)
1. Mechanism: Non-disjunction (failure of a homologous pair or
sister chromatids to separate)
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘aneuploidy’ (changes in chromosome number)
1. Mechanism: Non-disjunction (failure of a homologous pair or
sister chromatids to separate)
2. Human Examples
a. trisomies
Trisomy 21 – “Downs’ Syndrome”
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘aneuploidy’ (changes in chromosome number)
1. Mechanism: Non-disjunction (failure of a homologous pair or
sister chromatids to separate)
2. Human Examples
a. trisomies
Trisomy 21 – “Downs’ Syndrome”
Trisomy 18 – Edward’s Syndrome
Trisomy 13 – Patau Syndrome
Some survive to birth
Trisomy 9
Trisomy 8
Trisomy 22
Trisomy 16 – most common – 1% of pregnancies – always aborted
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘aneuploidy’ (changes in chromosome number)
1. Mechanism: Non-disjunction (failure of a homologous pair or
sister chromatids to separate)
Extreme effects listed below;
2. Human Examples
most show a phenotype within
a. trisomies
the typical range for XY males
47, XXY – “Klinefelter’s Syndrome”
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘aneuploidy’ (changes in chromosome number)
1. Mechanism: Non-disjunction (failure of a homologous pair or
sister chromatids to separate)
2. Human Examples
a. trisomies
47, XXX – “Triple-X Syndrome”
No dramatic effects on the
phenotype; may be taller.
In XX females, one X shuts
down anyway, in each cell
(Barr body).
In triple-X females, 2 X’s shut
down.
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘aneuploidy’ (changes in chromosome number)
1. Mechanism: Non-disjunction (failure of a homologous pair or
sister chromatids to separate)
2. Human Examples
a. trisomies
47, XYY – “Super-Y Syndrome”
Often taller, with scarring
acne, but within the
phenotypic range for XY males
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘aneuploidy’ (changes in chromosome number)
1. Mechanism: Non-disjunction (failure of a homologous pair or
sister chromatids to separate)
2. Human Examples
b. monosomies
45, XO– “Turner’s Syndrome” (the only human monosomy to survive to birth)