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Autopolyploidy
Colchicine is a naturally occurring alkaloid in the autumn
crocus that interferes with spindle formation
Homologous pairing of chromosomes in individuals that are
heterozygous for an inversion is achieved via an inversion loop
Paracentric inversion heterozygote (centromere not included)
Note that crossover events not observed
dicentric bridge
not viable
not viable
Pericentric inversion heterozygote (centromere included)
Note that crossover events not observed
not viable
not viable
Translocations
- usually, but not always, observed between different chromosomes
Nonreciprocal Translocation
A
B
C
D
E
F
H
I
J
K
L
M
A
B
C
D
E
H
I
J
K
L
G
M
F
G
Translocations
- usually, but not always, observed between different chromosomes
Reciprocal Translocation
A
B
C
D
E
F
H
I
J
K
L
M
A
B
C
D
E
H
I
J
K
G
L
F
M
G
Homologous chromosomes that are heterozygous for a
translocation form a crosslike configuration during synapsis
Segregation of chromsomes during meiosis produces some
gametes that are unbalanced (i. e. genes are missing).
These gametes cause infertility or lethality (if fertilization occurs).
Robertsonian Fusion is most common type of translocation
observed in humans (1 in 1000 live births)
break at centromere of two
acrocentric chromosomes
fusion of broken chromosomes
Robertsonian Fusion in one parent between chromosomes 14 and 21 is
responsible for a heritable form of Down Syndrome (<5% of cases)
normal,
despite
2N=45
A male Drosophila from a wild type stock was discovered to have
7 chromosomes, whereas the normal 2N number is 8. A karyotype
revealed that a nonreciprocal translocation had occurred so that one
copy of chromosome 4 had attached to the end of chromosome 2. It
lost its centromere.
Diagram all members of chromosomes
II and IV during synapsis in Meiosis I
A male Drosophila from a wild type stock was discovered to have
7 chromosomes, whereas the normal 2N number is 8. A karyotype
revealed that a nonreciprocal translocation had occurred so that one
copy of chromosome 4 had attached to the end of chromosome 2. It
lost its centromere.
Diagram all members of chromosomes
II and IV during synapsis in Meiosis I
-chromosomes replicated
-two pairs of sister chromatids for II
-one pair of sister chromatids for IV
If this male mates with a normal female who is homozygous for the
recessive chromosome IV mutation eyeless (ey), what chromosome
compositions will occur in the offspring regarding chromosomes II
and IV?
What phenotypic ratio will result
regarding the presence of eyes,
assuming all abnormal chromosome
compositions survive?
1) normal (heterozygous)
2) eyeless (monosomic)
3) normal (heterozygous)
4) normal (heterozygous with
translocated copy of chromosome 4)
II
IV
Pretend you are a genetic counselor. A woman was found to be
heterozygous for a chromosomal rearrangement between the
second and third chromosomes.
What type of chromosomal aberration is this?
How would these chromosomes pair during synapsis?
D
D
C
C
A
B
A
B G
H
F
E
G F
E
H
This woman is phenotypically normal. Why? Under
what circumstances might a phenotypic effect occur?
-breaks within genes
-if there are “position
effects”