Download Slides on chromosomal changes

Summary of large-scale chromosomal changes Chp 17
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2.
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Deletion of small or larger segments
Missing 1 or a few chromosomes
Additional chromosomes
Duplication of small to larger segments
Inversion of segments
Translocation of segment from one to anthor
See fig 17-2
Significance of chromosomal mutations or changes
1) They often characterize species differences and are often responsible for reproductive
isolation between species.
2) A number of crop plants have undergone such changes and chromosome
manipulation may be important in agriculture (breeding).
3) A number of such changes are responsible for human genetic diseases.
4) They may disrupt gene function directly if a break occurs in a gene.
5) Can lead to genetic redundancy, and the subsequent evolution of gene
function, differential expression, or gene loss
Polyploidy
Possession of more than two haploid sets of
chromosomes.
Diploids have two homologs of every
chromosome.
Polyploids might have 3, or 4, or 5 etc of every
homolog. So their chromosome number will
usually be a multiple of the haploid chromosome
number, n.
Some genera may have different species that form
a polyploid series such as:
2x (diploid); 3x (triploid), 4x tetraploid, 6x
hexaploid, 8x octoploid, where x is the basic
haploid number of chromosomes.
1st metaphase of meiosis in diploid Turnera subulata, 2n=10
1st metaphase of meiosis in autotetraploid Turnera subulata, 4x=20.
1st metaphase of meiosis in autotriploid Turnera subulata, 3x=15
1st metaphase of meiosis in allohexaploid Turnera velutina, 6x=30
Taraxacum officionale (Dandelion) is a triploid apomict
2n = 3x = 24
It flowers, and sets seeds, but circumvents meiosis producing
seeds through mitosis not meiosis and fertilization. All progeny
and the parental plants should be geneticially identical to
on another.
Triploid parthenogenetic salamanders and lizards are known to occur.
Musa acuminata x M. balbisiana 2n = 3x = 33.
Genome are AAA, AAB, or ABB
Triploid parthenocarps (fruits but no seeds).
Autopolyploids versus Allopolyploids
Autopolyploids: chromosome derived from a single
species.
- Exhibit tetrasomic inheritance
- Quadrivalents at MI
Allopolyploids: chromosomes derived from more
than one species
- Genome-wide gene duplication
- Bivalent formation at MI
Tetrasomic Inheritance in an autotetraploid
For two alleles, A and a, 5 possible genotypes occur
AAAA, AAAa, AAaa, Aaaa, aaaa
To deduce results of crosses determine gametic output of
each genotype:
AAAA yields diploid gametes all of which are AA
AAAa - ½ AA : ½ Aa
AAaa – 1/6AA : 4/6 Aa : 1/6 aa
Aaaa – ½ Aa : ½ aa
aaaa – all aa
So the cross AAaa x aaaa give 5 A- : 1 aaaa ratio
What does this produce AAaa x AAaa?
Tetrasomic inheritance of the aconitase-2 locus in
autotetraploid Turnera subulata 2n = 4x = 20.
From Shore JS. 1991. Heredity 66: 305—312
Origins of tetraploid Brassica species
See fig 17-8
Brassica oleraceae 2n=18
cabbage, cauliflower, broccoli, kale
kohlrabi, brussels sprouts
B. carinata, 2n = 34
B. napus, 2n = 38
Abyssinian mustard
Canola, Rutabaga
B. nigra, 2n = 16
B. juncea, 2n = 36
Black mustard
Leaf mustard
B. campestris, 2n = 20
Chinese cabbage, turnip, turnip
rape
Origin of allopolyploid Wheats, Triticum
See fig 17-9
T. urartu, 2n = 14
Aegilops speltoides, 2n = 14
AA genome donor
Wild diploid wheat
BB genome donor
Diploid goatgrass or extinct relative of it
Aegilops tauschii, 2n = 14
T. turgidum, 2n = 28
DD genome donor
Another wild grass species
AABB genomes
Emmer wheat ~10,000 year ago
T. aestivum, 2n = 42
AABBDD genomes
Common bread wheat
Genomic in situ hybridization (GISH) of allohexaploid wheat,
Triticum aestivum 2n = 42
Kato et al. 2005
A genome - yellow
D genome - red
B genome - brown
Green - chromosome
fragments of Thinopyrum
intermedium
GISH in a Gossypium hirsutum x G. sturtianum hybrid
2n = 3x = 39 (ADC triploid)
G. hirsutum (cotton) is an allotetraploid AADD
G. hirsutum A genome
pink
G. hirsutum D genome
dark blue
G. sturtianum C genome
light blue
Guan et al. 2007
Triploid Salamander, Ambystoma jeffersonianum-laterale
2n = 3x = 42 Genomes are JJL. Photo by Dr. L.E. Licht
CHANGES IN CHROMOSOME NUMBER AND
STRUCTURE
1. ANEUPLOIDY – MONOSOMY, TRISOMY
2. GENE “BALANCE”, DOSAGE, AND INVIABILITY
3. DELETIONS AND DUPLICATIONS
4. INVERSIONS
5. TRANSLOCATIONS
ANEUPLOIDY
Changes in chromosome number not involving complete
haploid sets
Humans: XXY, XYY, XXX, X0
Causes: Nondisjunction
Monosomy 2n-1 (lethal in humans with exception of X0)
X0 – Turner syndrome – phenotypic effects including some
level of congitive impairment
Trisomy 2n+1
XXY – Klinefelter syndrome, Males, lower IQ, sterile
XYY – once thought to have enhanced violence. Not clear
XXX – females normal
Trisomy 21 – Down syndrome
13- Patau syndrome – 130 day life expectancy
18 – Edwards syndrome – a few weeks
Gene balance may provide an explanation for
detrimental effects of trisomy and monosomy.
CHROMOSOME REARRANGEMENTS
Possible routes to chromosome rearrangements
See fig 17-19
Two break points are typically required.
If these occur on a single chromosome this can result in a deletion
or and inversion.
If two nonhomologous chromosome each has a break, a
Reciprocal translocation can occur.
If two homologous chromosomes each have break point this can
produce both duplications and deletions
Breakage an reunion leading to rearrangements
a
b
c
d
e
f
See fig 17-19
g
h
i
Breakage an reunion leading to rearrangements
a
a
b
b
c
c
d
d
e
e
f
f
See fig 17-19
g
g
h
h
i
i
Chromosome deletions and inversions may be visible
using microscopy especially in the giant/polytene
chromosomes of Drosophila or a pachytene
–see fig 17-20
A B
C
G H
I
F
D E
J
A B
C
D E
J
K
L
M
K
L
M
Possible routes to chromosome rearrangements
Via repetitve elements
See fig 17-19
It is possible that crossing over between repetitive elements
(e.g. transposable elements) generates the various rearrangements
Crossing over between elements on a single chromosome can
result in a deletion or inversion.
If two nonhomologous chromosome each have the same repeated
Element, crossing over between them can yield a reciprocal
translocation.
If two homologous chromosomes each have the same repeated
element, crossing over can produce both duplications and deletions
Chromosome 5 showing deletion responsible for Cri du chat syndrome
See fig 17-22.
Deletion
Note that deletions can be recognized and are often very useful in identifying the
locations of genes of interest.
a
A
b
c
d
e
f
g
h
E
F
G
H
Inversions
See Fig 17-27
a
b
c
d
e
f
g
h
i
A
B
C
F
E
D
G
H
I
Reciprocal translocations
See fig on page 589 for photograph of chromosomes
Fig 17-30 for pairing and results of segregation of reciprocal
Translocations
Fig 17-33 occurrence of Down syndrome when chromosome
21 is involved in a reciprocal translocation
Reciprocal translocation
Alternate disjuntion
- Viable gametes
Breaks and rejoining
Adjacent disjunction
-often inviable due to deletions
and duplications
Robertsonian fusion – asymmetrical reciprocal
translocation involving centromere loss see fig 17-33
Chr 14 chr 21
Normal
Robertsonian
Lost
Familial form of Down Syndrome involving
Robertsonian fused chromosome 21
Carrier – Normal
Down syndrome
Fate of human zygotes resulting from various chromosome
abnormalities see fig 17-37
Spontaneously aborted embryos (15% of conceptions)
Of 150,000 such embryos
75,000 due to chromosome abnormalities
39,000 of these are trisomics for various chromosomes
13,500 are XO
12,750 triploids
4,500 tetraploids
5,250 other chromosome abnormalities.
For live births (i.e. the remaining 85% of conceptions
For live births (i.e. the remaining 85% of conceptions)
About 0.6% of those result from chromosome abnormalities
Including various numbers of
Sex chromosome aneuploids
Trisomics for chromosomes 13, 18, 21
Robertsonian translocations and other reciprocal translocations
Inversions
Others, small deletions etc.