Topic 9, Chromosome mutations
(variations in
chromosome number and arrangement)
Variations in chromosome number
- Introduction:
A. Each organism has a typical number of chromosomes
B. Each organism has a typical number of sets of chromosomes
C. Ploidy = the number of sets of chromosomes per cell
D. Euploid: the chromosome number is an exact multiple of the haploid set
E. Aneuploid: the chromosome number is not an exact multiple of the haploid set
(there is the loss or addition of 1 or more of the chromosomes)
F. We will discuss
1) Variations in chromosome number (aneuploids and euploids)
2) Variations in chromosome structure
- Aneuploids
A. Aneuploid terminology
1) Definitions of diploid, monosome, trisome, tetrasome)
2) Plants are far more tolerant of aneuploidy than animals
3) Aneuploids are typically produced by non-disjunction
a. Definitions of disjunction and
non-disjunction (defined earlier in definition sheet 1)
b. Description of how nondisjunction of X
chromosome at the first or second meiotic division
can give rise to Turner
and Klinefelter syndrome individuals
B. Monosome (noun), monosomy (adjective), 2n-1
1) Can be caused by nondisjunction or chromosome loss
2) In humans, Turner syndrome, monosomics for other chromosomes do not
survive until birth
3) In Drosophila melanogaster, monosomy of the X and 4th
chromosomes are viable
4) In Zea mays, monosomics for all 10 chromosomes have been
recovered
5) Partial monosomy in humans, Cri-du-chat syndrome (46,-5p)
a. French for cry of the cat
b. Due to deletion of part of the short arm of
chromosome 5 in the heterozygous condition
C. Trisome (noun), trisomy (adjective), 2n+1
1) A trivalent (association of 3 chromosomes) is present in prophase of
meiosis in a trisome)
2) Trisomes for all of the chromosomes have been recovered in many
plants
3) In Drosophila melanogaster, trisomes for the X chromosome
and chromosome 4 have been recovered
4) In Homo sapiens, trisomes for the X chromosome (XXX) and 3
autosomes (21, 13, and 18) survive to birth
a. Down syndrome, trisomy 21 (47,+21)
(1) Frequency increases
greatly as the mother's age increases
b. Ptau syndrome, trisomy 13 (47,+13)
c. Edwards syndrome, trisomy 18 (47,+18)
- Euploids
A. Euploid terminology, haploid, diploid, triploid, tetraploid, polyploid,
autopolyploid, allopolyploid
B. Haploids (n)
1) Many lower plants and and animals are haploids
2) Haploids can be recovered in many higher plants
C. Diploids (2n), the normal type for most higher plants and animals
D. Autotriploids (3n)
1) The three copies of each chromosome form trivalents during prophase
I of meiosis
2) Seedless watermelons are triploid
a. Seedless watermelon seeds are produced by
crossing diploids (which produce n gametes) with tetraploids (which produce 2n gametes)
3) Bananas in our grocery stores are seedless because they
are triploids
4) Triploids occur in humans but do not survive until birth
(abort during fetal development)
E. Autotetraploids (4n)
1) Quite fertile with a slight reduction in fertility
2) The four copies of each chromosome form quadravalents during
prophase I of meiosis
3) Tetraploids are typically larger and more vigorous than their
diploid counterparts
4) Many ornamental plants are autotetraploids including the Easter lily
5) Doubling the number of chromosomes using a spindle poison (e.g.,
colchicine)
F. Allotetraploids (4n with the genomes coming from more than one species)
1) Raphanus (radish) x Brassica (cabbage) crosses to
produce an allotetraploid
2) The origin of American cotton, an allotetraploid
3) Triticale, an allopolyploid produced from crosses between
wheat (Triticum) and rye (Secale)
4) Polyploidy in evolution
a. 30-35% of higher plants are polyploid, and
75% of species have some polyploid forms
b. Many crop plants are polyploids including
wheat, potato, peanuts, cotton, strawberries, etc.
G. Endopolyploidy is produced by endomitosis
Variation in the structure and arrangement of chromosomes
- Deletions = deficiencies, loss of a segment of a chromosome
a. Terminal deletion: loss of the end of a chromosome
b. Internal = intercalary deletion: loss of an internal segment of a chromosome
c. Deficiency mapping: using deficiencies to determine the locations of genetic loci on
the chromosomes
d. Effects of deletions
1) Large deletions are often lethal in the heterozygous condition
(dominant lethals)
2) Small deletions are often lethal in the homozygous condition
(recessive lethals)
3) The genetic loci in the deleted segment are important
e. Cri-du-chat syndrome, a terminal deletion that was previously discussed
- Duplications, a segment of a chromosome is present in more than one copy
a. Classification of duplications (intrachromosomal, tandem, non-tandem,
interchromosomal)
b. Duplications usually do not have lethal effects
c. The bar eye mutation in Drosophila melanogaster is actually a duplication
1) Mutation from bar to wild-type or double bar occurs very frequently
and is associated with recombination
2) These mutations occur by recombination between duplicated regions
(16a segments) which increase or decrease the copy number of this segment
3) Position effect
d. Importance of duplication in the evolutionary process
- Inversions, a segment of a chromosome is present in reverse order
a. Paracentric inversion, both breakpoints are in the same chromosome arm and the
segment between the breakpoints is inverted
1) Pairing in prophase I of meiosis
2) Behavior at anaphase I of meiosis
3) Gametes produced
4) Some generalized statements
b. Pericentric inversion, one breakpoint occurs in each arm and the segment between the
breakpoints is inverted
1) Pairing in prophase I of meiosis
2) Behavior at anaphase I of meiosis
3) Gametes produced
4) Some generalized statements
c. Why inversions are crossover suppressors
- Translocations
a. Non-reciprocal translocations
b. Reciprocal translocations
1) Pairing in prophase I of meiosis
2) Behavior at anaphase I of meiosis
3) Gametes produced
4) Some generalized statements
c. Familial Down syndrome (due to a translocation)
- Ring chromosomes, two breaks occur in a chromosome and the broken ends join to
form a closed circle
a. Unstable and frequently lost
b. McClintock's 1938 study with a ring chromosome which showed
1) Centromeres are redundant structures
2) Sister chromatid exchange takes place
- Fragile-X syndrome (Martin-Bell syndrome)
a. Due to a CGG trinucleotide repeat
- Summary of chromosome aberrations
a. Quantitative changes (duplications and deficiencies)
b. Qualitative changes (inversion, translocations, ring chromosomes)
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Updated 10/18/00