Transcript Document

Chapter 15: Large-scale chromosomal changes
Fig. 15-2
Aberrant euploidy (usually polyploidy) and aneuploidy
Cell size typically
reflects ploidy
2N and 4N grapes
Fig. 15-4
Fig. 15-12
Types of polyploidy
Autopolyploidy: multiple copies of identical
chromosome sets; usually develop normally;
cells are proportionately larger than diploid
Alloploidy: multiple copies of non-identical
chromosome sets; includes genomes of two
different species; usually display “hybrid”
characteristics
Autotriploids routinely generate aneuploid gametes
(usually sterile)
Fig. 15-5
Autotetraploids are readily generated
by suppressing mitotic spindle
Fig. 15-6
Autotetraploids routinely generate aneuploid gametes
(usually sterile)
Fig. 15-7
Allopolyploids arise from interspecific
hybridization + genome duplication
Fig. 15-8
Likely origins of modern hexaploid wheat
Fig. 15-10
Aneuploidy: extra or missing chromosomes
(less than an entire haploid set)
Examples:
monosomy: 2n – 1
(one chromosome has no homolog)
trisomy: 2n + 1
(three homologs for one chromosome)
Aneuploidy arises from
meiotic nondisjunction,
forming aneuploid
gametes/spores
Fig. 15-13
Aneuploids produce aneuploid gametes/spores
Fig. 15-15
Viable human aneuploids are mostly limited to
the smallest chromosomes and to the sex
chromosomes
Examples:
trisomy-21: Down syndrome
XO (no Y): Turner syndrome; primarily female;
only viable human monosomic
XXY: Klinefelter syndrome; primarily male
Down syndrome: the clinical manifestations of trisomy-21
Fig. 15-17
The frequency of non-disjunction leading to trisomy-21
(and other aneuploidy) is correlated with maternal age
Fig. 15-18
Dosage compensation: mechanism for
making X-linked gene expression equal in
females (with two X chromosomes) and in
males (with one X chromosome)
In mammals: only one X chromosome is
active in each cell
In Drosophila: the activity of each X-linked
gene copy is reduced in multi-X cells
Thus, “gene balance” problems are alleviated in
commonly occurring sex chromosome aneuploids
Chromosomal rearrangements
•
•
•
•
Arise from double-strand DNA breaks
Such artificial ends are very transient and rapidly
join together
Rejoining may restore the chromosome or may result
in any imaginable combination of joined fragments
Recovery of those products follows certain rules:
1. Each product must have no more nor less than
one centromere
(a mitotic and meiotic “must”)
2. Viability of the gametes/spore/zygote following
meiosis is subject to gene balance effects
(segmental aneuploids are usually poorly
viable)
Types and origins of chromosomal rearrangements
Unbalanced
rearrangements
Balanced
rearrangements
Fig. 15-19
Consequences of inversions on neighboring genes
Fig. 15-20
Meiotic consequences of inversion heterozygosity
Fig. 15-21
Crossingover within inversion
loops result in chromosome
duplications/deletions
Paracentric/Pericentric
Crossover products yield
inviable gametes/progeny
• non-crossovers predominate
• outside markers appear
closer than they really are
• crossingover is suppressed
Fig. 15-22
Meiosis in translocation
heterozygotes can result
in duplication/deletion
gametes/spores
Fig. 15-24
Loops are also seen in synapsed homologs
in deletion heterozygotes
Fig. 15-28
Deletions behave genetically as multi-gene
loss-of-function mutations
Deletions are useful in physically mapping
small chromosome regions
Fig. 15-29
Incidence of
chromosome mutations
in humans
Fig. 15-33
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Fig. 15-