Download Chapter 11 Chromosome Mutations

Chapter 11
Chromosome Mutations
• Changes in chromosome number
• Chromosomal rearrangements
• Evolution of genomes
Aberrant chromosome constitutions of a normally diploid organism
Name
Monoploid
Designation
Constitution
Chromosome
number
n
ABC
3
Diploid
2n
AABBCC
6
Triploid
3n
AAABBBCCC
9
Tetraploid
4n
AAAABBBBCCCC
12
Monosomic
2n-1
ABBCC
5
Trisomic
2n+1
AABBCCC
7
Changes in chromosome number
About 70% of the plant species can be
considered as polyploids.
Polyploidization is often used in plant
breeding.
Euploidy
Organisms with multiples of the basic
chromosome set are called euploid.
The size of epidermal leaf cells of tobacco
increases with the ploidy level.
Generating monoploid plants by tissue culture
Pollen (haploid) are grown in tissue culture in the presence of certain plant
hormones. Haploid embryoids develop that can be grown into mature monoploid
plants. Advantage for plant breeding?
The use of colchicine to generate a diploid from a monoploid.
Colchicine disrupts spindle fibre formation during mitosis, preventing the migration
of chromatids after the centromere is split. A single cell results that contains pair of
identical chromosomes that are homozygous at all loci.
The use of colchicine to generate a diploid from a monoploid.
Why are monoploid plants produced? Is there any genetic advantage?
Monoploids
Male bees, wasps and ants are monoploid. These males develop
parthogenetically from unfertilized eggs.
However, in most species, monoploids are abnormal individuals.
Polyploids are distiguished in
autopolyploids,
that contain multiple chromosome sets from one species and
allopolyploids
that are composed of chromosome sets from two or more different
species. Allopolyploids form only between closely related species.
Triploids
are usually autopolyploids. They can result from a cross of a tetraploid
with a diploid. Polyploids with odd numbers of chromosome sets are
sterile, because their gametes are aneuploid. (Note that each chromosome
is really a pair of chromatids).
Meiotic pairing possibilities in tetraploids.
(Note that each chromosome is really a pair of chromatids).
Functional
gametes
Nonfunctional
gametes
Complex segregation ratios result from tetraploids
The amphidiploid (or allotetraploid) Raphanobrassica originates from
cabbage (Brassica) and radish (Raphanus)
The amphidiploid Triticale originates from
rye (Secale) and wheat (Triticum)
The evolution of modern
hexaploid wheat, in which
amphidiploids are produced
at two points. A, B, D are
different chromosome sets.
Aneuploidy
Chromosome number differs from wild
type by part of the chromosome set.
Aneuploid organisms are produced
mainly by nondisjunction at meiosis.
Monosomics (2n-1)
Turner syndrome (X0)
Trisomics (2n+1)
Klinefelter syndrome (XXY)
Trisomics (2n+1)
Down syndrome
(trisomy 21)
Maternal age and the incidence of trisomy 21
What is the reason that the sex chromosomes or chromosome 21
are often observed in human aneuploids?
The concept of gene balance.
Dosage compensation
X-chromosome inactivation
Chromosomal rearrangements
Origins of chromosomal rearrangements
Origins of chromosomal rearrangements
Inversions
A
B
C
D
E
F
paracentric
A
B
C
E
D
F
pericentric
A
D
B
E
F
normal sequence
C
Inversion heterozygotes pair in a loop at meiosis
Synaptonemal complexes in mouse heterozygous for a paracentric inversion
Meiotic products resulting from a
single crossover within a paracentric
inversion loop. Two nonsister
chromatids cross over within the loop.
Meiotic products resulting from a meiosis with a single crossover
within a pericentric inversion loop.
Message
Heterozygous inversions are characterized by reduced fertility
from unbalanced or deleted meiotic products.
Reciprocal translocations
The meiotic products resulting
from two most commonly
encountered chromosome
segregation patterns in a
reciprocal translocation
heterozygote.
A semisterile maize plant producing normal and aborted pollen. The clear pollen grains
contain chromosomally unballanced meiotic products of a reciprocal translocation
heterozygote. The opaque grains are normal.
Message
Heterozygous reciprocal
translocations are
characterized by semisterility.
Message
Chromosomal rearrangements lead to reduced fertility and
contribute to speciation.
Position-effect variegation
Evolution of genomes and speciation
1.
The genomes are made up of many duplicated segments
2.
The genomes of two related species are
characterized by many structural changes.
Chromosomal synteny
Chromosomal segments of two species that contain similar genes
in an identical order are said to be synthenic.
Syntheny of human and mouse chromosomes