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© 2012 W. H. Freeman and Company
CHAPTER 8 (p 209 – 240)[CHAPTER 10 (p 259 – 288)]
Study Unit 7Chromosome Variation
Study Unit 7Chromosome Variation
Benjamin A. PierceGENETICS
A Conceptual Approach
FOURTH EDITION
© 2012 W. H. Freeman and Company
Chromosome Mutations Include
Rearrangements, Aneuploids, and Polyploids
What to know?• The different types of chromosomal
mutations.
i.e.
- Chromosomal rearrangements:
Duplications, Deletions, Inversions and Translocations.
- Aneuploidy
- Polyploidy
• How they pair during Prophase I of
Meiosis.
• What effect the mutation has on the fertility of the organism.
Chromosome Rearrangements Alter
Chromosome Structure
Duplication:
Deletion:
Inversion:
Translocation:
Duplications
• A chromosome duplication is a mutation where part of the chromosome has been doubled (duplicated).
AB�CDEFG
AB�CDEFEFG AB�CDEFGEF AB�CDEFFEG
tandem displaced reverse
duplication duplication duplication
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Duplications Effects of chromosome duplications
Deletions
• A chromosome deletion is a mutation where part of the chromosome has been lost (deleted).
AB�CDEFG
AB�CDG
Effects of chromosome deletions
Consequences of chromosome deletions
• Phenotypic consequences depend on which genes arelocated on the deleted region.
•Homozygotes are usually lethal.
•Heterozygotes may produce imbalanced gene products.
•Pseudodominance = expression of a normally recessive
mutation on one homolog due to deletion of the dominantallele on the other homolog.
• Some genes need two copies for normal function. When a
single copy is insufficient for the wild-type pt =haploinsufficient gene. (e.g. Notch phenotype in
Drosophila melanogaster.)
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Inversions
• A chromosome inversion occurs when a chromosome segment is inverted – turned 180°.
AB�CDEFG
AB�CFEDG ADC�BEFG
Paracentric inversion Pericentric inversion
(does not include centromere) (includes centromere)
Paracentric inversion(does not include centromere)
Pericentric inversion(includes centromere)
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Consequences of chromosome inversions
• DNA neither lost nor gained, just rearranged.
•Often significant phenotypic effects.
•E.g.•Gene may be broken into 2 parts
•Position effect
Translocations
• A chromosome translocation occurs when genetic material moves between nonhomologous chromosomes
(translocates).
AB����CDEFG MN����OPQRS AB����CDEFG MN����OPQRS
AB����CDG MN����OPEFQRS AB����CDQRG MN����OPEFS
Nonreciprocal translocation Reciprocal translocation
Effects of chromosome translocations
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Robertsonian translocation Consequences of chromosome translocations
E.g. Philadelphia translocation chromosome
Reciprocal translocation between human chromosomes 9 and 22. Associated
with chronic myelogenous leukaemia (CML). Produces an oncogenic gene fusion.
Fragile sites
Fragile sites are constrictions or gaps at particular places on chromosome that are prone to breakage.
Common fragile sites: found in all humans, normal feature of chromosomes.
Often location of chromosome breakage and
rearrangement in cancer cellsLead to deletions, translocations and other
chromosome mutations.
Rare fragile sites:
Found in few people, inherited as Mendelian trait.
Often associated with mental retardationMost often expanding nucleotide repeats
E.g. Fragile-X-Syndrome C D
C D
a compensation loop
Synapsis configurations of chromosome rearrangements
Deletion heterozygote
Duplication heterozygote
Paracentric inversion heterozygote
Pericentric inversion heterozygote
Translocation heterozygote
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Problems
3. The following diagram illustrates a homozygous chromosome inversion during meiosis:
a. Name this type of inversion.
b. Draw the daughter chromosomes and/or fragments that would be found after segregation of the chromatids in the diagram below.
4. Diagram the chromosome synapsis during meiosis of thefollowing translocation heterozygote. The normal chromosome
1 carries loci abcdefg and chromosome 2 carries tuvwxyz.Following reciprocal translocation, the resulting translocated
chromosomes are abcdvwxyz and tuefg. Assume thecentromere is at the left end of all the chromosomes.
Aneuploidy Is an Increase or Decrease in the
Number of Individual Chromosomes.
• Causes of Aneuploidy:
– Deletion of centromere during mitosis and
meiosis
– Robertsonian Translocation
– Nondisjunction during meiosis and mitosis
• Fig. 10.19
Types of Aneuploidy
• Nullisomy: loss of both members of a
homologous pair of chromosomes. 2n − 2
• Monosomy: loss of a single chromosome.
2n − 1
• Trisomy: gain of a single chromosome. 2n + 1
• Tetrasomy: gain of two homologous
chromosomes. 2n + 2
Aneuploidy Is an Increase or Decrease in the
Number of Individual Chromosomes
• Effects of Aneuploidy:
– In plants: Fig. 10.20.
– In humans:
• Sex-chromosome aneuploids:
– Turner syndrome. XO
– Klinefelter sydrome. XXY
(Stine, 1989)
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Aneuploidy Is an Increase or Decrease in the
Number of Individual Chromosomes
• Effects of Aneuploidy:– In humans:
• Autosomal aneuploids:
– Trisomy 21 – down sydrome
» Primary Down Syndrome, 75% random
nondisjunction in egg formation. Fig. 10.21
» Familial Down Syndrome. Robertsonian translocation between Chromosome 14 and 21. Fig. 10.22 and
23.
9.3 Aneuploidy Is an Increase or Decrease in
the Number of Individual Chromosomes
• Effects of Aneuploidy:– In humans:
• Autosomal aneuploids:
– Trisomy 18 – Edward syndrome, 1/8000 live births
– Trisomy 13 – Patau syndrome, 1/15,000 live births
– Trisomy 8 1/25,000 ~ 1/50, 000 live births
– Why there is a drastic decrease in frequency of these
trisomic syndrome from Chromosome 18 to chromosome 8?
Gynandromorph
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9.3 Aneuploidy Is an Increase or Decrease in
the Number of Individual Chromosomes
• Effects of Aneuploidy:– In humans:
• Autosomal aneuploids:
– Aneuploidy and maternal age. Fig. 9.24
– Possible interpretation of this connection?
• Uniparental disomy: both chromosomes are inherited from
the same parent.
– Mosaicism and nondisjunction in mitotic division.
9.4 Polyploidy is the Presence of More Than
Two Sets of Chromosomes
• Autopolyploidy:
– From single species.
• Fig. 10.26
• Allopolyploidy
– From two species
• Fig. 10.28
Autotriploids (3n) arise in several ways:
1. 1st or 2nd division nondisjunction →→→→ 2n gamete
2n + n = 3n
2. n + n + n = 3n zygote
3. 2n x 4n = 3n
Triploids often produce genetically unbalanced
gametes with odd numbers of chromosomes
• infertile
Autopolyploids generally larger than diploid relatives
•Due to larger cells as opposed to larger number of cells
•Increased size → → → → increased commercial value
Triploids (3n):
Tetraploids (4n)
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Octoploids (8n)Allopolyploidy
The significance of polyploidy
• Polyploidy is less common in animals than plants.
• Allopolyploidy requires interspecific hybridization – less
frequent in animals – behaviour & complex development renders most hybrids nonviable.
• Parthenogenesis – greater opportunities for non-disjunction.
• Few human polyploid babies, die within days of birth.
• Polyploidy (usually 3n) found in 10% of all spontaneously
aborted human foetuses.
• Bread wheat = hexaploid = 6n = 42 (derived 3 different
species) (Fig. 10.29)
Polyploidy is the Presence of
More Than Two Sets of
Chromosomes
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Allopolyploidy
• Polyploidy can result from hybridization of two closely related species
• AA X BB
• A B
• AB
• Where A has a1; a2; a3; …an chromosomes, and
• B has b1; b2; b3; …bn chromosomes
Hybrid plant may be sterile – unable to form viable gametes
Some or all of a and b chromosomes not homologous – cannot synapse during meiosis ~ unbalanced genetic conditions
AB undergoes chromosome doubling → → → → fertile AABB tetraploid
Called an allotetraploid
When both original species are known = amphidiploid
If amphidiploids are formed from closely related species, some homology will exist between a and b chromosomes.
Called homeologous chromosomes
During synapsis:
Multivalents may form, produces unbalanced gametes
9.5 Chromosome Variation Plays an Important
Role in Evolution
• New and extra copies of genes give rise to new functions.
• New and extra sets of genes may give rise
to new species.
Problems
1. For a species with a diploid number of 20, how many chromosomes will be present in the somatic cells of
individuals who are:a. triploid
b. tetraploidc. monosomicd. Trisomic