Download Pierce5e_ch08_lecturePPT

Benjamin A. Pierce
GENETICS
A Conceptual Approach
FIFTH EDITION
CHAPTER 8
Chromosome Variation
© 2014 W. H. Freeman and Company
8.1 Chromosome Mutations Include
Rearrangements, Aneuploids, and
Polyploids
• Chromosome Morphology (position of the
centromere on the chromosome):
‒ Metacentric
‒ Submetacentric
‒ Acrocentric
‒ Telocentric
8.1 Chromosome Mutations Include
Rearrangements, Aneuploids, and
Polyploids
• Karyotyping
– Chromosomes prepared from actively dividing cells
– Halted in metaphase
– Chromosomes arranged according to size
• Banding
–
–
–
–
G bands: Giemsa stain
Q bands: quinacrine mustard
C bands: centromeric heterochromatin
R bands: rich in cytosine-guanine base pairs
8.1 Chromosome Mutations Include
Rearrangements, Aneuploids, and
Polyploids
• Types of Chromosome Mutations:
– Rearrangements:
• Aneuploidy
• Polyploidy
8.2 Chromosome Rearrangements Alter
Chromosome Structure
• Four types of chromosomal rearrangements:
Fig. 8.4
• Duplication: Fig. 8.5, Fig. 8.6, Fig. 8.7
• Unbalanced gene dosage: Fig. 8.8
8.2 Chromosome Rearrangements Alter
Chromosome Structure
• Deletions: loss of a chromosomal segment
• Large deletions easily detected; during pairing,
normal chromosome loops out (Fig. 8.9)
• Effects of deletions
‒ Imbalances in gene product
‒ Expression of a normally recessive gene
(pseudodominance)
‒ Haploinsufficiency
8.2 Chromosome Rearrangements Alter
Chromosome Structure
• Inversion (depends on the involvement of the
centromere in the inversion):
‒ Paracentric inversion
‒ Pericentric inversion
8.2 Chromosome Rearrangements Alter
Chromosome Structure
• Inversions in meiosis:
• Individuals homozygous: no problems arise
during meiosis
• Individuals heterozygous:
‒ homologous sequences align only if the two
chromosomes form an inversion loop
‒ Demonstrate reduced recombination in a
paracentric inversion, as gametes formed result in
nonviable offspring
‒ Have abnormal gametes formed in a pericentric
inversion
Concept Check 1
A dicentric chromosome is produced when
crossing over takes place in an individual
heterozygous for which type of chromosome
rearrangement?
a.
b.
c.
d.
Duplication
Deletion
Paracentric inversion
Pericentric inversion
Concept Check 1
A dicentric chromosome is produced when
crossing over takes place in an individual
heterozygous for which type of chromosome
rearrangement?
a.
b.
c.
d.
Duplication
Deletion
Paracentric inversion
Pericentric inversion
Translocations
• Nonreciprocal translocation
• Reciprocal translocation
• Robertsonian translocation
Translocations in Meiosis
Translocations in Evolution
Concept Check 3
What is the outcome of a Robertsonian
translocation?
a. Two acrocentric chromosomes
b. One metacentric chromosome and one
chromosome with two very short arms
c. One metacentric and one acrocentric
chromosome
d. Two metacentric chromosomes
Concept Check 3
What is the outcome of a Robertsonian
translocation?
a. Two acrocentric chromosomes
b. One metacentric chromosome and one
chromosome with two very short arms
c. One metacentric and one acrocentric
chromosome
d. Two metacentric chromosomes
8.3 Aneuploidy Is an Increase or
Decrease in the Number of Individual
Chromosomes
• Variations in copy number: aneuploidy and
polyploidy
• Causes of aneuploidy:
– Deletion of centromere during mitosis and
meiosis
– Robertsonian translocation
– Nondisjunction during meiosis and mitosis
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
Concept Check 3
A diploid organism has 2n = 36 chromosomes.
How many chromosomes will be found in a
trisomic member of this species?
Concept Check 3
A diploid organism has 2n = 36 chromosomes.
How many chromosomes will be found in a
trisomic member of this species?
2n + 1 = 36 + 1 = 37
8.3 Aneuploidy Is an Increase or
Decrease in the Number of Individual
Chromosomes
• Effects of Aneuploidy:
– In plants: Fig. 8.20.
– In humans:
• Sex-chromosome aneuploids:
– Turner syndrome. XO
– Klinefelter sydrome. XXY
8.3 Aneuploidy Is an Increase or
Decrease in the Number of Individual
Chromosomes
• Effects of Aneuploidy:
– In plants: mutants could actually be trisomics (Fig. 8.20)
– In humans:
• Autosomal aneuploids:
– Trisomy 21: Down syndrome
» Primary Down syndrome, 75% random
nondisjunction in egg formation (Fig. 8.21)
» Familial Down syndrome, Robertsonian translocation
between chromosomes 14 and 21 (Figs. 8.22 and
23)
8.3 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 is there a drastic decrease in frequency of this
trisomic syndrome from chromosome 18 to chromosome
8?
Concept Check 4
Briefly explain why in humans and mammals, sexchromosome aneuploids are more common than
autosomal aneuploids.
Concept Check 4
Briefly explain why in humans and mammals,
sex-chromosome aneuploids are more common
than autosomal aneuploids.
No mechanism of dosage compensation for
autosomal chromatids. Autosomes carry more
genes. Most autosomal aneuploids are
spontaneously aborted.
8.3 Aneuploidy Is an Increase or
Decrease in the Number of Individual
Chromosomes
• Effects of Aneuploidy:
– In humans:
• Autosomal aneuploids:
– Aneuploidy and maternal age. Fig. 8.24
– Possible interpretation of this connection?
• Uniparental disomy: both chromosomes are inherited from
the same parent.
– Mosaicism and nondisjunction in mitotic division.
8.4 Polyploidy is the Presence of More
Than Two Sets of Chromosomes
• Autopolyploidy:
– From single species
• Fig. 8.26
• Allopolyploidy
– From two species
• Fig. 8.28
8.4 Polyploidy is the Presence of More
Than Two Sets of Chromosomes
• The significance of polyploidy:
– Increase in cell size
– Larger plant attributes
– Evolution: may give rise to new species
Concept Check 5
Species A has 2n = 16 chromosomes and species
B has 2n = 14. How many chromosomes would be
found in an allotriploid of these two species?
a.
b.
c.
d.
21 or 24
42 or 48
22 or 23
45
Concept Check 5
Species A has 2n = 16 chromosomes and species
B has 2n = 14. How many chromosomes would be
found in an allotriploid of these two species?
a.
b.
c.
d.
21 or 24
42 or 48
22 or 23
45