Download Changes in chromosome number

Course Overview
Changes in chromosome
number
http://www.erin.utoronto.ca/~w3bio/bio207/index.htm
February 1
Outline
Week
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10
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Topic
Course objectives and Introduction to genetics
Human Pedigrees
Patterns of Inheritance: sex-linkage
Chromosomal basis of inheritance
Changes in chromosome number
Gene Mapping
Gene to Phenotype
Modified Mendelian ratios
Model organisms and mutants
Genetics of Plant Development (Arabidopsis)
Genetics of Animal Development (Drosophila)
Behaviour Genetics/Quantitative genetics
Drosophila
Calvin Bridges’ cross
• Drosophila
melanogaster has 4
chromosomes: 1 pair
of sex chromosomes
and 3 pairs of
autosomes
• semi-colons are used
to indicate when
genes are on different
chromosomes
• What is the expected ratio of progeny from the
cross:
w/w X +/Y
White-eyed females X red-eyed males ?
X+
– Eg: w;+;+;+
1;2;3;4
Chapter
Ch. 1 & Ch. 2
Ch. 2
Ch. 2
Ch. 3
Ch. 15
Ch. 4 (Ch. 16)
Ch. 6
Ch. 6
Ch. 6 (Ch. 16)
Ch. 18
Ch. 18
Ch. 16 + papers
4
3
2
Y
Xw Xw
Xw
X+
(red ♀)
Xw Y (white ♂)
Xw Xw
Xw X+ (red ♀)
Xw Y (white ♂)
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Sex determination
• In Drosophila sex is determined by the X:A ratio
– 1X:2A is male
– 2X:2A is female
– 2X:3A is intersex
• In Drosophila melanogaster dosage compensation is by
hyperactivation of the X in males
Text p. 79
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Text p. 491
Aneuploidy
• Aneuploid: an individual organism whose
chromosome number that differs from
wildtype by part of a chromosome set
– E.g. Drosophila nullo X (X0) sterile males
differ from wildtype males by the loss of Y
Text p.490-494
Stages in the production of mature
male and female gametes
2n Spermatogonium
2n Oogonium
2n Primary Oocyte
2n Primary Spermatocyte
Meiosis I
2n Secondary Spermatocytes
2n Secondary Oocyte 2n First Polar body
Meiosis II
n
n n
n
Spermatids
Spermatozoa (sperm)
n
n
n
n
n
Ootid
n
n
n
Three polar bodies
Ovum (egg)
n
Text p. 86
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Monosomic (2n-1)
• The absence of one
copy of a
chromosome from a
pair
– E.g. Turner syndrome
(45 X)
• Phenotype is female
Trisomy (2n+1)
• Trisomics have an extra copy of one
chromosome
• Autosomal trisomies:
– trisomy 13 Patau syndrome
– trisomy 18 Edwards syndrome
– trisomy 21 Down syndrome
• Sex chromosome trisomies:
– Klinefelter syndrome (XXY)
– XYY
– XXX
Trisomic: e.g. 47 XXY
• Klinefelter syndrome
47 XXY karyotype
– Phenotype male but
Barr bodies present
Trisomy 21
• Down syndrome
(Trisomy 21)
– have an extra copy of
chromosome 21
Text p.494
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Text p.493
Trisomic
• Gametic ratio : e.g.
for gene A on a
trivalent chromosome
–
–
–
–
1/6 A
2/6 a
2/6 A/a
1/6 a/a
Dosage compensation
• X-chromosome
inactivation:
– In early development
of female mammals
one X-chromosome
becomes in activated
and this inactivation
persists throughout all
subsequent mitotic
divisions
– Barr bodies: an
inactivated Xchromosome highly
condensed and visible
as a dark staining spot
XX
Mitosis
XX
XX
Mitosis
XX
XX
XX XX
Many Mitoses
Cell line with inactive
maternal X
X- inactivation
Cell line with inactive
paternal X
X- inactivation
E.g. Calico cats
– In early development
of female mammals
one X-chromosome
becomes in activated
and this inactivation
persists throughout all
subsequent mitotic
divisions
Text p.495 and p.324
Text p.495 and p.324
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12 possible trisomics
• The effects of one extra chromosome
– In humans trisomics form almost half of
chromosomally abnormal spontaneous
abortuses. For example trisomy 16 is most
common in abortions and not seen among live
births
– In Datura plants the effects an extra
chromosome can be seen from the shape of
the fruit
The jimsonweed plant (Datura stramonium)
Text p.77
Euploidy
• Euploid: having multiples of the basic
chromosome set
Text p.486
Text p.486
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Chromosome structure
• Duplication: the replication of a segment of chromosome
A. B C D E
becomes
A. B CC D E
• Inversion: a chromosomal rearrangement in which the
chromosome is broken twice and flipped 180 degrees
then rejoined
– A. B C D E
becomes
A. C B D E
• Translocation: the segment of one chromosome moved
to another chromosome
– A. B C D E
becomes
A. B C H I
• Deletion the loss of a segment within one chromosome
and the juxtaposition of the two segments on either side
of the deleted segment
– A. B C D E
becomes
A. B
E
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Inversions
• Inversion: a chromosomal
rearrangement in which
the chromosome is
broken twice and flipped
180 degrees then
rejoined
• The loops in these
polytene chromosomes
reveal the breakpoints of
the inversions
Translocation
• Translocation: the
segment of one
chromosome moved
to another
chromosome
Inversion picture from Text p.618
Deletions
• Deletion the loss of a
segment within one
chromosome and the
juxtaposition of the
two segments on
either side of the
deleted segment
– E.g. cri du chat
syndrome (5p15.2 and
15.3 are deleted)
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Naming chromosomal aberrations
• Drosophila melanogaster :
Duplication -Dp(3L) (i.e. a duplication in the left arm of chromosome-3)
Inversion- In(1) (i.e. Inversion in chromosome-1)
Translocation- T(3R) (i.e. translocation in the right arm of chromosome-3)
Transposition-Tp(2L) (i.e. transposition in the left arm of chromosome-2)
Deficiency- Df(3R) (i.e. deficiency in the right arm of chromosome-3)
More details can be found at:
http://fly.bio.indiana.edu/nomenclature.htm
Human sex chromosomes
SRY
Introduction to genetic analysis
Griffiths, A., Wessler, S.R., Lewontin,R.C., Gelbart, W.M.,Suzuki, D.T.
and Miller, J.H.
Eighth Edition, W.H. Freeman and Company NY
• Part I Transmission genetic analysis
– Chapter 1: all questions p. 24-26
– Chapter 2: all the questions p. 62-72
– Chapter 3: questions #1-12,18,19, 22, 25-27, 29, 30, 32,
40-42.
• Part IV The nature of heritable change
• Chapter 15: sections 15.1 and 15.3; questions #13,11-13,19,21,22, 32, 38, 52.
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