THE ROLE OF GENETICS IN MEDICINE & Chromosomal Basis of Heredity Dr S.M.B.Tabei.
Mendelian Genetics and the Chromosomal Basis for...
Transcript of Mendelian Genetics and the Chromosomal Basis for...
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Mendelian Genetics and the Chromosomal
Basis for Inheritance
Gregor Mendel
began the formal
study of genetics.
1866 - showed that
inherited factors are
passed down unchanged
from one generation to
the next.
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Mendelian Genetics and the Chromosomal
Basis for Inheritance
Mendel’s methods
were central to his
success:
• used pure
breeding lines • made controlled
crosses
• followed traits for
two generations
• counted many
progeny
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Monohybrid Cross
Pure breeding lines:
• multiple crosses
within strains to
insure pure breeding
• controlled crosses
between strains
• consistent,
repeatable outcomes Yellow and green peas appear in the F2
6022 yellow, 2001 green = 3:1
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Mendel’s discovery led to a genetic hypothesis
• Genes are passed
down unchanged
• Genes exist as pairs
• Members of pairs
separate evenly
during gamete
formation (Mendel’s
1st law) Mendel counted
the F2 of crosses
from seven
different traits
Repeatable 3:1
ratios led to three
conclusions
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A single gene model explains Mendel’s results
Yellow F2 selfed:
1/3 all yellow
2/3 yellow, green
Green F2 selfed:
all green
• Passed down
unchanged
• Exist as pairs
• Equal
segregation
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Mendel needed
a direct test of
his hypothesis
½ Y
½ y
?
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Test Cross
½ Y
½ y
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Figure 2-12 part 15
A test cross repeatably produces a 1:1 ratio
Test cross:
confirmed
Mendel’s
hypothesis
of even
segregation
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Figure 2-12 part 15
A single-gene model explains Mendel’s ratios
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Figure 2-12 part 15
A single-gene model explains Mendel’s ratios
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Summary of Mendel’s Results
Monohybrid Cross
Yellow green
YY yy
F1 all Yellow, Yy
gametes = ½Y , ½ y
F1 selfed =
Yy Yy
F2 = YY
Yy Yellow
yY
yy green
Test Cross
Yellow green
YY yy
F1 all Yellow, Yy
gametes = ½ Y , ½ y
F1 green
Yy yy
F2 = 1/2 Yy, Yellow
1/2 yy, green
Demonstrates even
segregation of alleles
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The chromosomal
basis for Mendel’s
law of segregation:
Genes are located on
chromosomes –
two copies of
each in each
diploid cell
(homologs)
Even separation of
homologs during
meiosis causes even
separation of alleles
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Molecular Alleles follow Mendelian Inheritance
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Genetics Symbols
Many organisms:
• dominant phenotype = C, active allele, expression+
• recessive phenotype = c, inactive allele, expression-
Microorganisms:
• wildtype = bio+, active allele
• mutant = bio-, inactive or null allele
Drosophila:
• recessive mutation = bw, wildtype allele = bw+
• recessive mutation is loss of function or null
• dominant mutation = Cy, wildtype allele = Cy+
• dominant mutation is gain of function
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A white eyed mutant male was discovered in a
wildtype culture – 1910, Thomas and Lilian Morgan
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Inheritance of white eyes in
Drosophila does not follow
expected Mendelian patterns
Inheritance patterns differ between the sexes
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Inheritance of white eyes
white eyed mutant male red eyed female
F1 = all red eyed
F2 = 3/4 red eyed, 1/4 white eyed
except that – males = 1/2 red, 1/2 white
females all red eyed
The reciprocal cross:
white eyed female red eyed male
F1 = all females red eyed, males white eyed
F2 = males and females both 1/2 red, 1/2 white
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Inheritance of Sex Chromosomes Autosomes: one pair
of each in male and
female
Sex chromosomes:
• male: X, Y
• female: X, X
Inheritance of sex
chromosomes
explains the
inheritance of white
eyes
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Inheritance of white eyes
white eyed male (XY) red eyed female (XX)
F1 = all red eyed (XX, XY)
F2 = 3/4 red eyed, 1/4 white eyed
females all red eyed (XX, XX)
males = 1/2 red (XY), 1/2 white (XY)
The reciprocal cross:
white eyed female (XX) red eyed male (XY)
F1 = females red eyed (XX)
males white eyed (XY)
F2 = males and females both 1/2 red, 1/2 white
(XY, XY; XX, XX)
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Chromosomal Basis for X-linked inheritance
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Chromosomal Basis for X-linked inheritance
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Inheritance of white eyes
white eyed male (Xw Y) red eyed female (Xw+ Xw+)
F1 = all red eyed (Xw+ Y, Xw+ Xw)
F2 = 3/4 red eyed, 1/4 white eyed
females all red eyed (Xw+ Xw+, Xw+ Xw)
males = 1/2 red (Xw+ Y), 1/2 white (Xw Y)
The reciprocal cross:
white eyed female (XwXw) red eyed male (Xw+Y)
F1 = females red eyed (Xw+ Xw)
males white eyed (Xw Y)
F2 = males and females both 1/2 red, 1/2 white
(Xw+ Y, Xw Y; Xw+ Xw, Xw Xw)
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Mendelian inheritance can be studied in humans
by analyzing pedigrees
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Figure 2-29 part 1
Rare recessives may become homozygous due
to inbreeding
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Figure 2-29 part 1
Genotypes may be inferred from inheritance
patterns
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Figure 2-31 part 1
Inheritance of an autosomal dominant character
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Inheritance of a rare X-linked character
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Inheritance of a rare X-linked character
Outside of line,
Unlikely to carry allele
(be heterozygous)
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Inheritance of a rare X-linked character
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X-linked recessive traits are much more
common in males than females:
Red-green colorblindness: about 8% males and
0.5% females of northern European ancestry
Explanation:
Xc frequency = 8% = 0.08, XC = 92% = 0.92
Males: one Xc needed for color blindness
XcY = 8%, XCY = 92%
Females: two Xc needed for color blindness
XcXc = 0.08 X 0.08 = 0.006, ≈ 0.5%
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How to Characterize Genes by
Inheritance Patterns in Pedigrees
• First, dominant or recessive ?
• A trait is recessive if there is any case
of two unaffected parents producing
at least one affected child – if not,
consider it dominant.
• A recessive trait will usually be rare
overall, a dominant trait will be
common in some lineages, absent in
others.
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How to Characterize Genes by
Inheritance Patterns in Pedigrees
• Second, autosomal or X-linked?
• A recessive X-linked trait will be
much more common in males than
in females – else autosomal.
• Also, affected females must have
affected fathers.
• A dominant X-linked trait will be
passed from an affected father to all daughters and no sons.
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?
Pedigrees can help estimate the chance of
having affected children
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?
Pedigrees can help estimate the chance of
having affected children
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Molecular Alleles follow Mendelian Inheritance
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Genomic Organization: Life on earth exhibits a wide range of
chromosome and genetic organization:
Viruses - small genomes, simple organization,
DNA or RNA, typically linear
~100 kb, ~100-200 genes
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41
NASA: “Life is a self-sustained chemical
system capable of undergoing Darwinian
evolution.”
Life is defined through its metabolism
and ability to reproduce itself and evolve.
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Prokaryotic genomes are larger, more complex
E. coli: 4600 kb, ~4300 genes F plasmid:
100kb,
~100 genes
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Inheritance occurs during cell division
Prokaryotic cells divide by binary fission
Rod-Shaped Bacterium, hemorrhagic E. coli, strain
0157:H7 (division) (SEM x22,810). This image is
copyright Dennis Kunkel at www.DennisKunkel.com
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Human mt:
16 kb, 37 genes
120-160 kb,
100+ genes
Eukaryotes have multiple genomes
Human nuclear:
3000 mbp, 21k genes
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Genomic Structure in Different Organisms:
simpler organisms → smaller genomes,
relatively more coding and less noncoding DNA
13 genes / 20 kb
11 genes / 20 kb
0.8 genes / 20 kb
0.33 genes / 20 kb
Total coding DNA per segment
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Eukaryotic Genome Comparison
• Baker’s Yeast (S. cerevisiae): microscopic
− 12,200 kb, ~6,000 coding genes,
− 73% coding
• Nematode (C. elegans) 1 mm long, 1,000 cells:
− 100,000 kb, ~19,000 coding genes
− ~28% coding
• Drosophila melanogaster
− 140,000 kb, ~17,000 coding genes
− ~18% coding
• Humans
− 3,100,000 kb, ~21,000 coding genes
− <2% coding
Gre
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Organization of Eukaryotic Chromosomes
DNA + histones = chromatin
Human Nucleus:
0.006 mm diameter
2,000 mm DNA
Equivalent:
40 km fine thread
into a tennis ball
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Another Representation
of a Nucleosome -
Crystal Structure
Side
Top
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Eukaryotic DNA -
histones removed
Scaffold
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Both Prokaryotic and Eukaryotic
Chromosomes are Mounted on Scaffolds
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Eukaryotic somatic cell division: mitosis + cytokinesis
Formation of identical daughter cells
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Prokaryotic / Eukaryotic
Chromosomes and Cell Reproduction
Prokaryotes
• Chromosome
continuous
• Introns lacking
• Intergenic spacers
lacking
• DNA “naked”
• Binary fission
Eukaryotes
• Chromosomes linear
• Introns usually
present
• Intergenic spacers
present, may be large
• DNA bound with
histone proteins
• Mitosis + cytokinesis
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Mitosis: asexual transmission of genetic information
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Mitosis results in even
distribution of replicates to
daughter cells
Marek Kultys
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Human
chromosomes are
best visualized at
mitotic metaphase
Coiling and folding during prophase
results in about a 7,500 fold
reduction in length of human
chromosomes (45mm to < 0.006 mm
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Organization of genes on chromosome 22
the second smallest autosome
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Human Female Karyotype
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Human Male Karyotype
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Light bands = euchromatin (active genes)
Dark bands = heterochromatin (~inactive)
Ideograms are used to depict
chromosome banding
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Chromosome 11
Chromosome 1
Chromosome ideograms
may show different
resolution
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Chromosomes can be stained using fluorescent probes
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Genes can be located on human mitotic
metaphase chromosomes using
fluorescent probes
\ Photo from Invitrogen
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Genes can be located on human mitotic
metaphase chromosomes and mapped to a
specific region using fluorescent probes
The EMBO Journal (1997) 16, 1093 - 1102
doi:10.1093/emboj/16.5.1093
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A diagram of Drosophila buzzatii polytene chromosomes
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Labeled DNA probes
can be used to locate
genes on polytene
chromosomes
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Eukaryotic sex cell division: meiosis + cytokinesis
Formation of haploid cells from diploid parents
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Meiosis I:
homologs
separate
Meiosis II:
replicates
(chromatids)
separate
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Meiosis: Stages of Prophase I under a microscope
Leptotene Zygotene Pachytene Diplotene
thin thread paired thread thick thread double thread
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CROSSING OVER
studyblue.com
T i m e X-overs hold
chromatids
together
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Meiosis: Stages of Prophase I
• Chromosomes pair,
beginning at the telomeres
• Synapsis occurs with the
formation of the
synaptonemal complex.
• Crossing over occurs –
required (~) for proper
alignment at metaphase
• Synaptonemal complex
dissolves, homologs repel
• Chiasmata appear
• Crossovers terminalize
(during anaphase)
• Leptotene –
thin thread
• Zygotene –
paired thread
• Pachytene –
thick thread
• Diplotene –
double thread
• Diakinesis –
moving through
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Chromosomal effects of
prophase I:
• alignment, segregation
of homologs
• chromosomal
recombination
Genetic effects:
• even segregation of
alleles
• genetic recombination
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Human (and Mammalian) Meiosis
Males – • Meiosis begins about puberty
• Initial wave of meiosis takes about 2 months
to complete
• Subsequent meiosis is a continuous process
throughout the life of the individual
Females – • Prophase I begins at ~10 weeks gestation
• At diplotene (~26 weeks) meiosis is arrested
until puberty
• At puberty, meiosis proceeds to metaphase II,
where it is arrested until fertilization
• Meiosis is completed upon fertilization
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X and Y are different over most of their
sequence, but pair during meiosis in a
pseudoautosomal region that is
homologous and contains equivalent
genes
Sex Chromosomes
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Sex Determination
• XY – male is the heterogametic sex
• Humans: Y determines sex by action of SRY (sex determining region)
• Drosophila: X:autosome ratio determines sex,
genes on Y determine male fertility
• ZW – female is the heterogametic sex
• Birds, some moths and butterflies, some
reptiles
• No sex chromosomes
• Crocodiles and alligators
• Developmental temperature determines sex
• 80-850 F, mostly female
• 90-950 F, mostly male
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Human X about 153 mb, 2000 genes
Human Y about 55 mb, 90 genes
mostly heterochromatin
evolved from X
X-linked genes potentially
imbalanced between sexes
One X in each adult female
cell is inactivated
X-inactivation provides
equal “doses” of X-linked
gene products for male
and female - gene dosage
compensation
Xist gene
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Mary Lyon 1961:
Barr bodies
represent inactivated
X chromosomes
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There is a direct
association
between the
number of X
chromosomes
and the number
of Barr bodies
XY
XX
XXXX
Hong B et al. PNAS 2001;98:8703-8708
XXX
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Enzymes and DNA often have allelic
differences that allow them to be
separated by electrophoresis
Electrophoretic alleles may be referred to
as “fast” or “slow”
For X-linked genes,
all males and all
homozygous females
show only fast or
slow bands.
Heterozygous
females show both.
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Glycolysis Glucose-6-phosphate
dehydrogenase (G6PD):
X-linked gene involved
in glycolysis, protection
of red blood cells from
oxidizing free radicals
Electrophoretic variants
exist that have little or
no effect on health
G6PD
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Electrophoresis gel confirming the patient to be hemizygote for M...
Mark Fast Fast Slow F/S Mark
100 bp 25 bp
G6PD Variation - DNA Electrophoresis
Burgoine, K. L. et al. 2010 Malaria Journal V9-1
DOI: 10.1186/1475-2875-9-376
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XfY XsY XfXf XsXs
Genotype of Individual
Electrophoretic Pattern
Well
Slow
Fast
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XfY XsY XfXf XsXs XfXs XfXs
Genotype of Individual
Heterozygous Female
Random Cells
Electrophoretic Pattern
Well
Slow
Fast
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XfY XsY XfXf XsXs XfXs XfXs
Genotype of Individual
Heterozygous Female
Random Cells
Clonal Colonies from
Individual Cells
Electrophoretic Pattern
Well
Slow
Fast
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XfY XsY XfXf XsXs XfXs XfXs XfXs XfXs XfXs XfXs XfXs
Genotype of Individual
Heterozygous Female
Random Cells
Clonal Colonies from
Individual Cells
Electrophoretic Pattern
Well
Slow
Fast
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X inactivation explains
female mosaicism for
X-linked genes
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X chromosome inactivation
XBXb Zygote
XBXb XBXb Early Divisions
Xb XB XB Xb Random Inactivation
(~ 20 cell stage)
Expression of Xist
Inactivated X passed to
all somatic descendants
→ somatic mosaicism
X reactivated during
oogenesis
XB = orange
Xb = non-orange
= black
Xb Xb XB XB XB XB Xb Xb
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Summary:
Males, homozgous
females are orange
or black
Random X
inactivation in
heterozygous
females produces
mosaics.
XB = orange
Xb = non orange
= black
http://www.bio.miami.edu/
dana/dox/calico.html
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Changes in Chromosome Number
Euploidy: increase or decrease in the entire genome
– Monoploidy - single chromosome set (2n to n)
– Polyploidy - multiple chromosome sets
– Example: 2n to 4n, 4n to 8n, 3n to 6n
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Changes in Chromosome Number
Euploidy: increase or decrease in the entire genome
– Monoploidy - single chromosome set (2n to n)
– Polyploidy - multiple chromosome sets
– Example: 2n to 4n, 4n to 8n, 3n to 6n
• allopolyploid - chromosome sets from different species
– results from polyploidy in a sterile monoploid hybrid
• autopolyploid – chromosome sets from same species
(most common)
– produces a new tetraploid species not interfertile with
parent
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Male donkey female horse = mule (sterile)
2n = 62 2n = 64 n = 31+32 = 63
Male lion female tiger = liger (partially fertile)
2n = 38 2n = 38 n = 19+19 = 38
Allopolyploid
©2014 Pearson Education, Inc.
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Autopolyploid
©2014 Pearson Education, Inc.
Autopolyploids:
• a new tetraploid species
• cannot interbreed with the parent species
• sterile triploids result from interbreeding
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Self Fertile
Ancestral
Self Fertile
Derived
Gametes Gametes
Sterile Triploid
Chromosomes from http://en.wikipedia.org/wiki/Polyploid
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Basis for sterility of triploids: meiosis
produces gametes with uneven numbers of
chromosomes
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Trivalents (or bivalents and univalents)
form during metaphase I
Chromosomes from http://en.wikipedia.org/wiki/Polyploid
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Trivalents (or bivalents and univalents)
form during metaphase I
and segregate unevenly during anaphase
Chromosomes from http://en.wikipedia.org/wiki/Polyploid
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Telophase I daughter cells have
unbalanced chromosome numbers
Chromosomes from http://en.wikipedia.org/wiki/Polyploid
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Final distribution of chromosomes in gametes:
uneven number leads to inviability
Chromosomes from http://en.wikipedia.org/wiki/Polyploid
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Aneuploidy is an increase
or decrease in number for
a single chromosome, all
others stay the same.
Changes in Chromosome Number Aneuploidy
Monosomy (one copy) Trisomy (three copies)
Chromosomes from http://en.wikipedia.org/wiki/Polyploid
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Aneuploidy results from
nondisjunction -
chromosomes do not
segregate normally
Nondisjunction produces
uneven distribution of
chromosomes in gametes
Nondisjunction in meiosis
I is more common due to
sensitivity of crossing
over
Changes in Chromosome Number Aneuploidy
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Aneuploidy: change in number for a single
chromosome
• Sex chromosome aneuploidy
– monosomy = one copy of the X chromosome
• X is the only monosomic that can survive
• XO, Turner syndrome
– trisomy = three copies of sex chromosome
• XXY, Klinefelter Syndrome
• XYY, Jacob Syndrome
• XXX, triplo X female, generally asymptomatic
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Human sex chromosome aneuploidy –
Turner and Klinefelter Syndrome
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Conceptions:
13,580/1,000,000
Survive – 80
Spontaneously abort -
13,500
Turner Syndrome
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Conceptions: 480/1,000,000
Survive - 440
Spontaneously abort - 40
Klinefelter Syndrome
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Akhenaten
Amenhotep IV
XXY ?
DNA evidence
says not
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Jacob or XYY Syndrome • Occurrence, survival ≈
Klinefelter syndrome
• Slightly taller than average
• Increased risk of learning
disabilities, no significant
effect on IQ
• Development of speech
language and motor skills may
be delayed
• Normal sexual development
and usually normal fertility
• No increase in aggression
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Human Autosomal Trisomy Trisomics that may survive to birth are circled
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Three Surviving Autosomal Trisomics
• Trisomy 13, Patau Syndrome
• 12% survive to birth
• 130 day life expectancy, >80% die within 1 year
• Trisomy 18, Edward Syndrome
• 6% survive to birth
• 50% die within 1 week, rarely live to teens • Survivors have serious medical and developmental
problems
• Trisomy 21, Down Syndrome – the most common
• 25% survive to birth
• Life expectancy variable, 8% live past 40
• Substantial medical intervention, parental care
likely • Low IQ (mean 50), heart disease, vision/hearing
disorders common
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Conceptions:
4,551/1,000,000
Survive – 1041
Abort – 3510
Down Syndrome
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Aneuploidy Total Live Abort
XO 13580 80 13500
XYY 500 460 40
XXY 480 440 40
Tri 21 4551 1041 3510
Tri 18 2360 130 2230
Tri 13 1450 170 1280