Transcript Mammalian X-chromosome inactivation

Proof for the chromosome theory of
inheritance
Sex chromosomes
Although Mendels data correlated with chromosome segregation
during meiosis and these were convincing correlations, actual
proof of the chromosome theory required the discovery of sex
linkage.
Remember, Mendel had found that reciprocal crosses produce
equal results with respect to the progeny. In general geneticists
confirmed his results.
However exceptions did arise. The most famous exception was
that discovered by Thomas Hunt Morgan in the fruit fly
Drosophila melanogaster. Drosophila eyes are normally bright red.
Morgan discovered an exceptional white-eyed male.
He performed the following crosses:
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Morgans crosses
CROSS P1
White
Red
F1
Both males and females were Red
Red is dominant to white
Selfing
3:1 red:white
(1 gene for eye color)
All white eyed flies were male!!!!
F2
Reciprocal cross
CROSS P2
White
Red
F1
Red
White
All females were red and males were white in the F1!!!
2
X and Y chromosomes
Somehow eye color was linked to sex
The key to understanding this pattern of inheritance arose from
work demonstrating that males and females of a given species
often differ in the chromosome constitution.
For example, they found that male and female Drosophila both
have four chromosome pairs. However in males one of the pairs
the members differed in size:
Female Drosophila:
Male Drosophila:
Sex
fourth
second
third
3
Sex chromosomes
Morgan realized that difference in chromosome constitution was
the basis of sex determination in Drosophila:
Females produce only X-bearing gametes, while males produce X
and Y-bearing gametes.
X
X
X
Y
XX
XY
XX
XY
2
:2
If the gene for eye color resides on a X chromosome
There is no counterpart for this gene on the Y chromosome4
Morgans crosses
CROSS1
White
Red
Red
Selfing
3:1 red:white
All white eyed flies were male!!!!
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Formal explanation
Females have 2 copies of the eye color gene and males have one copy
W (red) is dominant over w (white)
CROSS1
white
XwY
Red
XWXW
F1
Xw
XW
XW
XW Xw
y
XWY
Red
Red
XW Xw
XWY
Red
Red
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Formal explanation
Females have 2 copies of the eye color gene and males have one copy
W (red) is dominant over w (white)
Self cross
Red
XWY
Red
XWXw
F2
XW
XW
Xw
XW XW
y
XWY
Red
Red
XW Xw
XwY
Red
White
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Morgans crosses
Reciprocal cross
CROSS2
White
Red
Red
White
All females were red and males were white in the F1!!!
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Formal explanation
The reciprocal cross
Red
XWY
White
XwXw
F1
XW
Xw
Xw
XW Xw
Red
y
XwY
White
XW Xw
XwY
Red
White
In the F1 all the females are red and all the males are white
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Formal explanation
White
XwY
Red
XWXw
F2
Xw
XW
Xw
XW Xw
y
XWY
Red
Red
Xw Xw
XwY
White
White
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Equal numbers of male and female progeny are produced.
Morgan realized that he could explain the inheritance patterns of
eye color by assuming:
1.
The gene determining eye color resides on the X chromosome
(red and white eyes represent normal and mutant alleles of
this gene)
2. There is no counterpart for this gene on the Y chromosome
Thus females carry two copies of the gene, while males carry only a
single copy.
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Red-green color blindness
Red-green color blindness means that a person cannot distinguish
shades of red and green. Males are affected 16 times more often
than females, because the gene is located on the X chromosome.
In color-blind men, the green or red cones worked improperly.
The genes for the red and green receptors were altered in these
men
X-linked red-color blindness is a recessive trait. Females
heterozygous for this trait have normal vision. The color perception
defect manifests itself in females only when it is inherited from
both parents.
By contrast, males inherit their single X-chromosome from their
mothers and become red green color blind if this X-chromosome
has the color perception defect.
The dominant (normal) X chromosome is represented as XCB.
The recessive (mutant) chromosome is represented as Xcb.
Since males have only one X-chromosome, if this chromosome has
the red-green color blind allele, the males will have the color
perception defect.
Females have 2 X-chromosomes. Both X-chromosomes must carry
the mutant allele for the females to be color blind. Red-green color
blind females are homozygous for the recessive allele.
Females with one mutant allele and one normal allele are
heterozygous "carriers". They are not color blind, but they can pass
the color blindness to their children.
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Sex determination
Bridges a student of Morgan set up the cross outlined above in
large numbers
P cross:
white females
XwXw
x
x
red males
XWY
As expected, he obtained
red-eyed females (XwXW) and
white-eyed males (XwY)
About 1 in every 2500 progeny he obtained white-eyed fertile
female or a red-eyed sterile male
Cherish Your Exceptions
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Primary exception
About 1 in every 2500 progeny he obtained a white-eyed fertile
female or a red-eyed sterile male.
These were called primary exceptional progeny
How can these exceptional progeny be explained?
disjunction
Non-disjunction
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Primary exception
About 1 in every 2000 progeny he obtained a white-eyed fertile
female or a red-eyed sterile male.
These were called primary exceptional progeny
How can these exceptional progeny be explained?
autosome
X
autosome
X
disjunction
Non-disjunction
Bridges suggested that occasionally during meiosis the X
chromosomes fail to separate.
ｷ
Normal separation of the X chromosomes
produces Xw gametes
ｷ
Failure of X chromosome separation
(non disjunction) Creates XwXw and nullo gametes and
these gametes give rise to the sterile red eyed
males!
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Bridges and non-dysjunction
white
red
XWY
XwXw
F1
XW
Xw
Xw
Xw Xw
O
y
XW Xw
XwY
Red
white
XW Xw
XwY
Red
White
XW Xw Xw
Xw XwY
Lethal
white female
fertile
XW
Red male
Sterile
Y
Lethal
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Bridges assumed that XXX and Y0 progeny die
The only two viable progeny types were XXY and X0
In this model sex is determined by the number of X
chromosomes rather than the presence or absence of the Y
chromosome
This model makes a strong prediction -Hypothesis
Genes reside on chromosome
The exceptional red-eyed males should be X0
and
The exceptional white eyed females should be XXY
How do you show this?
Look at the chromosomes under the microscope
THAT IS WHAT BRIDGES SAW under the microscope in the
females!
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Non-Dysjunction in
Meiosis I
XaXA
x
XaY
Replication
XaXaXAXA
x
XaXaYY
Non Dysjunction in
Non Dysjunction in
meiosisI in mother
meiosis I in father
XaXaXAXA and O
XaXaYY and O
Normal meiosis II
XaXA and O
XaY and O
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Non Dysjunction in meiosis II
XaXA
x
XaY
Replication
XaXaXAXA
x
XaXaYY
Normal meiosisI in mom
XaXa and XAXA
Normal meiosis I in dad
XaXa and YY
Non Dysjunction in meiosis II
XaXa or XAXA
XaXa or YY
Aneuploid: Having a chromosome number that is not a
multiple of the haploid number for the species
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Quiz
What classes of progeny would be expected if you could do the
following cross
XwXwY
x
XWY
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Answer-- Triploids
White
XwXwY
XW
XwXw
Y
Xw
XwY
XW XwXw
lethal
XWY
Red male
XW Xw
red
XWY
Y
Y XwXw
white female
YY
lethal
Y Xw
Red female
White male
XW XwY
Y XwY
Red female
White male
Normal daughters are red eyed
Normal sons are white eyed
Non-disjunct daughters are white eyed
Non-disjunct sons are red eyed
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Sex in organisms
Sex chromosomes and sex:
In Drosophila, it is the number of X's that determine sex while
in mammals it is the presence or absence of a Y chromosome
that determines sex.
Homogametic sex- Producing gametes that contain one type of
chromosome (females in mammals and insects, males in birds and
reptiles)
Heterogametic sex- Producing gametes that contain two types
of chromosomes (males in mammals and insects, females in birds
and reptiles)
Species
XX
XY
XXY
XO
Drosophila
Female
male
female
male
Human
Female
male
male
female
Non-sex chromosomes are called autosomes
Humans have 22 autosomes, Drosophila has 3
Hemizygous
Gene present in one copy in a diploid organism
Human males are hemizygous for genes on the X-chromosome
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Karyotype
Bridges confirmed aneuploidy by visualizing abnormal
chromosome numbers in Drosophila using the microscope.
Karyotype gives species specific chromosome organization
It is usually a microscopic classification
The number of chromosomes
The size of each chromosome
Position of centromere on each chromosome
Telocentric
Acrocentric
Metacentric
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Chromosome characteristics
Centromere
Telomere
Centromere
Chromosome arms
Chromosome arms
Telomere
Unstained chromosome
Stained chromosome
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Chromosome number/size (haploid)
Organism
Yeast (S. cerevisiae)
Mold (Dictyostelium)
Arbidopsis
Lily
Nematode (C. elegans)
Fly (Drosophila)
Mouse
Human
number
16
7
5
12
6
4
20
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Evolutionary significance of variability in number is not known
Human chromosomes
Ch #
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
X
Y
Chromosome size
246.1
243.6
199.3
191.7
181.0
170.9
158.5
146.3
136.3
135.0
134.4
132.0
113.0
105.3
100.2
90.0
81.8
76.1
63.8
63.7
46.9
49.3
153.6
22.7
Chromosomes also vary in size
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Banding
Chromosomes can be stained
Cells in metaphase can be fixed and stained with dyes.
Dyes stain chromosomes and each chromosome has a
characteristic banding pattern.
In a diploid, homologous chromosomes have the same banding
pattern.
Stained chromosomes are photographed, cut and arranged in
decreasing size
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Karyotype
•
The human karyogram. The chromosomes are shown with the Gbanding pattern obtained after Giemsa staining. Chromosome
numbers and band numbers
•
Constitutive heterochromatin is very compact chromatin which has
few or no genes
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Karyotyping
Karyotyping provides a rapid means to identify alterations in the
number of chromosomes
Chromosome 21
In humans a very large number of conceptions are aneuploid
Over 70% of spontaneous abortions and early embryonic deaths
are caused due to Aneuploidy
~5-7% of early childhood deaths are to aneuploidy
Humans have a rate of aneuploidy that is 10 times greater than
other mammals
Non-dysjunction in meiosis is the primary cause
Monosomy- one chromosome of a pair is missing
Trisomy- extra chromosome is present
Only chromosome 21 trisomies survive to adulthood
Downs syndrome occurs in 1 in 200 conceptions and
1 in 900 live births
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A
Aneuploidy
a
A
a
A
A
a
A
A
a
A
Non-dysjunction
In MeiosisI
a
a
(Trisomy21)
a
A
A
A
A
A
a
a
a
A
A
a
Non-dysjunct
In MeiosisII
a
a
a
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Triploidy
Species that are triploid, reproduce asexually (plant species)
What are the consequences of triploidy during mitosis and
meiosis?
Haploid
Diploid
Triploid
Mitosis
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Triploidy
Species that are triploid, reproduce asexually (plant species)
What are the consequences of triploidy during mitosis and
meiosis?
Haploid
Diploid
Triploid
Mitosis in triploid
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Meiosis and triploids
MeiosisI
Meiosis I
Triploids produce
unbalanced gametes
This is for one chromosome. If there are n chromosomes in an
organism, then balanced gametes (equal copies of all
chromosomes) is very rare.
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What happens when you cross a triploid plant to a triploid plant?
4N
3N
3N
2N
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Seedless watermelons
Triploidy is useful in agriculture.
Take a diploid watermelon species and add colchicine- disrupt
microtubules. Chromosomes replicate but do not segregate
resulting in tetraploids
Cross a tetraploid watermelon with a diploid watermelon
Triploid watermelon seeds are produced by cross-pollination
between a tetraploid watermelon with a diploid watermelon.
The resulting triploid plants could be distinguished in the field
by the use of a genetic marker for fruit color.
The diploid parents have dark green (D) fruit,
Tetraploid parents have light green (d).
Triploid plants will have striped green (ds) fruit.
Tetraploid plants resulting from self-pollination will have light
green fruit and can be culled from production fields, leaving the
triploid plants with striped green fruit.
Triploid watermelon produced. This grows but gametes are
aneuploid- resulting in white seeds (after fertilization) which
are incapable of producing a plant.
Biological control: Triploid carp- eat weeds in waterways but
are unable to replicate and compete with beneficial fish
species.
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And triploid toads
Triploid toads
Nature Genetics 30, 325 - 328 (2002)
Tetraploid toads reproduce through diploid eggs and sperm cells.
A new taxon was discovered at an isolated site in the Karakoram
mountain range.
Every wild toad caught from eight localities was triploid
Did not find a single diploid or tetraploid Batura toad. Both males
and females were found to be triploid.
3N female
3N male
N
elimination
N
elimination
2N
2N
2N
N
N
3N
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Mendelian genetics in Humans: Autosomal and Sexlinked patterns of inheritance
Obviously examining inheritance patterns of specific traits in
humans is much more difficult than in Drosophila because defined
crosses cannot be constructed. In addition humans produce at most
a few offspring rather than the hundreds produced in experimental
genetic organisms such as Drosophila
It is important to study mendellian inheritance in humans because
of the practical relevance and availability of sophisticated
phenotypic analyses.
Therefore the basic methods of human genetics are observational
rather than experimental and require the analysis of matings that
have already taken place rather than the design and execution of
crosses to directly test a hypothesis
To understand inheritance patterns of a disease in human genetics
you often follow a trait for several generations to infer its mode of
inheritance --dominant or recessive?
Sex-linked or autosomal?
For this purpose the geneticist constructs family trees or
pedigrees (genetic analyses and interviews with family members)
Pedigrees trace the inheritance pattern of a particular trait
through many generations. Pedigrees enable geneticists to
determine whether a trait is genetically determined and its mode
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of inheritance (dominant/recessive, autosomal/sex-linked)
Pedigree symbols:
Male
Female
Sex Unknown
5
Affected individual
Spontaneous
abortion
Number of individuals
Deceased
Termination
of pregnancy
37
Pedigree symbols:
relationship line
Sibship line
line of descent
individual’s lines
consanguinity
Monozygotic
Dizygotic
38
Characteristics of an autosomal recessive trait:
There are several features in a pedigree that suggest a recessive
pattern of inheritance:
1.
Rare traits, the pedigree usually involves mating between two
unaffected heterozygotes with the production of one or more
homozygous offspring.
2. The probability of an affected child from a mating of two
heterozygotes is ~25%
3. Two affected individuals usually produce offspring all of whom
are affected
4. Males and females are at equal risk, since the trait is autosomal
5. In pedigrees involving rare traits, consanguinity is often involved.
In the pedigree shown below, an autosomal recessive inheritance
pattern is observed:
I
II:1
II:2
III:9
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Characteristics of an autosomal dominant trait:
1. Every affected individual should have at least one affected parent.
2. An affected individual has a 50% chance of transmitting the trait
3. Males and females should be affected with equal frequency
4. Two affected individuals may have unaffected children
40
The following pedigree outlines an inheritance pattern
Does this fit an autosomal recessive or autosomal dominant
pattern of inheritance?
41
Pedigree of Queen Victoria and the transmission of hemophilia.
Albert
Victoria
Alice
carrier
Irene
carrier
Beatrice
carrier
Alix
carrier
Alice
carrier
Victoria
carrier
carrier
carrier
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Characteristics of a X (sex)-linked recessive trait:
Hemizygous males and homozygous females are affected
Phenotypic expression is much more common in males than in
females, and in the case of rare alleles, males are almost
exclusively affected
Affected males transmit the gene to all daughters but not to any
sons
Daughters of affected males will usually be heterozygous and
therefore unaffected.
Sons of heterozygous females have a 50% chance of receiving the
recessive gene.
GG
gY
GY
gG
GY
GY
GY
gG
gG
GY
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Surname project
Y
Y
Y
Y
All males in this pedigree will have the SAME Y-chromosome!!!
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Surname project
Y
Y
Y
Y
All males in this pedigree will have the SAME Y-chromosome!!!
X1/Y1; A1/A2
(grandpa)
x
X2/X3; A3/A4
(grandma)
X2/Y1; A2/A4 (dad)
x
X4/X5; A5/A6 (mom)
X4/Y1
(son)
A4/A6
X5/X2 A2/A6
(daughter)
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Sex linkedGene Tree
After the death of a wealthy individual (II:3), a man claiming to
be his son (III:3) filed a paternity suit and claimed the
inheritance.
The deceased had only two living nephews (III:1 and III:2 who
were sons of his brothers (II:1 and II:2). In determining
whether the man was actually the son and had the rights to the
inheritance which of the following markers would be most useful
Autosomal
X-linked
II:1
Y-linked
mitochondr
$$$
III:1
III:2
III:3
???
46
Surnames/paternity
47
Y-chromosome migration
48
The Lemba
The Lemba in Africa, who practice
circumcision, keep one day a week holy
and avoid eating pork or pig-like
animals, have long asserted they are of
Jewish heritage.
An analysis of the male Y chromosome
found (1997) that a particular pattern
of DNA changes was much more
common among cohanim priests than
among lay Jews and very rare in nonJewish populations.
A team of geneticists have discovered
that Lemba men carry the same DNA
sequence that is distinctive to the
cohanim.
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Jefferson
50
Jefferson family tree
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