Proof for the chromosome theory of inheritance
Sex chromosomes
Although 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 Tomas 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:
1
Morgans crosses
CROSS1
Reciprocal cross
CROSS2
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:
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.
4
Formal explanation
Females have 2 copies of the eye color gene and males have one copy
W (red) is dominant over w (white)
CROSS1
F1
5
Formal explanation
Females have 2 copies of the eye color gene and males have one copy
W (red) is dominant over w (white)
Red
XWXw
Red
XWY
F2
6
Formal explanation
The reciprocal cross
White
XwXw
Red
XWY
F1
7
Formal explanation
Red
XWXw
White
XwY
F2
8
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.
9
Sex determination
Bridges a student of Morgan set up the cross outlined above in
large numbers
P cross:
As expected, he obtained
and
About 1 in every 2000 progeny he obtained white-eyed fertile female or a red-eyed sterile male.
10
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?
11
Bridges and non-dysjunction
white
XwXw
red
XWY
F1
12
0ne in 2500 eggs have non-dysjunction
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.
Genes reside on chromosome
The exceptional red-eyed males should be X0
and
The exceptional white eyed females should be XXY
THAT IS WHAT BRIDGES SAW under the microscope!
13
Dysjunction in Meiosis I
XaXA
x
XaY
XaXaXAXA
x
XaXaYY
Dysjunction in meiosisI in mother
meiosis I in father
Dysjunction in
XaXaXAXA and O
XaXaYY and O
Normal meiosis II
XaXA and O
XaY and O
14
Dysjunction in meiosis II
XaX A
x
XaY
XaXaXAXA
x
XaXaYY
Normal meiosisI in mother
father
XaXa and XAXA
Normal meiosis I in
XaXa and YY
Dysjunction in meiosis II
XaXa or XAXA
XaXa or YY
15
Chromosome
characteristics
Centromere
Telomere
Chromosome arms
16
Karyotype
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
Chromosomes can be stained
17
Chromosome number/size (haploid)
Organism
Yeast (S. cerevisiae)
Mold (Dictyostelium)
Arbidopsis
Lily
Nematode (C. elegans)
Fly (Drosophila)
Fugu
Mouse
Human
size
12
70
130
50,000
97
180
365
3000
3000
number
16
7
5
12
6
4
2
20
23
Evolutionary significance of variability in number and length is
not known
Chr
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
X
Y
Mb
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
18
Banding
Cells in metaphase can be fixed and stained with dyes
Some dyes that stain chromosomes give a characteristic
banding pattern.
In a diploid, homologous chromosomes have the same
banding pattern
Stained chromosomes are photographed,
cut and arranged in decreasing size
19
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
20
Karyotyping
Karyotyping provides a rapid means to identify alterations in the
number of chromosomes
Chromosome 21
In humans ~50% of conceptions are aneuploid
Over 70% of spontaneous abortions and early embryonic deaths
are caused due to Aneuploidy
1 in 170 live births is partially aneuploid
~5-7% of early childhood deaths are to aneuploidy
Humans have a rate of aneuploidy that is 10 times greater
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
21
Aneuploidy
A
A
a
Trisomy21 is
Non-dysjunction
In MeiosisI
a
22
Triploidy
Species that are triploid, reproduce asexually (plant species)
What are the consequences of triploidy during mitosis and
meiosis?
Haploid
Diploid
Triploid
Mitosis
23
Meiosis and triploids
MeiosisI
This is for one chromosome. If there are n chromosomes in
24an
organism, then balanced gametes (equal copies of all
chromosomes) are very rare.
Sex in organisms
Sex chromosomes and sex linkage:
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
Bridges could tell genotype by where the sex chromosome went
and therefore established that chromosomes carried genes
25
Non-sex chromosomes are called autosomes
Humans have 22 autosomes, Drosophila has 3
Homogametic sex-
XX-
females in humans
males in birds
Heterogametic sex-
XY-
males in humans
Hemizygous
gene present in one copy in a diploid
organism
Human males are hemizygous for
genes on the X-chromosome
26
Surname project
Y
Y
Y
Y
All males in this pedigree will have the SAME Y-chromosome!!!
27
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. Pedigrees trace the inheritance pattern of a particular
trait through many generations. Pedigrees enable geneticists to
determine whether a familial trait is genetically determined and its
mode of inheritance (dominant/recessive, autosomal/sex-linked)
28
Pedigree symbols:
Male
Female
Sex Unknown
5
Affected individual
Spontaneous
abortion
Number of individuals
Deceased
Termination
of pregnancy
29
Pedigree symbols:
relationship line
Sibship line
line of descent
individual’s lines
consanguinity
Monozygotic
Dizygotic
30
Characteristics of an autosomal recessive trait:
There are several features in a pedigree that suggest a recessive
pattern of inheritance:
nguinity is often involved.
In the pedigree shown below, an autosomal recessive inheritance
pattern is observed:
I
II:1
II:2
III:9
31
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
32
The following pedigree outlines an inheritance pattern
Does this fit an autosomal recessive or autosomal dominant
pattern of inheritance?
33
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
34
Characteristics of a X-linked 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
35
Mammalian X-chromosome inactivation
(epigenetics)
Mammalian males and females have one and two X chromosomes
respectively.
One would expect that X-linked genes should produce twice as
much gene product in females compared to males. Yet when one
measures gene product from X-linked genes in males and females
they are equivalent.
This phenomenon, known as dosage compensation, means that the
activity of X-linked genes is either down regulated in females or
up regulated in males.
The former proves to be the case:
X chromosome inactivation in females is the mechanism behind
dosage compensation.
In females, one of the X chromosomes in each cell is inactivated.
This is observed cytologically. One of the X-chromosomes in
females appears highly condensed. This inactivated chromosome is
called a Barr-body.
In Drosophila the genes on the single male X chromosome is upregulated 2-fold
36
X-inactivation
The inactivation of one of the two X-chromosomes means that
males and females each have one active X chromosome per cell.
X-chromosome inactivation is random. For a given cell in the
developing organism there is an equal probability of the female or
the male derived X chromosome being inactivated.
The embryo is a mosaic!
Once the decision is made in early development, then it is
stably inherited.
Patches of cells have the male X ON and patches of cells have
the female X ON
This is a Developmental rule that overlays on top of Mendellian
37
rules!
X-inactivation
The inactivation of one of the two X-chromosomes means that
males and females each have one active X chromosome per cell.
X-chromosome inactivation is random. For a given cell in the
developing organism there is an equal probability of the female or
the male derived X chromosome being inactivated.
zygote
Embryo
Inactivation
The embryo is a mosaic!
Once the decision is made in early development, then it is
stably inherited.
Patches of cells have the male X ON and patches of cells have
the female X ON
This is a Developmental rule that overlays on top of Mendellian
38
rules!
Barr bodies
·
XXX individuals have 2 Barr Bodies leaving one active X
·
XXXX individuals have 3 Barr Bodies leaving one active X
·
XXY individual have one Barr Body leaving one active X
(Klinefelter's syndrome)
·
X0 individuals have no Barr Bodies leaving one active X
(Turner's syndrome)
Given X-chromosome inactivation functions normally why are they
phenotypically abnormal?
Part of the explanation for the abnormal phenotypes is that the
entire X is not inactivated during Barr-Body formation (Escape loci)
Consequently an X0 individual is not genetically equivalent to an XX
individual.
39