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Transcript of 1 Proof for the chromosome theory of inheritance Although these were convincing correlations, actual...
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Proof for the chromosome theory of inheritance
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:
Sex chromosomes
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Morgans crosses
Reciprocal cross
CROSS1
CROSS2
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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:
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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.
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Formal explanation
Females have 2 copies of the eye color gene and males have one copyW (red) is dominant over w (white)
F1
CROSS1
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Formal explanation
Females have 2 copies of the eye color gene and males have one copyW (red) is dominant over w (white)
Red
XWXw
Red
XWY
F2
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Formal explanation
White
XwXw
Red
XWY
F1
The reciprocal cross
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Formal explanation
Red
XWXw
White
XwY
F2
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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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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.
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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?
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Bridges and non-dysjunction
white
XwXw
red
XWY
F1
0ne in 2500 eggs have non-dysjunction
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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.
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!
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XaXA x XaY
XaXaXAXA x XaXaYY
Dysjunction in meiosisI in mother Dysjunction in meiosis I in father
XaXaXAXA and OXaXaYY and O
Normal meiosis II
XaXA and O XaY and O
Dysjunction in Meiosis I
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XaXA x XaY
XaXaXAXA x XaXaYY
Normal meiosisI in mother Normal meiosis I in father
XaXa and XAXA XaXa and YY
Dysjunction in meiosis II
XaXa or XAXA XaXa or YY
Dysjunction in meiosis II
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Chromosome characteristics
Centromere
Telomere
Chromosome arms
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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
Chromosomes can be stained
Telocentric
Acrocentric
Metacentric
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Chromosome number/size (haploid)
Organism size numberYeast (S. cerevisiae) 12 16Mold (Dictyostelium) 70 7Arbidopsis 130 5Lily 50,000 12Nematode (C. elegans) 97 6Fly (Drosophila) 180 4Fugu 365 2Mouse 3000 20Human 3000 23
Evolutionary significance of variability in number and length is not known
Chr Mb1 246.12 243.63 199.34 191.75 181.06 170.97 158.58 146.39 136.310 135.011 134.412 132.013 113.014 105.315 100.216 90.017 81.818 76.119 63.820 63.721 46.922 49.3X 153.6Y 22.7
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Banding
Cells in metaphase can be fixed and stained with dyesSome 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
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Karyotype
• The human karyogram. The chromosomes are shown with the G-banding 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
In humans ~50% of conceptions are aneuploidOver 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 mammalsNon-dysjunction in meiosis is the primary cause
Monosomy- one chromosome of a pair is missingTrisomy- extra chromosome is present
Only chromosome 21 trisomies survive to adulthoodDowns syndrome occurs in 1 in 200 conceptions and 1 in 900 live births
Chromosome 21
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Aneuploidy
Trisomy21 is Non-dysjunction In MeiosisI
AAa
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 DiploidTriploid
Mitosis
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Meiosis and triploids
MeiosisI
This is for one chromosome. If there are n chromosomes in an organism, then balanced gametes (equal copies of all chromosomes) are very rare.
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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
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Non-sex chromosomes are called autosomesHumans have 22 autosomes, Drosophila has 3
Homogametic sex- XX- females in humansmales 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
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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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Mendelian genetics in Humans: Autosomal and Sex-linked 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)
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Male Female Sex Unknown
Affected individual
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Number of individuals Deceased
Spontaneous abortion
Termination of
pregnancy
Pedigree symbols:
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Pedigree symbols:
line of descent
individual’s lines
relationship line
Sibship line
consanguinity
Monozygotic Dizygotic
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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:
II:1 II:2
III:9
I
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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
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The following pedigree outlines an inheritance pattern
Does this fit an autosomal recessive or autosomal dominant pattern of inheritance?
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Pedigree of Queen Victoria and the transmission of hemophilia.
VictoriaAlbert
Alicecarrier
Beatricecarrier
Irenecarrier
Alixcarrier
Alicecarrier Victoria
carrier
carrier
carrier
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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.
gY GG
GY gG GY GY gG gGGY GY
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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 up-regulated 2-fold
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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 ONThis is a Developmental rule that overlays on top of Mendellian rules!
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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
EmbryoInactivation
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 ONThis is a Developmental rule that overlays on top of Mendellian rules!
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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.