1

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

2

Morgans crosses

Reciprocal cross

CROSS1

CROSS2

3

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:

4

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.

5

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

8

Formal explanation

Red

XWXw

White

XwY

F2

9

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

21

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

5

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!

38

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.