Chromosomes: Vessels For The Genes

©1999 Timothy G. Standish

Chromosomes:Chromosomes:Vessels For The GenesVessels For The Genes

Timothy G. Standish, Ph. D.


Reasons Mendel’s Reasons Mendel’s Work Was Ignored:Work Was Ignored:

There was no physical element in which Mendel’s inherited particles could be identified.

By the turn of the century, chromosomes had been discovered (physical particles) and biologists were better at math.


Chromosomes:Chromosomes:The Physical Basis of InheritanceThe Physical Basis of Inheritance 1866 Mendel published his work 1875 Mitosis was first described 1890s Meiosis was described 1900 Mendel's work was rediscovered 1902 Walter Sutton, Theodore Boveri

and others noted parallels between behavior of chromosomes and alleles.


Chromosomal Theory Chromosomal Theory of Inheritanceof Inheritance

Genes have specific loci on chromosomes.

Chromosomes undergo segregation (meiosis) and independent assortment,

Thus alleles of genes are independently assorted.


E

n

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Chromosomal Theory Chromosomal Theory of Inheritanceof Inheritance

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Father

Mother

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Telophase II

Replication

Telophase IProphase I

Crossing Over


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Sperm e ne NE nE N

EggsIndependent AssortmentIndependent Assortment

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EeNnEeNNEENnEENN

EennEeNnEEnnEENn

eeNneeNNEeNnEeNN

eenneeNnEennEeNn

As long as genes are on different chromosomes, they will assort independently


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Two Genes On One Two Genes On One ChromosomeChromosome

Telophase II

Father

Mother

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Telophase I

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Replication

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Prophase I

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As long as genes on the same chromosome are located a long distance apart, they will assort independently due to crossing over during Prophase I of meiosis


Thomas Hunt MorganThomas Hunt Morgan First to associate a trait (gene) with a

chromosome. Worked with fruit flies (Drosophila

melanogaster) Why fruit flies?

– Short generation time (≈ 2 weeks)– Survives and breeds well in the lab– Very large chromosomes in some cells– Many aspects of phenotype are genetically controlled.


DrosophilaDrosophila Mutations Mutations


More More DrosophilaDrosophila Mutations Mutations

ebony body ee

Wild Type ++

white eyes ww


X Chromosome Human and X Chromosome Human and DrosophilaDrosophila Genes Are Easy To Find Genes Are Easy To Find

In humans and Drosophila, males are XY Thus males are haploid for the X

chromosome Because of this, recessive genes on the X

chromosome show up far more commonly in male than female phenotypes


Morgan’s Discovery Of An X-Morgan’s Discovery Of An X-Linked Linked DrosophilaDrosophila Gene Gene

A white-eyed male was discovered

X+ X+

X+Y

Xw X+

XwY

X+ Xw

X+

Y

Xw X+

X+Y

Xw X+

X+Y

X+ X+

Xw

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X P

X F1

F2

1/21/4 1/4


The Key To Morgan’s DiscoveryThe Key To Morgan’s Discovery The key to Morgan’s discovery was the observation that all

the white-eyed individuals in the F2 generation were males

Without this vital data on the association of white eyes with being male, the gene for white eyes could have been seen as a simple recessive trait on an autosome

This illustrates the importance of recording all the data possible and being alert to the possibility of interesting things being present in the data

“Fate favors the prepared mind” (Louis Pasteur)


Human X-linked Recessive Human X-linked Recessive GenesGenes

Brown enamel - Tooth enamel appears brown rather than white

Hemophilia - Two types:– A - Classic hemophilia, deficiency of

blood clotting factor VIII

– B - Christmas disease, deficiency of blood clotting factor IX


X-linked Recessive GenesX-linked Recessive GenesRelated to sightRelated to sight

Coloboma iridis - A fissure in the eye’s iris Color Blindness - Two types:

– Deutan - Decreased sensitivity to green light– Protan - Decreased sensitivity to red light

Congenital night blindness - Not due to a deficiency of vitimin A

Mocrophthalmia - Eyes fail to develop Optic atrophy - Degeneration of the optic nerves


Royal PedigreeRoyal Pedigree

Albert of Saxe-Coburg (1819-1861)

Edward Duke of Kent (1767-1820) Victoria Princess of Saxe-Coburg (1786-1861)

Emperor Frederick III of Germany (1831-1888)

Victoria (1840-1901)

Victoria Queen of England (1819-1910)

Leopold Duke of Albany(1853-1884)

Victoria(1866-1953)

King Alfonso XIII of Spain (1841-1910)

Beatrice(1857-1944)

Alice(1843-1878)

Tsar Nicholas IIof Russia (1868-1918)

Irene(1866-1953)

King Edward VII of England (1841-1910)

Alix (Alexandra)(1872-1918)

Olga(1895-1918)

Alexis(1904-1918)

Anastasia(1901-1918)

Marie(1899-1918)

Tatiana(1897-1918)


Variation In Chromosome Variation In Chromosome Number - PolyploidyNumber - Polyploidy

Polyploid individuals have more than two sets of chromosomes

Many important commercial plants are polyploid:– Roses– Navel oranges– Seedless watermelons

Polyploid individuals usually result from some sort of interruption during meiosis

Pro or Metaphase I

Interruption

of meiosis

Metaphase II

2n Gametes

+ 1n Gamete

3n Zygote


Variation In Chromosome Variation In Chromosome Number - AneuploidyNumber - Aneuploidy

Polyploid humans are unknown, but individuals with extra individual chromosomes are known.

Having extra chromosomes or lacking some chromosomes is called aneuploidy

Aneuploid individuals result from nondisjunction during meiosis

Metaphase I Anaphase IZygote

+

Zygote

+


Aneuploidy In HumansAneuploidy In Humans Most human aneuploids spontaneously abort The most viable variations in chromosome number are those that deal with

the sex chromosomes: XO - Turner’s Syndrome - Phenotypically females XXX…- “Super” females XYY… - “Super” Males - On average tend to be larger and less intelligent XXY - Klinefelter’s Syndrome - Phenotypically male Of the non-sex chromosome aneuploidys, Down’s Syndrome, extra

chromosome 21, tends to be the most viable Down’s Syndrome is more common in children of mothers who gave birth

after age 40


Gene DosageGene Dosage There seem to be elegant mechanisms for

maintaining the correct dosage of genetic material in each cell

When aneuploidy causes a change in the relative dose of one chromosome, problems result


The Lyon HypothesisThe Lyon Hypothesis Having extra chromosomes causes problems (i.e.,

Downs Syndrome) Men have only one X chromosome and they are

normal (at least they think so) Women have two X chomosomes and they are normal Mary Lyon proposed that the extra dosage of X

chromosome that women have is compensated for by turning off one of the X chromosomes.

This turned off chromosome can be observed as a “Barr Body” in metaphase female nuclei


Consequences of X Chromosome Consequences of X Chromosome Dosage CompensationDosage Compensation

Early during development, X chromosomes are randomly turned off in female cells

All daughter cells have the same X chromosome inactivated as their parental cell.

Thus, females are a mosaic of patches of cells some patches expressing the genes on the paternal X chromosome, other patches expressing the maternal X chromosome


Consequences of X Chromosome Consequences of X Chromosome Dosage CompensationDosage Compensation

XXXXXXXX

At some pont (probably later than the 4 cell stage) half the X chromosomes are turned off

Daughter cells inherit the mother cell’s combination off and on X chromosomes

Because of dosage compensation, females are thought to be a mosaic of patches of cells with each patch expressing the same X chromosome, but none expressing both chromosomes

Different patches of cells inherit different act X chromosomes

XXXX

Cell division

XX

Zygote XXXXXXXX


Why Calico Cats Why Calico Cats Are Usually FemaleAre Usually Female

Orange coat color is a sex-linked trait in cats (it is on the X chromosome)

A female cat heterozygous for orange, has skin patches expressing the orange X with the other X chromosome turned off. In other patches the opposite occurs.


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Chromosomes: Vessels For The Genes