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©1999 Timothy G. Standish
Chromosomes:Chromosomes:Vessels For The GenesVessels For The Genes
Timothy G. Standish, Ph. D.
©1999 Timothy G. Standish
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.
©1999 Timothy G. Standish
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.
©1999 Timothy G. Standish
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.
©1999 Timothy G. Standish
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Chromosomal Theory Chromosomal Theory of Inheritanceof Inheritance
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Telophase II
Replication
Telophase IProphase I
Crossing Over
©1999 Timothy G. Standish
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Sperm e ne NE nE N
EggsIndependent AssortmentIndependent Assortment
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As long as genes are on different chromosomes, they will assort independently
©1999 Timothy G. Standish
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Two Genes On One Two Genes On One ChromosomeChromosome
Telophase II
Father
Mother
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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
©1999 Timothy G. Standish
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.
©1999 Timothy G. Standish
More More DrosophilaDrosophila Mutations Mutations
ebony body ee
Wild Type ++
white eyes ww
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
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
Y
X P
X F1
F2
1/21/4 1/4
©1999 Timothy G. Standish
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)
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
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)
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
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
+
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
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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