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©1999 Timothy G. Standish Chromosomes: Vessels For The Genes Timothy G. Standish, Ph. D.
©1999 Timothy G. Standish Reasons Mendel’s 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: The 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 of 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 E n e N Chromosomal Theory of Inheritance e N E n Father Mother N e E n N E n ee n E N e n E N e n E N e N E n Telophase II Replication Telophase I Prophase I Crossing Over
©1999 Timothy G. Standish EnEn eNeN enen ENEN Sperm e ne ne Ne NE nE nE NE N Eggs Independent Assortment n E e N e n N E n E e n e N N E EeNnEeNnEeNNEENnEENN EennEeNnEeNnEEnnEENn eeNneeNNEeNnEeNnEeNN eenneeNnEennEeNnEeNn As long as genes are on different chromosomes, they will assort independently
©1999 Timothy G. Standish E A a e Two Genes On One Chromosome Telophase II Father Mother e a E A Telophase I A e E A E a e a E AA e E aa e Replication e a E A Prophase I E A A a e a 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 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 Drosophila Mutations
©1999 Timothy G. Standish More Drosophila Mutations ebony body ee Wild Type ++ white eyes ww
©1999 Timothy G. Standish X Chromosome Human and Drosophila 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- Linked Drosophila Gene A white-eyed male was discovered X + X+YX+Y Xw X+Xw X+ XwYXwY X+X+ XwXw X+X+ Y Xw X+Xw X+ X+YX+Y Xw X+Xw X+ X+YX+Y X+X+ X+X+ XwXw Y X P X F1F1 F2F2 1/2 1/4
©1999 Timothy G. Standish The Key To Morgan’s Discovery The key to Morgan’s discovery was the observation that all the white-eyed individuals in the F 2 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 Genes 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 Genes Related 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 Pedigree Albert of Saxe-Coburg ( ) Edward Duke of Kent ( )Victoria Princess of Saxe-Coburg ( ) Emperor Frederick III of Germany ( ) Victoria ( ) Victoria Queen of England ( ) Leopold Duke of Albany ( ) Victoria ( ) King Alfonso XIII of Spain ( ) Beatrice ( ) Alice ( ) Tsar Nicholas II of Russia ( ) Irene ( ) King Edward VII of England ( ) Alix (Alexandra) ( ) Olga ( ) Alexis ( ) Anastasia ( ) Marie ( ) Tatiana ( )
©1999 Timothy G. Standish Variation In Chromosome Number - 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 Number - 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 IAnaphase I Zygote + +
©1999 Timothy G. Standish Aneuploidy 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 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 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 Dosage 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 Dosage Compensation XX 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 XX Cell division XX Zygote X X XX
©1999 Timothy G. Standish Why Calico Cats Are 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.
©1999 Timothy G. Standish
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