Danielle 5-6

Danielle 5-6 - Ch. 5 Opening: In 1910, T. H. Morgan...

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Unformatted text preview: Ch. 5 Opening: In 1910, T. H. Morgan discovered a white-eyed mutation with unusual sex-linked inheritance T.H. Morgan, Drosophila , & Sex- linkage T.H. Morgan started genetic studies with fruit flies ( Drosophila melanogaster ), 1909-1930s. Discovered sex linkage the phenotypic expression of an allele that is related to the chromosomal sex of the individual. This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes, where both sexes have the same probability of expressing the trait. Sex linkage of white-eyed mutant ( w ) was important proof that genes were on chromosomes. Fig. 5.2 Human Sex Chromosomes- Differ in size and morphology- Terminal regions similar, but few genes in common.- Y has few genes, X many genes (therefore there are more X-linked traits than Y-linked traits.- X and Y pair and disjoin in meiosis as if they were homologous chromosomes.- This occurs without a problem because, like homologous autosomes, the X & Y chromosome synapse during prophase of meiosis I. There is a small region of homology shared by the X & Y chromosome, and synapse occurs at that region. More on sex chromosomes Many organisms have sex chromosomes like X and Y. Chromosomes other than sex chr.= autosomes (Autosomes are non-sex chromosomes.) XY males are said to be hemizygous , have only one copy of X chr. genes. Hemizygous describes an individual who has only one member of a chromosome pair. Refers in particular to X-linked genes in males who under usual circumstances have only one X chromosome. XY males always show phenotype of recessive alleles of X chr. genes. (Ex. red-green color blindness and hemophilia.) X-linked inheritance X-linked inheritance means that the gene causing the trait or the disorder is located on the X chromosome. Remember, females have two X chromosomes, while males have one X and one Y. Genes on the X can be recessive or dominant, and their expression in females and males is not the same because the genes on the Y chromosome do not exactly pair up with the genes on the X. X-linked recessive genes are expressed in females only if there are two copies of the gene (one on each X chromosome). However, for males, there only needs to be one copy of an X-linked recessive gene in order for the trait or disorder to be expressed. For example, a woman can carry a recessive gene on one of the X chromosomes unknowingly, and pass it on to a son, who will express the trait: Morgans observation :-In 1910 Morgan noticed a white-eyed mutant male among the red-eyed wild types....
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This note was uploaded on 03/30/2008 for the course BIOL 2156 taught by Professor Larkin during the Spring '08 term at LSU.

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Danielle 5-6 - Ch. 5 Opening: In 1910, T. H. Morgan...

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