Unformatted text preview: Last Class Mitosis and Meiosis Plants Two Lives Haploid Diploid How about us? Sexual Reproduction Sex Determination Chromosomally Genetically Environmentally Chromosomally 1891, Hermann Henking Viewing insect nuclei Oddity in male insects X body Clarence E. McClung Grasshoppers, looked for Henking's observation Noted that X body was a chromosome ("accessory") Females had one more chromosome than males Chromosomally 1905, Nattie Stevens and Edmund Wilson Females had two accessory "X" chromosomes and males had only one Sex Chromosomes XX-XO sex determination Female XX (Homogametic) Male XO (Heterogametic) Sex Determination Humans Diploid Dioecious Species in which there exists either male of female reproductive organs (structures) (Monoecious species that bear both male and female reproductive organs/structures [hermaphrodite]) XX-XY Sex Determination How do chromosomes pair? Turner Syndrome
1:3000 births Klinefelter syndrome
1:1000 births XXXY, XXXXY, XXYY Triplo-X (Poly-X) syndrome XXX normal female >3 Mental defects Physical ailments Dosage Compensation Females have 2 X chromosomes males only 1, cells have to compensate for this discrepancy Male drosophila increase the activity of their X chromosome Female C. elegans each X chromosome is active but proteins reduce activity of each by 1949, Murray Barr Darkly stained body in female cell Cat nuclei In human females, one of the 2 X-chromosomes is inactivated early in development 1949, Murray Barr Darkly stained body 1961, Mary Lyon Inactive X chrom. X-inactivation Takes place in all embryos with two or more X chromosomes X-inactivation center (XIC) site centromerically located (Xq11.2-Xq21.1) Within XIC is Xist gene X-Inactivation-specific transcript To date only gene known to be transcribed from inactive X chromosome Functional RNA not a protein X-inactivation Within XIC is Xist gene X-Inactivation-specific transcript To date only gene known to be transcribed from inactive X chromosome Functional RNA not a protein Coats X chromosome Chromatin condensation Histone deacytlation, methylation, composition Pseudoautosomal region not affected X-inactivation Which chromosome to inactivate Random inactivation Some cells will have different x inactivated leading to areas of similarity and areas of dissimilarity mosaic X-inactivation Best example is the cat 2 sex-linked alleles for coat color Orange (X0) and Black (X+, nonorange) Males either orange or black, only have one chromosome (XoY or X+Y) Females can be heterozgous leading to a mosaic coat of orange and black patches "tortoiseshell" or "calico" (X+Xo) White patches on a calico are due to autosomal gene ZZ-ZW Sex Determination Richard Goldschmitt Chickens Barred Feathers Inheritance was opposite of a sex-linked pattern (fathers to daughters) Hypothesized that the male birds were the homogametic sex ZZ-ZW Sex Determination Richard Goldschmitt Chickens Barred Feathers Inheritance was opposite of a sex-linked pattern in seen in flies (fathers to daughters) Hypothesized that the male birds were the homogametic sex Males ZZ Females ZW Z-Linked Traits Genetic Sex Determining Systems Haplodiploidy Sex determined by ploidity Number of chromosome sets Hymenoptera Bees, Wasps, ants Males haploid (unfertilized eggs) Male sperm all the same (mitosis) Females diploid (fertilized eggs) Often used to explain social cooperation Genetic Sex Determining Systems Genic sex determination Genotype at one loci (one gene) or many loci determine sex of an organism Humans XX males XY females Master Sex Determine Gene Genetic Sex Determining Systems Genic sex determination Genotype at one loci (one gene) or many loci determine sex of an organism Humans XX males XY females Androgen-insensitive syndrome Outward appearance female XY, non-functional androgen receptor Environmental Sex Determination Environment influences sex Crepidula fornicata Larva settles becomes female Female attracts larva top larva becomes male mate for female below Male switches sex and becomes female to attract more larva Sequential hermaphroditism Slipper limpet Environmental Sex Determination Environment influences sex Male switches sex and becomes female to attract more larva Sequential hermaphroditism Protandric (born male change to female) Typically 15 in a stack Middle are hermaprodites Bottom one often dead Thomas Hunt Morgan The Fly Room: Home to students
Alfred Sturtevant and Calvin Bridges Proof of Chromosome Theory 1909 Student found a rare white eyed variant (mutant) male fly amongst wild-type (normal) red eyes. Crossed that fly with red eyed female In an example of exceptional science Inbred F2 flies to produce true-breeding flies Morgan's Experiment Used true breeding white eye flies Concluding Evidence for Chromosome Theory Calvin Bridges Morgan's student Using white-eyed flies, got a few whiteeyed females and red-eyed males Concluding Evidence for Chromosome Theory Calvin Bridges Morgan's student Using white-eyed flies, got a few whiteeyed females and red-eyed males Karyotyped flies The chromosome composition of male and female Drosophila Concluding Evidence for Chromosome Theory Calvin Bridges Morgan's student Using white-eyed flies, got a few whiteeyed females and red-eyed males Karyotyped flies Theorized that X-chromosome failed to segregate during anaphase I of meiosis Occurred at a calculable rate Called it non-disjunction Consequences of nondisjunction (lack of segregation) of the X-chromosomes during meiosis in Drosophila Sex Determination of Drosophila Bridges continued on XO are sterile males Y only required for fertility Found how sex was determined in flies X-chromosome to Autosome ratio > 1 = metafemales < = metamales > <1 = intersex Theorized that sex determination controlled by genes located on both X chromosome and autosomes Mechanism of sex-determination by X-chromosome/autosome ratio in Drosophila Sense the number of chromosomes The X-chromosome/autosome ratio controls the switching on/off of the master control gene, sex lethal (Sxl) (rna-binding protein) Nondisjunction and Humans Failure of chromosomes to separate during meiosis creates cells with extra or missing chromosomes Down syndrome 13,18,21, X, Y Meiosis Forms Haploid Gametes Turner syndrome and Klinefelter syndrome On a different note: development ...
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- Summer '08
- chromosome theory, Environmental Sex Determination, Genetic Sex Determining Systems