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BIOL 239 (290)


4 Pages

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BIOL 239
Heidi Engelhardt

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Chromosomes  Chromosomes and reproduction o Earliest indications :  1667  studies showing semen contains spermatozoa  1854  studies with frogs and sea urchins showing union of male and female gametes. Equal contribution.  1879  observation of threadlike structures in the nuclei of salamander cells during cell division  1880  advances in microscopic techniques  use of dyes for differential staining of cytoplasm, nucleus and nuclear structures, including chromosomes  Early 1900s  linking of chromosomes to the Mendelian principles of segregation and independent assortment  proposed that each egg or sperm contains only one of each chromosome of a pair.  Chromosomal Theory of Inheritance o States that hereditary information is on genes and that genes are located on chromosomes o Also that egg and sperm contribute equally to the genetic endowment of offspring through their nuclei  1901  the X and Y chromosomes determine sex in grasshoppers  Soon others confirmed presence of separate sex chromosomes in other species o one sex carries 2 of the same sex chromosome, the other carries one of each (not true in all species)  Platypus: Studies in PNAS and Nature; females are (XXXXX)x2, males are XXXXXYYYYY  not all Xs and Ys are homologous  Most mammals:  Males XY  Females XX  Birds, butterflies, some reptiles:  Males ZZ  Females ZW  Sex Determination o Humans vs Drosophila  In humans, presence of Y chromosome determines maleness: in Drosophila, it does not rather, sex is determined by ratio of X chromosomes to autosomes
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