Extra credit problem for Lecture 6 For extra credit question please use the index cards provided Print your name and section at top of card Thanks How is sex determined in Drosophila melanogaster Sex Determination in Drosophila and other organisms X chromosome Autosome Balance In Drosophila sex is determined by the ratio of the number of X chromosomes to the number of sets of autosomes Other kinds of sex determination Grasshoppers crickets roaches XO Bees ants wasps haplodiploidy Snakes ZZ ZW Lizards both XX XY and ZZ ZW systems Crocodiles most turtles and some lizards sex is determined by the incubation temperature during a critical period of embryo development This is known as temperature dependent sex determination TSD Some lizards Turtles Lizards Turtles Crocodilians In some organisms sex is not determined at all Some fish change sex as they get older or larger In clown fish largest group member is female second largest is male and all others are non breeding Sometimes the largest fish in the group will become male all others will be female wrasses Coral Goby Gobiodon histrio Monogamous pairs If mate dies or leaves resident will court next fish to come by one will change sex to accommodate And some species are all female Asexual Cnemidophorus species are all female reproduce via parthenogenesis Diploid or triploid eggs develop directly from oocytes via premeiotic endomitosis no cellular nuclear division At meiosis identical rather than homologous chromosomes pair and then segregate All offspring are females genetically identical to their mothers Cnemidophorus species tend to have extremely low levels of genetic diversity From A J Collum Creighton Univ http biology creighton edu faculty cullum CnemmyInfo html Genomic Imprinting Differential expression of genetic traits depending on whether the trait has been inherited from a mother or a father Disease can result from imprinting QuickTime and a TIFF LZW decompressor are needed to see this picture QuickTime and a TIFF LZW decompressor are needed to see this picture QuickTime and a TIFF LZW decompressor are needed to see this picture Paternal SNRPN gene active chrom 15 Short stature Mild retardation Poor muscle tone Compulsive eating Maternal SNRPN Paternal SNRPN Gene inactive by Gene inactive by imprinting mutation Fertilization Prader Willi Syndrome QuickTime and a TIFF LZW decompressor are needed to see this picture Both copies inactive Genetic Linkage and Mapping Meiosis Prophase I Closer look Notation Aa Diploid Adult Haploid gametes single chromatid A a A A a a Two homologous chromosomes four chromatids total 1 2 A 1 2 a Dihybrid Cross AA BB Female A B a b P aa bb Male A Gametes B a Egg A B A b Sperm a b B a b F1 adult after the S phase A F1 B a b A F1 a A A 4 gamete types equally frequent B a a B b b Parental Recombinant B Recombinant b Parental F2 of dihybrid cross when genes not linked What proportion of the gametes of the double heterozygote are recombinant 50
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