Extra credit problem for Lecture #6 How is sex determined in Drosophila melanogaster? For extra credit question, please use the index cards provided. Print your name and section # at top of card. Thanks!

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 pre- meiotic 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,

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 Paternal SNRPN Gene inactive by mutation Paternal SNRPN gene active (chrom. 15) Maternal SNRPN Gene inactive by imprinting Fertilization Both copies inactive Prader-Willi Syndrome Short stature Mild retardation Poor muscle tone Compulsive eating

Genetic Linkage and Mapping

Meiosis: Prophase I

Closer look

Notation — ————— A _________ A a Aa Diploid Adult Haploid gametes (single chromatid) — ————— Two homologous chromosomes, four chromatids total _________ A a a 1/2 A : 1/2 a

Dihybrid Cross AA BB Female aa bb Male A B a b — ——————— —————— ——————— ————— P: — ——————— —————— ——————— ————— A B a b Gametes: Egg (A B)Sperm (a b) A B a b — ——————— ————— ——————— ————

A B — ——————— ————— F1: — ——————— ————— a b after the S phase F1 adult

4 gamete types, equally frequent A B — ——————— ———— A b — ——————— ———— a B — ——————— ———— a b — ——————— ———— A B — ——————— ————— F1: — ——————— ————— a b Parental Recombinant Parental

F2 of dihybrid cross when genes not linked What proportion of the gametes of the double heterozygote are recombinant? 50 %