Sex-Determining Systems

• XX-XO


  Females are homogametic, males have a single X
• ZZ-ZW


  Females are heterogametic, males are homogametic
• Haploidiploidy


  Sex of individual is determined by number of chromosomes inherited, females- haploid, males- diploid
• Genic


  Determined by genotype, no specific sex chromosomes, have sex loci on autosomal chromosomes
• Environment


  Temperature may favor one sex over the other, ex. fish and reptiles
• Drosophila sex determination


  1. A ratio: <1.0 = male, <0.5 = metamale, 0.55-0.99 = intersex, 1 = female, >1.0 = metafemale
• SRY gene


  On Y chromosome determines male phenotype, encodes for DNA-binding protein that expresses genes that influence testes formation
• Testes


  Secrete testosterone and Mullerian-inhibiting substance that causes degeneration of female reproductive system
• One X is required for human sex determination