Announcements

1. Announcements We have decided to halt our efforts with the yw crosses, due to uncooperative yw females, frustration, and lack of motivation (we KNOW the expected results). There will be no assignment due for this lab activity. You are responsible for understanding the expected results and how to calculate recombination frequencies and map distance (ie. flylab 6). You should continue your X-linked crosses. Vote on when to review for exam 2 (10/17, 10/18) - Mon. evening or in class Wednesday. If we review in class Wed, then class meets Fri. If we review Mon., then class meets Wed. Is there a serious problem taking the exam on Thurs. or Fri.??? Reminder that homework is due next Tues., Wed. Quiz in lab next week; be prepared - pick up lab7 overview and read it. Practice problems for Ch.8: 1,3,9,11; Ch.9: 4,8,10

2. Review of Last Lecture 1.Quick review of conjugation: F-, F+, Hfr 2. Transformation: a different process of recombination, can be used to map genes 3. Bacteriophages are viruses that use bacteria as hosts; they can mediate bacterial DNA transfer - transduction 4. Extrachromosomal inheritance: Phenotype of maternal parent is solely responsible for phenotype of offspring - example of maternal inheritance of chloroplasts

3. Outline of Lecture 16 I. Two more examples of extrachromosomal inheritance: mitochondria and “maternal effect” II. Modes of sex determination III. Humans and sex determination IV. Dosage compensation

4. I. Mitochondrial Heredity • About 30,000 genes on 23 human chromosomes (3.3 billion base pairs/haploid cell) • Mitochondrial Genome contains 37 genes: • 13 code for some of the proteins involved in oxidative respiration • 22 tRNA genes • 2 rRNA genes • some proteins involved in mt protein synthesis • 16,569 base pairs, circular

5. Human Mitochondrial Diseases • Are maternally inherited: only offspring of affected mothers are affected • Show deficiency in mitochondrial function • Are caused by a mutation in a mitochondrial gene • Example: myoclonic epilepsy and ragged red fiber disease (MERRF) • Deafness, dementia, seizures • Point Mutation in a mitochondrial tRNA

6. Other Human Mitochondrial Diseases • Leber’s Hereditary Optic Neuropathy (LHON) • Sudden bilateral blindness • Point mutation in small subunit of NADH dehydrogenase • Kearns-Sayre Syndrome (KSS) • Symptoms in eyes, muscles, heart, brain • Deletion mutation in mtDNA

7. Maternal Effect: Snail Coiling • Pattern of offspring determined by genotype of mother, regardless of phenotype DD dd dextral sinistral D egg, d sperm D sperm, d egg Dd Dd dextral sinistral

8. Maternal Effect Genes in Drosophila Development • Discovered by Edward Lewis, Christiane Nusslein-Volhard and Eric Weischaus (Nobel Prize in Physiology or Medicine, 1995) • Gene products deposited in egg by cells of mother’s ovary. • These genes set up the first embryonic axes: dorsal-ventral and anterior-posterior and control later zygotic genes.

9. II. Sex Determination Gene Mapof theY Chromosome

10. Modes of Sex Determination • XX/XO (Protenor) • absence of second X chromosome determines maleness • as in Caenorhabditis elegans • XX/XY (Lygaeus) • presence of Y chromosome determines maleness • in mammals • sometimes females are heterogametic sex

11. XX/XO(Protenor) Mode Male Female 1:1 SEX RATIO

12. C. elegans Sex Determination XX XO X/Autosome Sets = 0.5 X/Autosome Sets = 1

13. III. Normal Human Karyotypes Female Male

14. Klinefelter Syndrome (47, XXY) Some male dev. but no sperm, and some female dev. too. 2 in 1000 male births

15. Turner Syndrome (45, X) Female dev. but no eggs. 1 in 3000 female births

16. The Odyssey of the Germ Cells • Primordial Germ Cells are set aside early in embryonic development • PMG’s migrate to embryonic kidney ridges: • If cells are XY, medulla develops into testes under direction of male hormones around week 7 • If cells lack a Y, cortex develops into ovary, also under active hormonal control • Active gene expression required in either case • Duct differentiation: • If XY, Wolffian ducts > epididymis and vas deferens • If no Y, Mullerian ducts > oviduct

17. The Human Y Chromosome NRY is non-recombining region of Y

18. Testis-Determining Factor • Was the object of an intense search. • SRY gene on the Y chromosome was identified as the gene that codes for TDF: • SRY is translocated to X in rare XX males • SRY is absent from Y in rare XY females • The “home run” experiment by Koopman et al. used transgenic mice.

19. The Transgenic Sry Experiment:How It Was Done • Reference: Nature 351:117 (1991) • Nuclei of fertilized XX eggs were injected with Sry gene, then the eggs were transplanted to surrogate mothers. • Sry gene then randomly incorporated into a chromosome and was inherited in subsequent cell divisions. • Animals karyotyped after development to adult.

20. Genotypically Female Mice Transgenic for SRY are Phenotypically Male XY male XX male

21. IV. Dosage Compensation • Shouldn’t XX females produce twice the amount of X-linked gene products as XY males? • No, because XX females “compensate” by inactivating one of their X chromosomes to make a single “dosage” of X-linked genes.

22. Barr Bodies are Inactivated X Chromosomes in Females

23. The Lyon Hypothesis of X Inactivation • Proposed by Mary Lyon and Liane Russell (1961) • Inactivation of X chromosome occurs randomly in somatic cells during embryogenesis • Progeny of cells all have same inactivated X chromosome as original, creating mosaic individual

24. Lyon-Hypothesis: X-inactivation A precursor cell to all coat color cells Random inactivation

25. Mosaicism Reveals the Random Inactivation of one X chromosome Regions where sweat glands are absent.

26. Bilateral Gyandromorph (both Male and Female Genotypes) Occurs from loss of one X (with wildtype alleles) at 1st mitotic division during development, oriented bilaterally. Female (XX) half: heterozygous for both markers Male (XO) half: white, miniature wing