Today…

1. Genome 351, 21 April 2014, Lecture 7 Today… • Sex chromosomes (X & Y) • Inheritance patterns of sex-linked traits • Mitochondrial inheritance • Sex determination in humans

2. Chromosomes - a reminder How many do humans have? • 22 pairs of autosomes • 2 sex chromosomes • Each parent contributes one chromosome to each pair • Chromosomes of the same pair are called homologs • Others are called non-homologous Photo: David McDonald, Laboratory of Pathology of Seattle

3. The Sex Chromosomes • X chromosome • contains more than 1,500 genes • larger than the Y chromosome (~150x106bp vs. ~60x106bp) • Y chromosome • contains ~130 genes, including the SRY gene responsible for male development • genes for sperm formation • long palindromes (madam I’m Adam)

4. The Y chromosome • Much of the Y-chromosome lacks genes altogether • ends match the ends of the X-chromosome • the rest has no match to the X or anywhere else in the genome. p q Y

5. Recently discovered Y-linked genes Channel Flipping (FLP) Catching & Throwing (BLZ-1) Self-confidence (BLZ-2) (note-unlinked to ability) Ability to Remember & Tell Jokes (GOT-1) Sports Page (BUD-E) Addiction to Death & Destruction Movies (T-2) Air Guitar (RIF) Ability to identify aircraft (DC10) Preadolescent fascination with Arachnida & Reptilia (MOM-4U) Spitting (P2E) Sitting on the john reading (SIT) Selective hearing loss (HUH?) Total lack of recall for dates (OOPS) p q Y Science261, 679 (1993)

6. Recently discovered Y-linked genes • The gene, SRY, that determines maleness. • A few decaying genes from the ancestral chromosome. • 78 genes involved in sperm formation. • No genes affecting visible traits! p more in a bit… q more in a bit… Y

7. Origins of the human Y chromosome AA ~1500 genes Ordinary chromosomes (autosomes) from which the X and Y evolved Timeline Several rodent species have completely lost the Y-chromosome!! A gene acquires ability to cause maleness Y X X-Y split ………....... Cynognathus (a cynodont) 150-300 million years Most genes from ancestral X lost or damaged (5-7 genes lost/1x106 years) few if any genes lost Is the human Y-chromosome also heading for extinction? human-chimp split 6 million years No more genes damaged 5 genes damaged Chimp Y Human Y Human X ~130 genes ~1500 genes

8. Crossing over between homologs is required for meiosis Meiosis I How does this work in males? The crossover’s hold the homologs together until all the homologs are joined; absence of crossovers can result in daughter cells with too many or too few homologs locations where crossing over occurs

9. The X and Y chromosomes pair and crossover in the pseudoautosmal region crossovers only in pseudo-autosomal region Sex chromosomes during meiosis I sequences the same here in X & Y; behave like autosomes (2 copies per cell in males & females) X X X Y no crossovers— X and Y sequences diverged crossovers anywhere along the length females males

10. X X X X Y Y X X = XX =XY X X X XX XX Y XY XY Inheritance of sex chromosomes meiosis 1 meiosis 1 meiosis 2 meiosis 2 Each daughter gets the same paternal X X comes from mother; Y from father

11. The Y chromosome serves as a paternal marker through history DNA mutations on the Y can serve as markers for the male lineage

12. The Y chromosome serves as a paternal marker through history DNA mutations on the Y can serve as markers for the male lineage

13. Inheritance of X-linked traits -X-linked recessive traits Example: Hemophilia - a disease where blood clotting is severely impaired so that an affected individual may bleed severely from minor injuries. Most commonly due to a defect in clotting factor VIII, made from a gene on the X chromosome.

14. Blood clotting cascade

15. XH XH XH Xh Xh Y Y = XHXh XH =XHY XH Xh XH XHXH XhXH Y XHY XhY Inheritance of X-linked recessive traits meiosis 1 meiosis 1 meiosis 2 meiosis 2

17. Features of X-linked recessive traits • Always expressed in the male. • Males pass their allele to their daughters, never their sons.

19. Features of X-linked recessive traits • Always expressed in the male. • Males pass their allele to their daughters, never their sons. • Half the sons of a carrier mother have the trait. Half of her daughters are carriers.

20. 1/4 2/5 6/9 9/18 or 1/2

21. Features of X-linked recessive traits • Always expressed in the male. • Males pass their allele to their daughters, never their sons. • Half the sons of a carrier mother have the trait. Half of her daughters are carriers. • Expressed in the homozygous mother, but only rarely in a heterozygote. More on this later

22. opsin genes… 98% identical red green unequal exchange Red opsin defective Green opsin defective Another X-linked disorder:Red-green color blindness • Red and green opsin genes lie near the tip of the X-chromosome

23. Color blindness tests

24. Another X-linked disorder:Red-green color blindness • Red and green opsin genes lie near the tip of the X-chromosome • 8% of males of European ancestry are color-blind as are 4% of males of African descent • Only 0.2-0.8% of females of both groups are color-blind

25. Inheritance of red-green color blindness

26. XdY XdXD Inheritance patterns of X-linked traits X-linked recessive traits… • more affected men than women (rare traits… almost exclusively affect men) - sons of affected women will be affected X-linked dominant traits… • affected women: each child has 50% chance of being affected • affected man: will transmit trait to all his daughters and none of his sons

27. XdXd XdXd XDY XDY Questions In general… is the pedigree inconsistent with any particular mode of inheritance? Which of these pedigrees is/are not consistent with X-linked recessive inheritance? if X-linked recessive… Not consistent Consistent Consistent Xd = affected XD = not affected

28. XDX? XdY XdXd XdXd XDY XDY Questions (continued) Which of these pedigrees is/are not consistent with X-linked dominant inheritance? Consistent Not consistent Not consistent Consistent XD = affected Xd = not affected

29. Another pattern of inheritance! What features characterize this pattern of inheritance? Mother’s children all inherit the trait. Father’s children never inherit the trait!

30. Maternal Inheritance: Mitochondria mitochondria Generates most of the cells energy, but also does much more inner membrane outer membrane ~1,000-1,500 mitochondrial proteins encoded in the nucleus and imported post-translationally cristae Inter-membrane space matrix mitochondrial genome (~40 genes; < 1% of all human genes ) 100 - >100,000 copies/cell

31. Maternal Inheritance: Mitochondria egg embryo Why? Quality control filter in female germline? Sperm mitochondrial DNA prone to damage? sperm mitochondria • All the embryo’s mitochondria come from oocyte (egg); sperm have little mitochondrial DNA; sperm mitochondrial DNA is rapidly destroyed in the embryo • Mitochondrial genes are transmitted from mother to all of her offspring • Mitochondrial DNA mutation rate is much higher than nuclear DNA • Mitochondrial DNA does not recombine

32. Mitochondria act as a simple tracer of the maternal lineage through history • DNA sequence alterations can be used in evolutionary studies • Diseases caused by mitochondrial DNA mutations affect ~1/4,000 children

33. Heteroplasmy • Many copies of the mitochondrial genome per cell (100 - >100,000) • May have more than one allele for the same gene in the same cell • Heteroplasmyis the condition where mitochondrial DNA sequence is not the same in all copies eggs normal mutant

34. Heteroplasmy

35. incomplete chromosome sets Sex determination in humans Clues from aneuploidy XXY individuals (Klinefelter syndrome; 1/1,000)… male XO individuals (Turner syndrome; 1/2,500)… female Conclusion: In humans (and all mammals), presence of Y causes male developmental pathway Not universal… Birds: females are heterogametic, WZ; males are ZZ Bees: unfertilized eggs  males; fertilized eggs females

36. x sex reversal x Sex determination in humans What is it about the Y that causes male-ness? Again, clues from exceptional meioses… Karyotyping of infertile couples revealed sex reversal XY XY XX XX how does this happen?

37. The X and Y chromosomes pair and crossover in the pseudoautosmal region crossovers only in pseudo-autosomal region Sex chromosomes during meiosis I in many sex-reversed individuals—illegitimate recombination X X X Y no crossovers— X and Y sequences diverged crossovers anywhere along the length females males

38. egg Aberrant crossover: portion of Y transferred to X! sperm Normal crossover: XY female XX male

39. Conclusion Portion of Y that causes male-ness must lie just next to pseudoautosomal region SRY gene (sex reversal on Y) aka TDF (testis determining factor) The SRY protein has a modular design and functions as a transcription factor

40. XY XX; SRY Is SRY really the male-determining factor? An experiment in mice Add SRY gene SRY Embryo showing just X and autosomal 14 chromosomes An embryo with the SRY gene inserted on one of the two copies of chromosome 14 Embryo develops into a male!

41. SRY - Another experiment • Introduction of the mouse SRYgene (protein coding and flanking regions) causes sex reversal of XX mice • Can human SRYgene introduced into mice do the same? No!!!

42. SRY - Another experiment Mouse and human SRY regulatory regions

43. Is it the human SRY protein that doesn’t work in mice? • Test this by taking the human coding sequence and combining this with the mouse regulatory regions It works!!

44. Gonadal sex determination Before Sry expression -- week 6 • In early embryos unspecialized gonads and two sets of reproductive ducts exist until week 6 • An embryo gonad develops as a testis if it has the Sry gene and an ovary if it does not Indifferent stage Indifferent gonad Müllerian duct Wolffian duct

45. Early events SRY absent SRY present Müllerian inhibiting factor (MIF) testosterone estrogen

46. Sexual differentiation estrogen testosterone

47. Steps in Sex Determination 1. Chromosomal sex (XX or XY) fertilization SRY -> testis 2. Gonadal sex Primitive gonad 6 weeks Ovary Female primary and secondary sexual characteristics Estrogen 3. Phenotypic sex Ovary 8-12 weeks Male primary and secondary sexual characteristics Testosterone Testis oviduct/uterus MIF

48. chromosomal sex gonadal sex phenotypic sex = female testis XYSry = testosterone MIF Testosterone action (pseudohermaphrodites) Absence of androgen receptor -> Androgen (testosterone) Insensitivity Syndrome (AIS; testicular feminization) X-linked recessive 1/65,000 births external female, testes, no internal reproductive tract, little sexual body hair No receptor so no somatic male development No uterus

49. The search for other genes … Chromosomal region Gene Phenotype OMIM* SRY Yp11 Sex reversal 480000 SOX9 17q25 sex reversal 114290 DAX1 Xp21 sex reversal 300018 SF1 9q33 sex reversal with adrenal failure 184757 Conclusion: Sex determination is a sensitive switch influenced by many genes *Online Mendelian inheritance in man: http://www.ncbi.nlm.nih.gov/omim

50. Germline formation during embryogenesis - males zygote 5 weeks - primordial germ cell formation 10 weeks - Mitotic arrest - 10-14 years Mitosis Meiosis I Meiosis II All functional