Sex Determination and Sex Chromosomes
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This document discusses sex determination and sex chromosomes across different organisms. It begins by describing primary and secondary sexual differentiation. It then provides examples of sex determination in various species like maize, nematodes, butterflies and humans. In humans, the presence of an X or Y chromosome determines sex, with males having XY and females having XX. Abnormal sex chromosome combinations can result in conditions like Klinefelter syndrome or Turner syndrome. The document also discusses dosage compensation that balances X chromosome expression between males and females. Finally, it notes that temperature during egg incubation can determine sex in some reptiles.
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Sex Determination and Sex Chromosomes
- 1. Sex Determination and Sex Chromosomes Dr. Dinesh C. Sharma
- 2. Copyright © 2009 Pearson Education, Inc. Life Cycles Depend on Sexual Differentiation •In multicellular organisms, it is important to distinguish between: • primary sexual differentiation • involves only the gonads where gametes are produced • secondary sexual differentiation • involves the overall appearance of the organism
- 3. Copyright © 2009 Pearson Education, Inc. • Some organisms (e.g. Chlamydomonas) spend most of their life cycle in the haploid phase, asexually producing daughter cells by mitotic division.
- 4. Copyright © 2009 Pearson Education, Inc.
- 5. Copyright © 2009 Pearson Education, Inc.
- 6. Copyright © 2009 Pearson Education, Inc. • In maize (Zea mays), the diploid sporophyte stage predominates and both male and female structures are present on the adult plant. • This indicates that sex determination must occur differently in different tissues of the same plant
- 7. Copyright © 2009 Pearson Education, Inc.
- 8. Copyright © 2009 Pearson Education, Inc. • The nematode worm Caenorhabditis elegans has 2 sexual phenotypes: • Males, which have only testes • Hermaphrodites, which have both testes and ovaries
- 9. Copyright © 2009 Pearson Education, Inc.
- 10. Copyright © 2009 Pearson Education, Inc. • XX/XO (Protenor) mode of sex determination • depends on random distribution of the X chromosome into half of the male gametes. • Presence of two X chromosomes in the zygote = female offspring • Presence of only one X chromosome = male offspring http://www.natureproducts.net/Anim als/Insects/Butterflies/Papilio_prote nor2.jpg
- 11. Copyright © 2009 Pearson Education, Inc.
- 12. Copyright © 2009 Pearson Education, Inc. • XX/XY (Lygaeus) mode of sex determination: • female gametes all have an X chromosome • male gametes have either an X or a Y chromosome • Zygotes with two X chromosomes (homogametous) = female offspring • Zygotes with one X and one Y chromosome (heterogametous) = male offspring http://bugguide.net/node/view/193531/bgimage
- 13. Copyright © 2009 Pearson Education, Inc.
- 14. Copyright © 2009 Pearson Education, Inc. • ZZ/ZW sex determination: • females are the heterogametic (ZW) sex • males are the homogametic (ZZ) sex
- 15. Copyright © 2009 Pearson Education, Inc. • The Y Chromosome Determines Maleness in Humans • Human karyotype revealed that one pair of chromosomes differs in males and females: • females have two X chromosomes • males have one X and one Y chromosome
- 16. Copyright © 2009 Pearson Education, Inc. Normal Human Karyotypes
- 17. Copyright © 2009 Pearson Education, Inc. • Persons with Klinefelter syndrome have: • male genitalia • more than one X chromosome (usually XXY, or a 47,XXY karyotype) Klinefelter syndrome http://all4freehere.com/2009/07/what-is-klinefelters-syndrome/
- 18. Copyright © 2009 Pearson Education, Inc.
- 19. Copyright © 2009 Pearson Education, Inc. • Persons with Turner syndrome usually have: • a single X chromosome • no Y chromosome (45,X karyotype) • female genitalia Turner syndrome http://www.lucinafoundation.org/assets/turner-syndrome.jpg
- 20. Copyright © 2009 Pearson Education, Inc.
- 21. Copyright © 2009 Pearson Education, Inc. • 47, XXX – Trisomy X • 3 X chromosomes along with a normal set of autosomes results in female differentiation. • Usually normal • In other cases, underdeveloped secondary sex characteristics, sterility, and mental retardation may occur. Trisomy X
- 22. Copyright © 2009 Pearson Education, Inc. • 47, XYY – Jacobs Syndrome • Males are usually over 6 feet tall. Jacobs Syndrome
- 23. Copyright © 2009 Pearson Education, Inc.
- 24. Copyright © 2009 Pearson Education, Inc. • The Y chromosome contains far fewer genes than the X chromosome. Chromosome http://blogs.discovermagazine.c om/notrocketscience/files/2010/ 07/XY.jpg
- 25. Copyright © 2009 Pearson Education, Inc. • Pseudoautosomal Regions (PARs) • regions on Y chromosome that share homology with regions on the X chromosome • synapse and recombine with it during meiosis • Presence of such a pairing region is critical to segregation of the X and Y chromosomes during male gametogenesis Pseudoautosomal Regions
- 26. Copyright © 2009 Pearson Education, Inc. • Y chromosome contains: • the male- specific region of the Y (MSY) • a sex- determining region of the Y (SRY) Y chromosome
- 27. Copyright © 2009 Pearson Education, Inc. • Testis-determining factor (TDF) • a protein encoded by a gene in the SRY that triggers testes formation. • The MSY consists of three regions: • X-transposed region • X-degenerative region • ampliconic region TDF
- 28. Copyright © 2009 Pearson Education, Inc. • The Ratio of Males to Females in Humans is not 1.0 • Primary sex ratio reflects the proportion of males to females conceived in a population. • Secondary sex ratio reflects the proportion of each sex that is born. sex ratio http://partywarehouse.co.nz/zen/images
- 29. Copyright © 2009 Pearson Education, Inc. • Dosage Compensation Prevents Excessive Expression of X-Linked Genes in Humans and Other Mammals • Dosage compensation balances the dose of X chromosome gene expression in females and males. Dosage Compensation
- 30. Copyright © 2009 Pearson Education, Inc. • The inactive X is highly condensed, can be observed in stained interphase cells, and are referred to as Barr bodies Barr bodies
- 31. Copyright © 2009 Pearson Education, Inc.
- 32. Copyright © 2009 Pearson Education, Inc. • The Lyon hypothesis states that X-inactivation occurs randomly in somatic cells. • This is evident in the calico cat (Figure 7.12). Lyon hypothesis
- 33. Copyright © 2009 Pearson Education, Inc. Lyonization in Humans: - Red-green color blindness - Anhidrotic ectodermal dysplasia
- 34. Copyright © 2009 Pearson Education, Inc. • The X-inactivation center (Xic) is active on the inactive X. • It consists of the X-inactive specific transcript (XIST) gene. X-inactivation center
- 35. Copyright © 2009 Pearson Education, Inc. The Ratio of X Chromosom es to Sets of Autosomes Determines Sex in Drosophila
- 36. Copyright © 2009 Pearson Education, Inc. • For all crocodiles, most turtles, and some lizards, sex determination is achieved according to the incubation temperature of eggs during a critical period of embryonic development. Temperature Variation Controls Sex Determination in Reptiles
- 37. Copyright © 2009 Pearson Education, Inc. • There are three different patterns of temperature sex determination in reptiles
Editor's Notes
- Figure 7-1 The life cycle of Chlamydomonas. Unfavorable conditions stimulate the formation of isogametes of opposite mating type that may fuse in fertilization. The resulting zygote undergoes meiosis, producing two haploid cells of each mating type. The photograph shows vegetative cells of this green alga.
- Figure 7-2 llustration of mating types during fertilization in Chlamydomonas. Mating will occur only when plus and minus cells are together.
- Figure 7-3-02 The life cycle of maize (Zea mays). The diploid sporophyte bears stamens and pistils that give rise to haploid microspores and megaspores, which develop into the pollen grain and the embryo sac that ultimately house the sperm and oocyte, respectively. Following fertilization, the embryo develops within the kernel and is nourished by the endosperm. Germination of the kernel gives rise to a new sporophyte (the mature corn plant), and the cycle repeats itself.
- Figure 7-4 (a) Photomicrograph of a hermaphroditic nematode, C. elegans; (b) The outcomes of self-fertilization in a hermaphrodite and a mating of a hermaphrodite and a male worm.
- Figure 7-5a (a) The Protenor mode of sex determination where the heterogametic sex (the male in this example) is XO and produces gametes with or without the X chromosome; (b) The Lygaeus mode of sex determination, where the heterogametic sex (again, the male in this example) is XY and produces gametes with either an X or a Y chromosome. In both cases, the chromosome composition of the offspring determines its sex.
- Figure 7-5b (a) The Protenor mode of sex determination where the heterogametic sex (the male in this example) is XO and produces gametes with or without the X chromosome; (b) The Lygaeus mode of sex determination, where the heterogametic sex (again, the male in this example) is XY and produces gametes with either an X or a Y chromosome. In both cases, the chromosome composition of the offspring determines its sex.
- Figure 7-6 The traditional human karyotypes derived from a normal female and a normal male. Each contains 22 pairs of autosomes and two sex chromosomes. The female (a) contains two X chromosomes, while the male (b) contains one X and one Y chromosome (see arrows).
- Figure 7-7a The karyotypes and phenotypic depictions of individuals with (a) Klinefelter syndrome (47,XXY) and (b) Turner syndrome (45,X).
- Figure 7-7b The karyotypes and phenotypic depictions of individuals with (a) Klinefelter syndrome (47,XXY) and (b) Turner syndrome (45,X).
- Figure 7-8 The presence or absence of the Y chromosome and SRY determines sexual differentiation in humans. Early human embryos are sexually indifferent. Bipotential gonads can form either ovaries or testes depending on the presence or absence of the SRY genotype. With its presence in XY embryos, bipotential gonads form testes and, subsequently, male reproductive organs and ducts. In the absence of SRY, in XX embryos, the female reproductive organs and ducts are formed.
- Figure 7-11 Barr body occurrence in various human karyotypes, where all X chromosomes except one (N –1) are inactivated.
- Figure 7-13 Depiction of the absence of sweat glands (shaded regions) in a female heterozygous for the X-linked condition anhidrotic ectodermal dysplasia. The locations vary from female to female, based on the random pattern of X chromosome inactivation during early development, resulting in unique mosaic distributions of sweat glands in heterozygotes.