Clemson GEN 3000 - Test 2 Study Guide

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Genetics Test 2 Study GuideCH 5 Heteromorphic chromosomes (XY chromosomes) characterize one sex or the other  Results in sex chromosomes  Genes (not chromosomes) that results in sex determination Some are present on sex chromosomes, some are autosomal  Sexual dimorphism: differences between males and females  Primary sexual differentiation: involves only the gonads (where gametes are produced)  Secondary sexual differentiation: involves overall appearance of organism (external genitalia) Unisexual = dioecious = gonochoric: all refer to an individual containing only male or only female reproductive organs  Bisexual = monoecious = hermaphroditic: all refer to individuals containing both male and female reproductive organs  Common in plants and animals  Can produce both egg and sperm Intersex: individuals with intermediate sexual condition, often sterile  Life cycles of plants alternate between haploid gametophyte stage and diploid sporophyte state  Meiosis and fertilization link  Sex determinism occurs differently in different tissues of the same organism  Nematode Worm Popular in genetic studies because adults consist of 1000 cells and the linage can be easily traced to specific origins  Two sexual phenotypes: males (have testes) and hermaphrodites (have testes and ovaries)  Hermaphrodites: testes produce sperm that is stored and the ovaries do not undergo oogenesis until reaching adulthood. The ovaries are then self fertilized by the stored sperm - Most organisms that results are hermaphrodites (less than 1% are males) but males can mate with hermaphrodites and produce ½ males and ½ hermaphrodites  Lack a Y chromosome so males have one X and hermaphrodites have XX  Sex Determinism H. Henking: identified a nuclear structure in the sperm of insects: X-body  Clarence McClung: female grasshopper have 1 more chromosome than males  Edmund B. Wilson: Female somatic cells in butterfly contain 14 chromosomes, including two X chromosomes while male somatic cells in butterflies only had 13 chromosomes, including 1 X chromosome During spermatogenesis, gametes are produced with either 6 chromosomes, without an X, or 7 chromosomes with an X. If the gamete received an X it would be a female but if they did not (and were X deficient) they would be a male - Called the XX/XO or Protenor Mode of Sex Determination  Depends on random distribution of X chromosome into ½ of male gametes during segregation  Wilson also saw that females of milkweed bugs had two X chromosomes while males have only a single X chromosome and a smaller chromosome (labeled Y)- XX/XY or Lygaeus Mode of Sex Determination  Heterogametic sex: males produce unlike gametes (XO or XY) which determines the sex of the progeny  Males may not always have heterogametic sex, some female species exhibit XX/XO or XX/XY Denoted as ZZ/ZW where the female is the heterogametic sex (ZW) Homogametic sex: females produce like gametes (XX)  Joe Hin Tijo and Albert Levan Discovered a way to prepare chromosomes for accurate viewing during the metaphase stage and saw the diploid number of 46  Human male: XY Human female: XX  Klinefelter Syndrome (47, XXY): have an extra X chromosome  Can also be (48,XXXY), (48XXYY), (49, XXXXY), (49, XXXYY)- All about severity  Generally tall with long legs and arms and large feet and hands  Have genitalia and internal ducts that are male but testes do not produce sperm and female sexual development is not entirely suppressed  Slight enlargement of breasts and rounded hips  Intersexual  Happens 1/200 male births  Turner Syndrome  (45, X): only a single X  Can also result from mosaics (somatic cells display two different genetic cell lines, each exhibiting a different karyotype) - (45, X/46X,Y) and (45,X/46,XX)  embryo that began life as a normal karyotype can give rise to an individual whose cells show a mixture of karyotypes and who exhibits various aspects of the disease  Has female genitalia and internal ducts but ovaries are rudimentary  Normally short stature, skin flaps on back on the neck and underdeveloped breasts  Happens 1/2000 female births, majority of 45,X fetuses die in utero  Nondisjunction: the failure of sex chromosomes to segregate properly during meiosis  Causes Turner and Klinefelter Y chromosome causes maleness in humans while in its absence, even if the human only has 1 X, they will be female  Poly X Syndrome (47, XXX)  Also called triplo-X Results in female differentiation  Usually perfectly normal and unaware but other cases results underdeveloped secondary sex characteristics and sterility  (47, XYY)  Results in males that are taller than normal with a subnormal intelligence  High (but not constant) correlation between an extra Y chromosome and the predisposition of males to exhibit behavioral problems  Sexual differentiation  During early development, every human embryo undergoes a period when it is potentially hermaphroditic. By the fifth week, gonadal primordial (tissue that will form gonads) arise asa pair of gonadal ridges associated with each embryonic kidney. At this stage, the gonadal phenotype is sexually indifferent and the gonadal ridges can form into male or female gonads. As development progresses, primordial germ cells migrate to these ridges, where an outer cortex and inner medulla form. Cortex is capable of developing into a ovary while medulla may develop into testis. In addition, two sets of undifferentiated cells called Wolffian and Mullerian ducts. Wolffian differentiate into male reproductive while Mullerian differentiates into female. Bipotential gonads: because gonadal ridges can form either ovaries or testes- Presence or absence of Y chromosome is the trigger to go testes or ovary Y chromosome contains less genes than X PARs (Pseudoautosomal regions): share homology with regions on the X chromosome and synapse and recombine during meiosis  SRY (sex-determining region) - Controls male sexual development - Becomes active at 6 weeks of development and encodes a protein (testis-determining factor) that causes undifferentiated gonadal tissue to form testis  Sex Ratio: actual proportion of male to female offspring Primary: The proportion of males to females conceived in population- Could possibly be a 1:1 ratio  Secondary: proportion that are actually born - Not a 1:1

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Clemson GEN 3000 - Test 2 Study Guide

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