Genetics Test 2 Study Guide CH 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 traced to specific origins Popular in genetic studies because adults consist of 1000 cells and the linage can be easily 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 males and 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 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 Y chromosome causes maleness in humans while in its absence even if the human only has 1 X Causes Turner and Klinefelter 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 as a 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 Could possibly be a 1 1 ratio Primary The proportion of males to females conceived in population Secondary proportion that are actually born Not a 1 1 ratio because of the alterations between conception and birth more males conceived and born Dosage Compensation because females have two larger X chromosomes there would theoretically be a genetic dosage difference in males and females but the dosage compensation balances the dose of X chromosome gene expression in males and females Barr Bodies Sex chromatin structure in female cells Inactivated X