Bio 240 Chapter 7 notes!
Bio 240 Chapter 7 notes! Bio 240
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This 6 page Class Notes was uploaded by Izabella Nill Gomez on Saturday October 10, 2015. The Class Notes belongs to Bio 240 at University of Tennessee - Knoxville taught by Dr. Hughes in Summer 2015. Since its upload, it has received 31 views. For similar materials see General Genetics (Bio 240) in Biology at University of Tennessee - Knoxville.
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Date Created: 10/10/15
Biology 240 Chapter 7 notes Meiosis produces haploid gametes after fertilization a diploid zygote Meiosis ensures genetic constancy within members of the same species These events depend ultimately on an ef cient union of gametes during fertilization In turn successful fertilization depends on some form of sexual differentiation Heteromorphic chromosomes dissimilar chromosomes such as the XY pair in mammals characterize one sex or the other in a wide range of species resulting in sex chromosomes Genes rather than chromosomes ultimately serve as the base for sex determination Some of these genes are present on sex chromosomes but others are autosomal Primary sexual differentiation involves only gonads where gametes are produced Secondary sexual differentiation involves the overall appearance of the organism including clear differences in mammary glands external genitalia and other non reproductive organs Differentiation of sexes is evident via phenotypic dimorphism Homogametes produce like chromosomes ex zygotes with two X sresults in female Heterogametes produce two different chromosomes ex zygotes with X and Y results in male Humans are a heterogametic system Unisexual diecious gonochoic refer to an individual containing only male or only female reproductive organs Bisexual monoecious hermaphrodite individuals with both male and female reproductive organs common in plants lntersex refer to those with intermediate conditions mostly sterile Chlamydomonas green algae with infrequent periods of sexual reproduction Stays mostly in the haploid phase asexually reproducing daughter cells by mitosis Under unfavorable nutrient conditions certain daughter cells function as gametes joining in fertilization and a diploid zygote is formed to withstand unfavorable conditions There is little difference between haploids and gametes lsogametes are two gametes that fuse and are not distinguishable from one anotherthe species producing them are isogamous Gametes in a Chlamydomenac has two mating typespus and minusthese form zoospores They differ chemically to tell their sexuality Zea mays these plants undergo both haploid gametophyte stage and diploid sporophyte stage Meiosis and fertilization link the processes together Maize is a monoecious plant sporophyte phase dominate the life cycle Both male and female structures are present Sexual differentiation occurs in the tissues of the plant The stamen produces the mother cells which produce the male microspores pollen grain The pistil produces female cells Pollination occurs in the pistil and double fertilization takes placeproducing kernels C elegans nematode is a popular organism in genetic studies particularly for investigating genetic control of development Males only have testes hermaphrodites during larval development form two gonads which produce sperm and eggsthey selffertilize Less than 1 of offspring become just male Heterogametic sex malesXY Homogametic sex femalesXX The presence or absence of the X chromosome in male gametes provides an efficient mechanism for sex determination The male is not always the heterogametic sex this depends on the species Ex the female chicken is ZW and males are 22 The Y chromosome determines maleness in humans Klinefelter syndrome 47 XXY individuals with this abnormality are generally tall with long arms and legs long handsfeet Usually have internal male genitaliaducts but cannot produce sperm The feminine sexual development of the individual is not entirely suppressed rounded hips slight breasts are produced The IQ level is lower 1660 male births have this syndrome Turner syndrome 45 X the individual has female externalinternal genitaliaducts but the ovaries are rudimentary Marked by short stature skin folds on the neck lower IQ and a childish development This occurs in 12000 female births Both conditions result from nondisjunction or the failure of X chromosome to segregate during meiosis Even with two X39s one Y determines maleness A lack of Y de nes female development lndividuas with more XXY39s leads to more severe conditions of Klinefelter39s Turner39s syndrome can also result from karyotypes other than 45 X including individuals called mosaics whose somatic cells display two different genetic cell lines each with a different karyotype due to mitotic error in early development Ex 45 X 46XY 45X 46 XX A majority of 45 X die in utero and are aborted spontaneously 47 XXX syndrome results in female differentiation triploX occurs in 11000 female births Some have perfectly normal development In 48 XXXX this results in underdeveloped sex characteristics sterility and lower IQ The same occurs in 49 XXXXX only the conditions are more pronounced Additional X s disrupt the delicate balance of genetic information essential to normal female development 47 XYY results in above average height used to be associated with huge antisocial criminal acts Subnormal intelligence and personality disorders are other possible characteristics No real case study has ever been performed on this condition due to ethical reasons Mode of sex determination ProtenorXXXO mode of sex determination depends on the random distribution of X chromosome into half of male gametes The presence of two X39s results in female offspring and X0 determines male common in butter ies Lygaeus milkweed bug XXXY mode of sex determination female gametes have one X chromosome male gametes have either an X or Y chromosome humans follow this mode of determination During early development every human embryo undergoes a hermaphroditic stage By the 5th week of gestation period gonadal primordia arise as a pair of gonadal genital ridges associated with the embryonic kidney As development progresses germ cells migrate to the right where the outer cortex and inner medulla form The cortex can develop into an ovary and the medulla into testes bi potential gonads Presence or absence of the Y chromosome is the key to gonad development By the 7th week XY39s develop into testes The presence of the Y also inhibits the formation of female reproductive organs The Y chromosome has about 50 genes compared to 1000 on the X Both are believed to have originated from two homologous autosomal chromosomes 200 million years ago At both of the Y are pseudoautosomal regions PARS that share homology with the X chromosome and synapse and recombine during meiosis Pairing is critical for segregation during male gametogenesis X and Y The rest of the chromosome 95 does not synapseit is a nonrecombining region of the Y NRYor malespeci c region of the Y MSY Some proteins share homology with genes on the X chromosome and are divided into euchromatic with functional genes and heterochromatic lacking genes regions The sex determining region Y SRY within euchromatin are adjacent to PAK of the short arm Absence of the Y almost always leads to female development lack of SRY At 68 weeks of development the SRY gene is active in XY it encodes a protein that triggers testes developmentTestisdetermining factor TDF SRY is present in all mammals TDF is a transcription factor a DNA binding protein that interacts directly with regulatory sequences of other genes to stimulate expression TDF acts as a maser switch that controls other genes Xtransposed region comprises about 15 of MSY and is originally derived from the X chromosome in the course of human evolution 99 of it is identical to Xq21 of the modern chromosometow genes both with X chromosome homologs are present in this region Paindrones are sequences of base pairs that read the same but in the opposite direction on the complimentary strands and are present through MSY Recombination between palindrones occurs on sister chromatid is the Y during replication for the purpose of mutation repair The second area of the MSY is the Xdegenerative region which comprises 20 of the MSY and contains DNA sequences that are more distantly related to those present on the X chromosome It contains 27 single copy genes and a number of pseudogenes nonfunctional All share some homology on the X chromosome One is the SRY gene The third area is the ampliconic region which comprises 30 of the MSY and most genes are related to development of the testes They lack counterparts on the X chromosome 60 transcription units are divided among 9 families Almost all are identical Each repeat unit is an amplicon They encode proteins speci c to the development and function of the testes lncreased paternal age is now associated with an increased risk in offspring of congenital disorders with a genetic basiscancer schizophrenia autism etc paternal age effects PAE Sex ratio actual ratio of male to females there are more males than femalesthe reason is unknown Primary sex ratio re ects the proportion of males to females conceived in a population Secondary sex ratio the proportion of each sex that is born This is much easier to determine but has the disadvantage of not accounting for any disproportionate embryonic or fetal mortality Theoretical assumptions of the sex ratio 1 Because of segregation males produce an equal number of X and Y bearing sperm 2 Each type of sperm has equivalent viability and motility in the female reproductive tract 3 The egg surface is equally receptive to both X and Y bearing sperm The disparity between the X and Y chromosomes creates a quotgenetic dosagequot difference between males and females for all Xlinked genes Dosage compensation balances the dose of the X chromosome gene expression in males and females Barr bodysex chromatin body A highly condensed structure that lies against the nuclear envelope of interphase cells It is an inactivated X chromosome It suggests the female dosage of X is equivalent to males Evidence lies in the Klinefelter males XXY that have one none in Turner X two in XXX females 3 in XXXX females and so on Follows the N1 rule where N is the number of X chromosomes present However the presence of Barr bodies does not stop the effects of each syndrome The explanation could be that chromosome inactivation does not occur in early stages of development of cells destined to be gonadal tissues The other explanation is that not all of each X chromosome forming a Barr body is inactivated 15 of human X genes actually escape X inactivation Lyon hypothesis the inactivation of the X chromosomes occurs randomly in somatic cells at a point early in embryonic development most likely sometime during the blastocyst stage of development Once inactivation has occurs all descendant cells have the same X chromosome inactivated as their initial progenitor cell Based on observations of female mice heterozygous for Xlinked coat color genes Pigmentation is mottled Other observations lie in female Calico catsmosaic pattern Redgreen colorblindness Xlinked recessive disorder In humans hemizygous males are fully colorblind in all retinal cells However heterozygous females have mosaic retinas with patches of defective color vision and surrounding areas with normal perception Random inactivation of the X chromosomes led to this phenotype Imprinting process in which one homolog but not the other it affected DNA histones or both silence the chromosome and its genes Xic X inactivation center region of the mammalian X chromosome at the proximal end of the p arm in humans lts genetic composition occurs only on the X chromosome that is inactivated Contains several regulatory units and 4 genes One is the Xinactive speci c transcript XIST and is a critical gene for Xinactivation The RNA product is large and does not encode a protein and not translated RNA products of XIST spread and coat the chromosome that produced them Two other noncoding genes at the Xic locus Tsix antisense partner of XIST and Xite also play a role Transcription of XIST begins at low levels on all X chromosomes and as inactivation begins it is enhanced on the X that is inactivated In cells with more than two X chromosomes the paternal and maternal X pair brie y and align at Xic loci to count the number of X chromosomes prior to inactivation In Drosophila the Y chromosome does not determine maleness X chromosomes and autosomes together play a role XXY ies are normal females and X0 ies are sterile males Y does not determine maleness but plays a part in male fertility A critical factor in determining sex is the ratio of X chromosomes to the number of haploid sets of autosomes A present Normal 2x2A and triploid 3X3A females have a ratio of 1 and are both fertile As the ratio exceeds unity a metafemale is formed Normal XY2A and sterile XO2A males have a 5 ratio When the ratio decreases to 33 infertile metamales are produced Between 5 and 10 are intersexes The genic balance theory results and a ratio of 12 is maleness X2A but XX2A results in females Dosage compensation complex DCC cluster of gene activating proteins that enhances genetic expression along sites of the X chromosome related to maleness in Drosophila The Master switch le gene plays are role in dosage compensation ln XY ies le is inactive and autosomal genes are active causing enhanced X chromosome activity In females le is active and functions to inactivate one or more male speci c autosomal genes like ml for maleness Ensures no double expression of the Xlinked genes in females le acts as a sensor for several other X linked genes by counting X chromosomes Genotypic sex determination GSD where genetically sex is determined Temperature dependent sex determination TSD sex is determined by temperature In many species of reptiles GSD is involved at conception based on sexchromosome composition as is the case in most organisms However in other reptiles TSD is the norm where sex determination is achieved according to incubation temperature of eggs during a critical point in embryonic development In some cases high temperatures yield males in others both high and low temperatures yield females and intermediates yield different proportions of males Possible exam questions 1 A male with two X chromosomes is often the result of SRY relocating to an X 2 A Drosophila with XXYAAA is what sex intersex 3 An individual with Turner syndrome has no Barr bodies True