Presentation on theme: "SEX DETERMINATION from these studies concluded that genes are on"— Presentation transcript:
1SEX DETERMINATION from these studies concluded that genes are on chromosomes and specialized chromosomesdetermine sex
2Modes of Sex Determination XX/XY (ZZ/ZW) (Lygaeus)- heteromorphic sex chromosomesXX/XO (Protenor)
3heteromorphic chromosomes - a chromosome pair with some homology, butdiffering in size, shape or staining propertiesheterogametic sex (e.g. XY OR ZW)- the sex with different sex chromosomes; thegametes have different sex chromosomes…..the opposite of homogametic sex
5XY Method of Sex Determination XXXYXY method common in higher organismsY-chromosome evolved to give X a pairing partnerA = autosome, any non sex-chromosome
6Sex Determination in Humans Primary non-disjunctionXXX XXY XO YO
7Aneuploid Offspring XXX -- Trisomy-X XXY -- Klinefelter XO -- Turner YO -- Diesaneuploidy involving the sex chromosomesis much better tolerated than autosomalaneuploidy
8XXX Syndrome 47, XXX female (also see XXXX and XXXXX) 1/1200 female live birthsVariable phenotype, frequently appear normal, sterile,may show mental retardation. Under-developedsecondary sex characteristics
9Klinefelter Syndrome 47, XXY male (XXXY, XXXXY, XXYY) 1/500 male live birthsMale genitalia, testes undeveloped,fail to produce spermFemale secondary-sex characteristics
10Turner Syndrome 45, XO female 1/2000 female live births Ovaries rudimentary, sterileShort stature, shield-like chest,webbed neckNormal intelligenceMany XO fetuses miscarry
11XYY Condition 47, XYY male 1/1000 male live births Above-average height, 2% in maximum securityprisons are XYY, personality disorders, subnormalintelligenceFertileMost XYY males socially normal
12Human Sex-Determination Mechanism XY XOXXY XXXYY XXXXXYY XXXXXXXY XXXXXXXXXYCONCLUSION:Presence of Y is male-determiningLack of Y is female-determiningY produces testis-determining factor (TDF)
13crossing over between X and Y during meiosis occurs in pseudoautosomalregion (PAR)……necessary for normal disjunctionPAR homologous to region ofX-chromosomenon-recombining region of Y (NRY)unique to Y chromosomehuman Y chromosome contains geneSRY (sex-determining region Y)which makes TDFXY females deleted for SRY regionare observed and XX males with TDFgene translocated to X chromosome
14hemizygous genea gene present in only one copy in adiploid organisme.g. any gene on the X or Y (NRY) chromosomein human males
15Primary and Secondary Sex-Ratio Maturity 1 : 1Old Age 1 : <1Explanationfor primarysex-ratio:Y sperm more able to fertilize egg, or,better survival in reproductive tractY lacks most genes on X, any subvital/lethal genes uncovered in hemizygousmales, higher lethality
16Autosomal genes are inherited in equal frequencies by both sexes Aa x Aa1AA : 2Aa : 1aa(if allele a is recessive equalnumbers of females and malesshould be affected)
17Y-Chromosome Linkage in Man Holandric Inheritance Hairy PinnaY always inherited father to sonGene found only on Y-chromosome
18Inheritance of Sex (X)-Linked Traits Color Blindness ==colorblindccCYColor-blindness is asex-linkedrecessive traitCCcYCccYCcCYCYCcCcCc or CCCYCc or CCCc or CCcYtrait more common in males because of hemizygosity
19Dosage Compensation in Mammals Observation -- Trisomys-XXX, nearly normalXXXY looks fairly normal21/21/21 Down’s quite abnormal13/13/13 Severely affectedWhy can individuals tolerate additional X-chromosomesbut not additional autosomes?How is X-chromosome gene expression controlled?
20Dosage Compensation Background X-chromosome inactivation in mammals1. Barr (1949) -- Observed chromatin bodies in the nervecells of female cats. Somatic cells only.2. Ohno -- Proposed Barr body is X-chromosome3. The # of chromatin bodies is one less than the # ofX-chromosomes. Restricted to somatic cells.InterphaseCells ofCells ofXXX =XXXX =XY =XXY =
21Dosage Compensation Explained Lyon Hypothesis (1962) (English Mouse Geneticist)1. The chromatin body is an X-chromosomeOne X-chromosome is inactivated - visible as a differentiallystained Barr body during interphase. Genes inactivated.3. The inactive X can be either maternal or paternal in origin.Inactive X-chromosome is late-replicating. It lags on themetaphase plate during mitosis.5. X-inactivation in humans begins about 16 days intodevelopment.6. Once an X-chromosome becomes inactive it will always beinactive in all cells derived from that cell.
22mosaic / chimera- an individual composed of two or moregenetically different cell types
24Dosage Compensation Example Calico Cat Black BB BYCalico Bb ----Yellow bb bYException: XXY Klinefeltermale calico cat
25Bridges Sex-Balance Theory Drosophila melanogaster The white eye gene in Drosophila was the first gene to belocated on a chromosome.a) Crossed XW XW (x) X+ YXWX+YXW X+XW YWild Type White(red eye)Exceptional offspring occurred at low frequency1. White2. Wild type (sterile)}Primary Exceptions
26Hypothesis XW XW (x) X+ Y [Primary non-disjunction] XW XW X+ Y XW XWX+ X+ OXW XWYOYFertileSterileDiesMetafemaledies
27Chromosomes of Drosophila centromereXIIIIIIVXIIIIIIVX & Y = Sex-chromosomesAutosomes= XXAA II + III + IV= XYAA = 1 Set Autosomes (A)Autosome = any chromosome that is not a sex-chromosome
28Non-disjunction shown for X-chromosome and complete sets of autosomes. Mutation of a third chromosome gene, C3G, leads to a high degree of non-disjunction.Non-disjunction shown for X-chromosome and complete sets of autosomes.XXXAAA (X) XYAAChromosome Segregation during MeiosisX XXX XXXAXXAAXAAXXAA AAAA A
29XA XXAA XAA XXA (Triploid ) XXXAAA (Intersex) XXAAA (Meta ) XXXAA XA XXYAAA(Meta )XYAAAXXYAAYAXYAACONCLUSIONS:Female and male determining factors onX-chromosome and autosomesY-chromosome carries male fertility genesX:A ratio determines sex
31External Environment and Sex Determination Bonellia Worm -- Marine Worm1. and have same chromosomes.2. If young worm is raised from an isolated larva it becomes afemale.3. Drop larva in water with females. If larva comes in contact withfemale then larva turns into male and eventually migratesto female reproductive tract.4. Possibly a hormone is secreted from the proboscis of female toinitiate male sexual differentiation.
33Sex Determination in Alligators and Crocodiles all femalesat 31oC;all malesat 33oC;at 35oCtemperature affects levels of male and femalesex hormones
34Solving Pedigrees5 basic types - autosomal recessive- autosomal dominant- sex-linked recessive- sex-linked dominant- Hollandric inheritance1. Recessive characters often skip generations2. Dominant characters are usually observed in every generation.3. Autosomal characters are inherited equally by females and males.4. Recessive sex-linked characters are inherited at a much higherfrequency by males.5. Y-linked characters are only inherited by males.
35Hints:- when determining inheritance of a rare mutation - if anindividual marries into the family assume that the individualis homozygous for the wild-type allele (non-disease allele)unless there is evidence suggesting otherwise- dominant mutations are usually lethal when homozygous;assume that individuals carrying dominant mutations areheterozygotes- when determining the mode of inheritance in a pedigreebe sure to rule out other possibilities
36Duchenne Muscular Dystrophy : X - linked dominant inheritance
37Rare autosomal recessive kidney disease What is the probability that individuals 1 and 2 will have an affected child?