Lecture – 7Sex determination, Sexlinked, Sex influencedand Sex limited traitsbyDr. Praveen KumarAsst. Prof. GPBMSSSoA, CUTM, Odisha
Sex Determination in PlantThree mechanisms Environmental Chromosomal and Genic Mechanism
Environmental Sex DeterminationEquisetum plantsUnder optimum condition- Under adverse condition- Sex of flower affected by temperature, day-length,ethylene, gibberellic acid (GA3), some ions (Ca , Mg etc.). E.g. Melons, Cucumber, Cannabis etc. Usually GA3 promotes female flower Generally environment has more pronounced effect onmales than in females.
Chromosomal Sex determination In several plant species Sex is determined on the basis ofsex chromosome morphology Homomorphic HeteromorphicSpeciesFeatures of YchromosomesMechanism of sexdetermination1. Homomorphic sex chromosomeA. Homogametic (XX) - Female, Heterogametic (XY)- MaleAsparagus officinalisSimilar to X chromosome Y- active (?)Spinacca oleraceaSimilar to X chromosome Y- active (?)B. Heterogametic (XY)- Female, Homogametic (XX) - MaleFragaria elateria(Wild strawberry)------May be genic
SpeciesFeatures of YchromosomesMechanism of sexdetermination1. Heteromorphic sex chromosomeA. Homogametic (XX) - Female, Heterogametic (XY)- MaleCannabis sativaLargeX/ autosome balanceCoccinia indicaLarge, HeterochromaticY- activeHumulus lupulusSmall, euchromaticX/ autosome balanceRumex hastatulusLarge, euchromaticX/ autosome balanceSalina latifoliaLarge, euchromaticY- activeSalina dioicaLarge, euchromaticY- activeB. XX- Female, XY1Y2 - MaleHumulus japonicusLarge, HeterochromaticX/ autosome balanceRumex hastatulusLarge, euchromaticActive Y and X/autosome balanceC. X1X1X2X2- Female, X1X2Y1Y2 - MaleHumulus lupulusModerate, euchromaticX/ autosome balance
(Female Supressor Region)(Male promoter Region)(Differential Region)Of X Chromosome(Male Fertility Region)(Pairing Region)
Deletion in segment of YchromosomeOutcomeI. Female Suppressor RegionBisexual FlowerII. Male promoter RegionAsexual FlowerIII. Male Fertility RegionMale Sterile FlowerIV. Pairing RegionIrregular separation of X and Ychromosome at AI of meiosis In S. latifolia single Y is able to produce fertile male flowereven in the presence of 4 X chromosomes (XXXXY) In mammals, where single gene SRY regulates sexdetermination, at least two genes are necessary for sexdetermination in plants: one gene suppress carpeldevelopment, while other gene is essential for stamendevelopment.
Genic Sex Determination Found in both monoecious and dioecious plants Sex determination may depends on a single orseveral genes. Eg. Papaya, Vitis, cineria etc.1. Single Gene Sex Determination- eg. In papayasingle gene with three alleles.1. Multigenic Sex Determination- eg. Annualmercury plant (Mercurialis annua), sex isdetermined by three different genes.
Inheritance For a number of traits, gene expression differs inmales and females The causes fall under 3 categories: Sex-Linked Sex-Limited Sex-Influenced
SEX LINKED INHERITANCE The characters for which genes are located on sex or‘X’ chromosomes which occurs in different numbersin two sexes and the absence of its allele in the ‘Y’chromosome are known as sex linked traits. Such genes are called sex linked genes and linkageof such genes is referred to as sex linkage. Inheritance of such genes or characters is known assex linked inheritance. Clear-cut explanation for this phenomenon waspresented by T.H. Morgan in Drosophila for arecessive gene ‘w’ responsible for white eye colour.
X and Y are homologous chromosomes X and Y are an exceptionto the homologydefinition. Human X-chromosome islarger than Y and hasabout 2000 genescompared to about 450. X and Y are homologousbecause they pair upduring meiosis I. Pairing is due to a smallarea of homology aroundthe centromere. TheY-chromosome lacks manygenes found on its homologousX-chromosome. Thisleads to a pattern ofinheritance called sex linkage. InXX females, a recessiveallele on one X can be maskedby a dominant allele on theother X. InXY males, a recessive alleleon the X has no second copyto mask its effects.
Homogametic and heterogametic Becausethey are homologous, the sex chromosomes areseparated during meiosis into different gametes.44 XX22 X44 XY22 X22 X22 Y Human females produce all gametes with the samecombination of chromosomes homogametic. Human males produce gametes with two possiblecombinations of chromosomes heterogametic.
Sex Chromosomes Y chromosome:– Contains 90 genes– Majority of genes Male Specific Region(MSR)– SRY gene – determines “maleness” X chromosome:– Contains 2000 genes– Some dealing with sexual development– Most genes encoding proteins that havenothing to do with sex
SRY Gene SRY Sex-determining Region of Y A transcription factor (TF) TF’s are genes that control the expression ofother genes (turn on/off) SRY turns on “male” genes “Male” genes activate male hormones(testosterone) end up producing male structures Also, destroy female structures
Sex linkage- White-eyed males In the early 1900s, T. H. Morgan studiedinheritance in Drosophila melanogaster totry to disprove Mendel’s theory of the 3:1ratioΧ It took two years to find any variation inhis vast fly breeding programme. Eventually a male fly was found whichhad white eyes. (His wife found it.)XRXRXrYXR This white-eyed male was crossed with anormal red-eyed female.XrXRXr We will use the notation XR to show thered eye allele is on the X-chromosomeand is dominant to white eye, Xr.YXRY
Conclusion White eye gene was located on ‘X’ chromosome ofDrosophila Y-chromosome does not carry an allele for this gene. Female flies will have two copies of w gene (WW, Ww orww), while male will have only one copy (W or w) i.ehemizygous for w This was first conclusive evidence demonstrated that aspecific gene located in a specific chromosome of anorganism
Sex linkage in human and other organism Man, Mice, Cat, insect, Poultry, Cattle, Guinnea pig etc. In human over 200 genes exhibited sex linkage and mostof these causes genetic diseasesSex linked trait inhumenEffect1. HemophiliaInability of blood to clot2. Colour blindnessInability to perceive one or the othercolour3. Optic atrophyDegeneration of optic nerve4. Juvenile glaucomaHardening of eye ball5. MyopiaNearsightedness6. Mitral stenosisAbnormality in mitral valve in theheart
Hemophilia A blood disorder where the blood does not clotproperly. A minor cut can cause serious injury and demandmedical attention. Bleeding into the joints, internal bleeding and deepcuts can be fatal for hemophiliacs. Genetic lack of one of the clotting factorsproduced by the liver. There is no cure for hemophilia but treatmentoptions with clotting factor transfusions areavailable.
Complications from hemophilia include: bruising andbleeding into the muscles, bleeding into the joints,infection, adverse reaction to transfusions and seriousbleeding.
Genetics of Hemophilia The gene forhemophilia is found onthe X chromosome It is a recessivedisorder. It is referred to as asex-linked recessivedisorder. Males are more likely toget hemophilia. Females have thepossibility of beingheterozygous forhemophilia. (This makesthem a carrier)
In this example: Thefather has hemophilia.He cannot give his sonhemophilia because hegives his son the Ychromosome.He can give his daughterthe recessive gene, but ifher mother does not giveher the recessive gene,she will not havehemophilia. She will bea carrier.
Color Blindness Males are morelikely to be colorblind due to thefact they onlyhave one Xchromosome. Color Blindnessis a sex-linkedtrait found on theX chromosome.
In this example:the mother is acarrier of thecolorblind gene.There is a 50%chance her son willbe colorblind butunless the father iscolorblind thedaughter cannotend up colorblind.
Sex-Linked Inheritance- PunnettSquare In the punnett squarethe mother is a carrierand the father isnormal. Male offspring:50% normal &50% hemophiliac Female offspring:50% normal50% carrier
Complete the followingpunnett square: Cross a normal mother with a hemophiliacfather.Results:Genotypes: 50%50%Phenotypes: 50%50%
Sex- Influence Inheritance Genes are inherited from both parents Either autosomal or X chromosome Modes of gene expression differ between males & females An allele may be expressed as a dominant in one sexand a recessive in the other Male pattern baldness Scurs (degrowing of horn after disbudding operation) oncattle is a sex-influenced inheritance The allele for scurs is dominant in males and recessive infemales A male with one copy will be scurred, but a female musthave 2 copies
Genotype MalesPP SS PolledPP Ss PolledPP ss PolledPp SS ScursPp Ss ScursPp ss Polledpp SS Hornedpp Ss rnedHorned
Sex-Limited inheritance Genesare inherited from both parents Either autosomal or X chromosome Yet, affect a structure that is only present in onesex, therefore phenotype shows a sex “difference” Horns Milkproduction Genitalia anatomy/function Thesegenes are not necessarily on the sexchromosomes but are only expressed in the maleor female Thought to be hormonally conditioned
SEX LINKED INHERITANCE The characters for which genes are located on sex or ‘X’ chromosomes which occurs in different numbers in two sexes and the absence of its allele in the ‘Y’ chromosome are known as sex linked traits. Such genes are called sex linked genes and linkage of such genes is referred to as sex linkage.
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Chapter 15. Sex Determination and Differentiation Barry Sinervo 1997-2004 Index Sex Determination Chromosomal Sex Determination . the organism develops into a female with ovaries and a system of endocrine glands that regulate female reproduction and behaviors. If the gene is present, the organism develops testes rather than ovaries, and the
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