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Sex Determnation and sex based inheritance(Genetic)


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Sex Determnation and sex based inheritance(Genetic)

  2. 2. SEX DETERMINATION SEX DETERMINATION The natural event by which an individual of a dioecious species become male or female Enviromental Sex In some species sex is determined after fertilization by environment factors Two main mechanisms Genetic sex determination Sex is determined at fertilization by the combination of genes that the zygote recieves
  3. 3. SEX DETERMINATION MECHANISMS  Monoecious -Both male and female reproductive structures in the same organism  Hermaphoditism -both sex in the same organism  Dioecious -either male or female reproductive structures in one organism
  4. 4. Environmental Sex Determination  Sex Determination : Enviroment • Sex Determination : social • Sex Determination : chance • Sex Determination : number of offspring
  5. 5. Sex Determination : Enviroment  Sex is determined by extrinsic factors after the process of fertilization -temperature -population size -sex of others
  6. 6. Sex Determination : Social  For example: Clown fish  Coral reef fish may start out as one sex but later change to the other  The trigger may be a social change , such as the dissapearance of a dominant male or female
  7. 7. Sex Determination : Chance  For example: Bonellia verdis  A marine worm , females are large and attach to rocks in the sea, males are small.  The larvae float in the ocean  When they settle down and land on land a female worm, they become male  If they land elsewhere (on the sea floor),they become female
  8. 8. Sex Determination : Number of offspring  For example : Meloidogyne incognita is a nematode plant parasite.  If nutrient are sparse, they bcome male  If plentifil, the worms become females, which enchances the reproductive potential of the population
  9. 9. Modes of sex Determination Protenor mode(XX-XO system) FEMALE ( XX) -Presence of two X chromosomes in the zygote MALE ( XO) -Presence of only one X chromosome Example:some insects:grasshoppers
  10. 10. The modes of sex determination: Lygaeus Mode(XX-XY System) FEMALE -XX(Homogametic sex) -Female gametes all have an X chromosome -Zygotes with two X chromosomes MALE -XY(Heterogametic sex) -Male gametes have either X and Y chromosome -Zygote with one X and Y chromosome Example: Drosophila, mammals and some plants
  11. 11. The modes of sex determination ZZ-ZW System FEMALE -Heterogametic sex(ZW) MALE -Homogametic sex(ZZ) -The Z chromosome is larger and has more genes -Example:birds,buttrflies,some fishes
  12. 12. The modes of sex determination X-Y-XY -Female gametophytes-Y -Male gametophytes-X -Sporophytes-XY -Example:Occurs in alternation of generation (liverworts,vascular plants)
  13. 13. THE MODES OF SEX DETERMINATION • • • • Haplodiploidy System(Arrhenotoky) Arrhen=male Tokos=childbirth -Diplod set-Female -Females arise from fertilized eggs -Haploid set-Male -Male develop from unfertilized eggs -Example:Bees,ants,wasps
  14. 14. SEX DETERMINATION IN HUMANS XX-XY • • • • • Females have two X chromosomes Males have one Y chromosome The male-determining gene is located on the Y chromosome A single Y, even in the presence of several X, still produces a male phenotype. The absence of Y results in a female phenotype
  15. 15. GENOTYPIC SEX DETERMINATION  Sex ratio always remains close to 0.5, and its not depend on the environment.  Individuals are heterogametic or homogametic.  Fertilization by X sperm yields female, Y sperm produces males.
  16. 16. Sex Determination in Humans XX-XY  Y chromosome contains less genes than X chromosome.
  17. 17. Y Chromosome  Contains:    The male-specific region of the Y (MSY) A sex-determining of the Y (SRY) SRY gene  Located near the end of the short arm of the Y chromosome .  Important in causing the undifferentiated gonad to develop into a testis.  Determine maleness.
  18. 18. Y Chromosome  Testis-determining factor (TDF)   A protein encoded by a gene in the SRY that triggers testes formation. The MSY consist of three regions:  X-transposed region.  X-degenerative region.  Ampliconic region.
  19. 19. TDF Factor
  20. 20. Pseudoautosomal Regions (PARs)  Region Y chromosome-share homology with regions on the X chromosome  Synapses and recombine with it during meiosis  Presence of such pairing region is critical to segregation of the X and Y chromosomes during male gametogenesis
  21. 21. Equalizing the expression of X chromosomes in Males and Females  Human females have one X chromosome inactivated in their somatic cells to balance the expression of X-linked genes in male and female  Females have two X-chromosomes,male have one  X chromosomes contain many genes that are unrelated to sex determination,but not present in Y chromosomes
  22. 22. Dosage compensation occurs by X chromosome inactivation  Dosage compensation  -A mechanism that regulates the expression of sex-linked gene products  -Proposed by Marry Lyon(1961)  -Random inactivation of one X chromosome in females equalizes the activity of X-linked genes in males and females  Prevents excessive expression of X-linked genes in humans and other mammals  Balance the dose of X chromosome gene expression in female and males
  23. 23. Barr Body  Barr body The condensed, single X-chromosome, appearing as a densely staining mass, that is found in the nuclei of somatic cells of female mammals.  This occurs in almost all cells of females at blastocyst stage  An inactivated X chromosomes,tightly coiled
  24. 24. X-inactivation  X-inactivation is a process by which one of the two copies of the X chromosome present in female mammals is inactivated.  The inactive X chromosome is silenced by it being packaged in such a way that it has a transcriptionally inactive structure called heterochromatin  As female mammals have two X chromosomes, Xinactivation prevents them from having twice as many X chromosome genes products as males, which only possess a single copy of the X chromosome
  25. 25. FemALE MAMMALS ARE ACTUALY MOSAIC FOR X CHROMOSOME EXPRESSION Inactivated chromosome can come from either mother/father  Inactivation occurs early in development  nactivation-permanent 
  26. 26. Effects of random xchromosome inactivation  Can causes twins with identical genotypes to have different phenotypes  Calico cats
  27. 27. Sex-Related Phenotypic Effects  In sex-influenced and sex-limited inheritance, the sex of the individual affects:    Whether the trait is expressed The degree to which the trait is expressed Autosomal and sex-linked genes  Sex hormone levels modify the expression of genes  Altered phenotypic ratios
  28. 28. Sex-linked gene  Colour blindness, an X-linked vision disorder
  29. 29. Sex-linked gene  Hemophilia, an X-linked blood-clotting disorder in Russian imperial family
  30. 30. Sex-Limited Traits Genes that produce a phenotype in only one sex E.g: genes influence how much milk produce by lactating mother Traits expressed only in females because males die before birth Examples: Male-lethal X-linked dominant traits Traits expressed only in males Example: Duchenne muscular dystrophy (X-linked recessive); males do not have offspring and don’t pass their X onto daughters Very rare in females
  31. 31. Sex-Influenced Trait Sex-influenced traits  Traits controlled by autosomal genes that are usually dominant in one sex but recessive in the other sex. E.g: Pattern Baldness  Acts like an autosomal dominant trait in males and an autosomal recessive trait in females