Exam 3

• Sexual reproduction allows offspring that vary from parents
• Fertilization: mechanism by which new organisms are produced via sexual reproduction
• In animals, including humans, sexual differentiation is evident as phenotypic dimorphism of males and females
• Phenotypic sexual dimorphism example: presence of an Adam’s apple in male humans
• Sex chromosomes: characterize one sex or the other in a wide range of species
• Heteromorphic chromosomes: dissimilar chromosomes (example is sex chromosomes X and Y)
• Sex determination is done by specific genes instead of an entire chromosome
• Homogametic sex:

  • producing like/same chromosomes
  • zygotes with two X chromosomes
  • results in female offspring
• Heterogametic sex:

  • producing unlike/different chromosomes
  • zygotes with one X and one Y chromosome
  • results in male offspring
• XX/XO mode of sex determination:

  • Protenor (butterfly)
  • depends on random distribution of X chromosome into half of male gametes
  • presence of two X chromosomes in zygote results in female offspring
  • presence of one X chromosome results in male offspring
• A hermaphrodite is also considered a homogametic sex. They also have XX. this can be seen in C. Elegans. (worm) these hermaphrodites are capable of making eggs and sperm at the same time and are thus capable of self mating.
• ZZ/ZW sex determination:

  • Xenopus laevis (frog)
  • females are actually the heterogametic sex (ZW)
  • males are the homogametic sex (ZZ)
• Danio rerio (fish) have no determinate sex chromosomes despite possessing male and female counterparts of the species
• Y chromosome determines maleness
• Human karyotype:

  • 22 pairs of autosomal chromosomes
  • 2 sex chromosomes
  • reveals on pair of chromosomes differs in males and females
• (Human) Female: XX
• (Human) Male: XY
• Klinefelter and Turner syndrome:

  • two human abnormalities
  • characterized by aberrant sexual development
  • both syndromes result from nondisjunction which is the failure of the X chromosome to segregate during meiosis
• Normal meiotic division will result in 4 gametes with a single “n”
• Nondisjunction in meiosis 1 will result in four gametes that are n+1, n+1, n-1, and n-1
• Nondisjunction in meiosis will result in four gametes that are n+1, n-1, n, and n
• Klinefelter syndrome (47, XXY):

  • Tall, long arms and legs
  • large hands and feet
  • internal ducts are male, rudimentary testes fail to produce sperm
  • feminine development is not suppressed so enlarged breasts are common as are rounded hips
  • occurs in about 1 of every 660 male births
• Turner syndrome (45, X):

  • Phenotypically female
  • female external genitalia and internal ducts
  • ovaries are rudimentary
  • underdeveloped breasts
  • short stature
  • occurs in about 1 in 2,000 female births
• 47, XXX syndrome: Triplo-X

  • three x Chromosomes
  • normal set of autosomes
  • results in female differentiation
  • sometimes women are perfectly normal
  • sometimes underdeveloped secondary sec characteristics occur and sterility and mental retardation presents
• 47, XYY condition:

  • the only consistently shared characteristic is that males are over 6 feet tall.
  • subnormal intelligence
  • personality disorders
• Sexual differentiation in humans:

  1. Early development
  2. Fifth Week of development
  3. further into development
  4. similar time point as ovary and testis development
  5. completion of development
• (Sex determination) Early development: embryo is potentially hermaphroditic
• (Sex determination) Fifth week of development: gonadal (genital) ridges form but the individual is still potentially hermaphroditic
• (Sex determination) further into development: germ cells move to the gonadal ridges and begin developing ovaries or testis. if cells have a y, testis form. If cells have no y, ovaries form.
• (Sex determintaion) Similar time point as ovary and testis development: reproductive tracts develop from undifferentiated ducts. if cells have a y, male tract forms. If cells have no y, female tract develops.
• (Sex determintaion) Completion of development: the development of male or female organs continues to drive sexual differentiation throughout the rest of the developmental process
• SRY gene produces the testis determine SRY protein which then initiates the production of multiple proteins that cause gonad medulla to differentiate into a testis
• Testis have leydig cells which secrete testosterone which controls the development of wolffian duct into accessory structures and development of male external genitalia
• Testis have Sertoli cells which secrete anti-müllerian hormone which causes the reduction of the müllerian duct
• Müllerian duct is the fallopian tubes and uterine tubes
• Y chromosom:

  • has at least 75 genes
  • fewer genes than the X chromosome
  • some of these genes are homologous to genes on the x chromosome and some are not
• PARs: Pseudoautosomal regions

  • present on both ends of Y Chromosome
  • share Homology with regions on X chromosome
• MSY: Male specific region of the Y chromosome

  • non-recombining region of Y chromosome
  • contains euchromatin (SRY: Sex-determining region of Y chromosome, located adjacent to PAR of the short arm of Y chromosome. Controls male development. Encodes protein: testis-determining factor (TDF)
  • heterochromatin
• Dosage compensations:

  • genetic mechanism
  • balances dose of X chromosome gene expression in males and females
  • prevents excessive expression of X-linked genes in humans and other mammals
• Barr bodies (sex chromatin bodies):

  • genetic mechanism compensates for X dosage disparities
  • inactive X chromosome, highly condensed
  • darkly stained body in interphase nerve cells observed: Barr bodies
  • random inactivation
  • occurs early in embryonic development