Prader-willi syndrome



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PRADER-WILLI Syndrome

  • PRADER-WILLI Syndrome





In Prader-Willi missing piece of #15 was from father

  • In Prader-Willi missing piece of #15 was from father

  • In Angelman’s, missing piece of #15 was from the mother





Concept 15.1: Mendelian inheritance has its physical basis in the behavior of chromosomes

  • Concept 15.1: Mendelian inheritance has its physical basis in the behavior of chromosomes

  • Several researchers proposed in the early 1900s that genes are located on chromosomes

  • The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment























The sex chromosomes

  • The sex chromosomes

    • Have genes for many characters unrelated to sex
  • A gene located on either sex chromosome

    • Is called a sex-linked gene






The X chromosome in males . . .

  • The X chromosome in males . . .

  • flies WITHOUT a copilot!



Breakage of a chromosome can lead to four types of changes in chromosome structure

  • Breakage of a chromosome can lead to four types of changes in chromosome structure

    • Deletion
    • Duplication
    • Inversion
    • Translocation


A deletion occurs when a chromosome fragment lacking a centromere is lost during cell division.

  • A deletion occurs when a chromosome fragment lacking a centromere is lost during cell division.

    • This chromosome will be missing certain genes.
  • A duplication occurs when a fragment becomes attached as an extra segment to a sister chromatid.



An inversion occurs when a chromosomal fragment reattaches to the original chromosome but in the reverse orientation.

  • An inversion occurs when a chromosomal fragment reattaches to the original chromosome but in the reverse orientation.

  • In translocation, a chromosomal fragment joins a nonhomologous chromosome.





Some individuals with Down syndrome have the normal number of chromosomes but have all or part of a third chromosome 21 attached to another chromosome by translocation.

  • Some individuals with Down syndrome have the normal number of chromosomes but have all or part of a third chromosome 21 attached to another chromosome by translocation.





The frequency of aneuploid zygotes may be quite high in humans, most of these alterations are so disastrous that the embryos are spontaneously aborted long before birth.

  • The frequency of aneuploid zygotes may be quite high in humans, most of these alterations are so disastrous that the embryos are spontaneously aborted long before birth.

  • Certain aneuploid conditions upset the balance less, leading to survival to birth and beyond.

  • These individuals have a set of symptoms - a syndrome - characteristic of the type of aneuploidy.























TRIPLE X syndrome (TRISOMY X)

  • TRIPLE X syndrome (TRISOMY X)

  • 1 in 1000 females

  • Tall stature (height)

  • Normal IQ, but may be 10-15 points below siblings

  • Learning disabilities (70%):

  • Speech and language delays (50%)

  • Delayed motor skills: poor coordination, awkwardness, clumsiness

  • Behavioral: introverted, difficulty with interpersonal relationships



Xyy Males (1 in 1000 males)

  • Xyy Males (1 in 1000 males)

  • Causes no unusual physical features or medical problems.

  • Usually taller than average and several centimeters taller than their parents and siblings.

  • Most 47,XYY males have normal sexual development and usually have normal fertility.

  • Usually detected only during genetic analysis for another reason.

  • Increased risk for learning disabilities (50%)



Organisms with more than two complete sets of chromosomes, have undergone polypoidy.

  • Organisms with more than two complete sets of chromosomes, have undergone polypoidy.

  • This may occur when a normal gamete fertilizes another gamete in which there has been nondisjunction of all its chromosomes.

  • The resulting zygote would be triploid (3n).

  • Alternatively, if a 2n zygote failed to divide after replicating its chromosomes, a tetraploid (4n) embryo would result from subsequent successful cycles of mitosis.



Polyploidy is relatively common among plants and much less common among animals.

  • Polyploidy is relatively common among plants and much less common among animals.

  • Polyploids are more nearly normal in phenotype than aneuploids.

  • One extra or missing chromosome apparently upsets the genetic balance during development more than does an entire extra set of chromosomes.











Prader-Willi syndrome- Born floppy and pale; Mildly retarded; Obesity;|Tiny hands and feet; Short stature Underdeveloped sex organs, Spectacular temper tantrums especially if refused food

  • Prader-Willi syndrome- Born floppy and pale; Mildly retarded; Obesity;|Tiny hands and feet; Short stature Underdeveloped sex organs, Spectacular temper tantrums especially if refused food

  • Angelman syndrome - spontaneous laughter, jerky movements, severe retardation; and other motor and mental symptoms.





In the new generation, both maternal and paternal imprints are apparently “erased” in gamete-producing cells.

  • In the new generation, both maternal and paternal imprints are apparently “erased” in gamete-producing cells.

  • Then, all chromosomes are reimprinted according to the sex of the individual in which they reside.

  • Imprinting is critical for normal development.







Fragile X syndrome, which leads to various degrees of mental retardation, also appears to be subject to genomic imprinting.

  • Fragile X syndrome, which leads to various degrees of mental retardation, also appears to be subject to genomic imprinting.

    • This disorder is named for an abnormal X chromosome in which the tip hangs on by a thin thread of DNA.
    • This disorder affects one in every 1,500 males and one in every 2,500 females.
  • Inheritance of fragile X is complex, but the syndrome is more common when the abnormal chromosome is inherited from the mother.

    • This is consistent with the higher frequency in males.
    • Imprinting by the mother somehow causes it.









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