LIFESCI 4 Lecture Notes - Lecture 5: Down Syndrome, Turner Syndrome, Trisomy
Video 5
X-chromosome inactivation
In the case of a female embryo:
• In early embryogenesis, each cell of embryo inactivates one of the X chromosomes
• Could be the maternal/paternal one
• This process happens randomly -> half of cells in embryo has paternal X chromosome,
other cells have maternal X chromosome
Ploidy
• n = the number of chromosomes
• haploid genome = 1n, example: vegetative life cycle of some yeast and slime molds
• diploid genome = 2n, example: most animals
• tetraploid genome = 4n, example: Xenopus (frog)
• Aneuploid: a numerical change in part of the genome
- E.g. Trisomy:an individual carries one extra chromosome (2n + 1)
➢ most trisomies in humans are lethal.
➢ 3 chromosomes at chromosome 21 -> Down Syndrome
➢ caused by nondisjunction
- e.g. Turner Syndrome: XO -> Turner female
why are sex chromosomes aneuploidy less deleterious than autosomes?
nondisjunction
• can occur at all of the chromosomes at a low frequency .
• Most trisomies (three chromosomes) and monsomies (one chromosome) are
embryonically lethal
- With one of the smallest chromosomes (#21), they can live (Down Syndrome)
• Trisomy 21 -> Down syndrome
- 47 chromosomes are present instead of 46 from duplication of chromosome #21
- Extra level of gene product causes phenotype.
- Trisomy 21 related to maternal age (greater age, esp. after 35, greater frequency)
Egg produced at early age & stored, spindle fibers fall apart with longer time
• Amniocentesis
- Extract amniotic fluid & cells in it -> grow cells -> karyotype -> count chromosome
Sex determination
• sex chromosomes
- humans; silkworms - presence of Y chromosome
- Drosophila; nematodes - X to autosome ratio
• Ploidy
- In bees: males haploid, females diploid
• Environment
- sea worm: Larva lands on the ocean floor -> develops into a female; lands on a
female -> male.
• Human sex determination
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Document Summary
In early embryogenesis, each cell of embryo inactivates one of the x chromosomes: could be the maternal/paternal one, this process happens randomly -> half of cells in embryo has paternal x chromosome, other cells have maternal x chromosome. Trisomy: an individual carries one extra chromosome (2n + 1) 3 chromosomes at chromosome 21 -> down syndrome. With one of the smallest chromosomes (#21), they can live (down syndrome: trisomy 21 -> down syndrome. 47 chromosomes are present instead of 46 from duplication of chromosome #21. Extra level of gene product causes phenotype. Trisomy 21 related to maternal age (greater age, esp. after 35, greater frequency) Egg produced at early age & stored, spindle fibers fall apart with longer time: amniocentesis. Extract amniotic fluid & cells in it -> grow cells -> karyotype -> count chromosome. Humans; silkworms - presence of y chromosome. Drosophila; nematodes - x to autosome ratio: ploidy.
