aneuploidy Flashcards
aneuploidy
diploid genome which lacks a chromosome or has an extra chromosome
how is aneuploidy different from polyploidy
organism gains or is missing a single chromosome whereas in polyploidy all sets are affected
trisomies
2N+1
- diploid for all chromosomes but +1 chromosome for one of the homologous groups
- some result in aborted embryo
monosomies
2N-1
- diploid for all chromosomes but -1 for one of the homologous groups
when is it a possibility for trisomies or monosomies to be viable
when they are sex chromosomes
cause of aneuploidy
meitotic or meiosis non disjunction
aneuploidy from nondisjunction in meiosis I
- all gametes are wrong
- homologous pairs do not disjoin
- final zygotes are 2 trisomies and 2 monosomies
- all monosomies are aborted except sex chromosomes
aneuploidy from nondisjunction in meiosis II
- half the gametes are wrong
- sister chromatids will not disjoin
- final zygotes are 1 trisomie, 1 monosomie and 2 normal diploids
- if happens to one cell usually doesn’t happen to the other - low frequency event
autosomal trisomies that are seen as live births
trisomy 21 (down syndrome)
trisomy 13 (Patau syndrome)
trisomy 18 (Edwards syndrome)
- only those with trisomy 21 can live into adulthood
down syndrome karyotype
- chromosome 21 is one of the smallest chromosomes
- most trisomy can be tolerated because it is the smallest amount of extra genes - still bad but the least bad
Aneuploidy in sex chromosomes
- trisomy and monosomy can be tolerated
- comes from X inactivation
XO - turner syndrome
- female with a single X
- can be caused by the mom or dad
- 2n - 1
XXY - klein felter syndrome
- male with an extra X
- 1 barr body
- mom could give XX or day could give XY
- if from dad, nondisjunction in meiosis I
- some female hormones are heightened
XXX - triple X syndrome
- female with an extra X
- mom gives XX
- 2 barr bodies
XYY - double Y males
- male with an extra Y
- gets YY from dad, nondisjunction in meiosis II
- most common phenotype = very tall (due to extra hormones)
why are older mothers more likely to give birth to down syndrome children
frequency of trisomies increases due to prophase arrest
- in the human female fetus meiosis begins and arrests, germ cells remain in an arrested prophase I until ovulation
- stalled gametes are constantly exchanging genetic material through recombination which causes them to break apart from mitotic spindle
prophase arrest
problems with pairing of homologous chromosomes
correlation between trisomies and maternal age
- bivalents with crossovers near the ends of chromosomes are fragile and increasingly unstable over time
- if the bivalent dissociates, non-disjunction of the homologues occur
cohesion
non-disjunction between sister chromatids
what sorts of chemicals can cause non-disjunction
colchicine and Bisphenol A
causes of down syndrome
- 96% due to meiotic nondisjunction
- of these, 80% from oogenesis in the mother and 20% from spermatogenesis in the father
- in males due to random chance, in females due to prolonged prophase I
deletions / deficiency in chromosome segment
- missing part of the chromosome
- could be due to mistakes in how they are copied
duplication in chromosome segment
- an extra part of the chromosome
- hyperploidy
hyperploidy and hypoploidy
hypoploidy: less genetic material
hyperploidy: more genetic material
Hypoploidy example: cri-du-chat syndrome
- missing the tip of one chromosome 5, due to mistake in DNA replication
- grow into adulthood with developmental disorder
