15) Ploidy Flashcards
What is ploidy?
Some organisms have extra copies of some or all chromosomes
What is aneuploidy?
Some organic
Loss or gain of individual chromosomes or large chunks of them
Usually happens through chromosomal non-disjunction- Sister chromatids do not separate properly during Meiosis I or Meiosis II
Associated with developmental conditions
What are the different subdivisions of aneuploidy?
1) Monosomy= Loss of 1 individual chromosome e.g. Y chromosome or part of one chromosome
2) Trisonomy= Gain of one individual chromosome= Down’s syndrome
3) Tetrasomy, pentasomy and so on…gain of 2 or 3 etc individual chromosomes
Why are aneuploidies so damaging?
Humans in particular are very sensitive to genome loss= losses or gains of only 1-5% of genome can have hugely deleterious effects
Gene dosage: Gene expression is balanced across pair of homologous chromosomes= If chromosomes numbers aren’t quite right= Gene expression will be out of balance
Could be responsible for spontaneous abortions
Chromosomal disorders can be heritable= 5% of Down’s syndrome are inherited but 95% are spontaneous
What is polyploidy?
Organisms gain another copy of their entire genome (lots of chromosomes)
Whole genome is duplicated, common in plants
What is the problem with polyploidy?
Cannot get through meiosis- prophase I
Homologous chromosomes need to segregate correctly or gametes produced will not be viable
What is autopolyploidy?
Type of polyploidy where the multiple sets of genomes come from the same species or individual
What is the problem with autopolyploids?
Have too many homologous chromosomes= Instead of 2 chromosomes pairing up at meiosis, get multivalents= 3 chromosomes tripling up if they’re triploid etc..)
Creates jumble at spindle= Hard for cell to untangle and separate during meiosis
What is allopolyploidy?
Type of polyploidy where the extra genome comes from a different species
What is the problem with allopolyploidy?
Formation starts with 2 games from different species= Fuse –> forms hybrid zygote
Interspecific hybridisation happens in animals e.g. mules
BUT: Chromsomes form each parent aren’t similar enough to form homologous pairs at meiosis= Chromosome segregation into gametes end up being random
THEREFORE: Chances of getting a full set in any one gamete are pretty low
Can grow by mitosis but cannot produce gametes= Sterile
What can plants do to overcome the problem of allopolyploidy?
Somatic doubling
S-phse but without cell division afterwards= Hybrid tetraploid (2n x 2=4n) zygote
Now each chromosome has a homologue= Own copy produced during the somatic doubling= Zygote can grow up in a plant that can undergo meiosis= Produce gametes
Example: domestication of bread wheat
Incorrect mitosis where 2 chromosomes are replicated= can now find homologous chromosome
What are the advantages of polyploidy?
1) Allows genes to be brought in from closely related species through hybridisation= Broaden gene pool
2) Allows extremely rapid speciation (over course of one generation)= Allow plants to expand into other ecological niches
3) Gene expression is often related to amount of DNA in cell= Polyploid organisms can produce more gene product per cell= More productive e.g. wheat
4) More copies of a gene in cell= Allow some copies to change= raw material for evolution
