BMS2042 Lecture Notes - Lecture 14: Brassicoraphanus, Gene Pool, Aneuploidy
30 May 2018
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Week 5. Changes in chromosome number: Polyploidy
and Population genetics
CHANGES IN CHROMOSOME NUMBER: POLYPLOIDY
• Euploidy: having an equal number of all the chromosomes of the haploid set
• Polyploid: an individual with more than 2 sets of haploid chromosomes in their somatic cells
• Most species are diploid with haploid gametes but polyploids can occur (mostly in plants)
• Two major categories of germline polyploidy:
o Autopolyploids: more than two sets of chromosomes all derived from one ancestral
species
o Allopolyploids: more than two sets of chromosomes derived from more than one
ancestral species
o Can also find normal variation in euploidy in certain tissues called somatic polyploidy
• Autopolyploidy:
All chromosomes derived from one ancestral species
Autotetraploidy (4n)
o Arises due to accident during mitosis: chromosomes replicate but cell
division does not occur
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o Plants:
If occurs in one cell, all descendants are autotetraploid
Individuals will then be mosaic of 2n and 4n cells
If 4n cells are part of germline -> gametes will be diploid ->
can fertilise each other to give fully autotetraploid individual
Most plants are autotetraploid
Phenotype: everything is larger, slower growing, otherwise
closely resembles diploid
Usually fully fertile/viable as chromosomal complement is
balanced
-plants self-fertilise hence zygotes = 4n
o 4n zygote is not viable in mammals
Autotriploidy (3n)
o Arises if diploid gamete (2n) joins with a haploid (1n) gamete
-> 3n zygote
o Haploid gamete comes from normal diploid
o Diploid gamete can come from:
An autotetraploid
A diploid unreduced gamete -> during meiosis when all
chromosomes at either anaphase I or II are incorporated
-> rare mis-division
o Phenotype: intermediate between diploid and tetraploid
o Fully viable in plants but not mammals
o Usually completely sterile (Meiosis = unbalanced gametes, if n>10 =
sterile)
o This sterility may be an advantage in some crop plants (no seeds)
eg. seedless watermelon (3n=33) are generated from 2n x 4n crosses
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• Evolution of autopolyploid:
o Once produced = instant new species
o If 4n x 2n => 3n = sterile
o Tetraploid is reproductively isolated from diploid ancestor
o New 4n resembles 2n but larger and may colonise new habitats
• Allopolyploidy:
Chromosome sets derived from more than one ancestral species
o First step in origin is formation of inter-species hybrids
o Usually only closely related species will for a viable hybrid (others will not breed or
resulting zygotes are inviable)
o Most interspecies hybrids that survive are sterile
-> either meiosis does not occur or two sets of chromosomes do not pair (all univalent,
no balanced gametes)
o Not usually viable in mammals however possible
eg. mare and donkey = mule is viable but sterile (AB = 63, if made into polyploid AABB =
126 would be inviable)
Allodiploid
o Hybrid is allodiploid
-> one set of chromosomes from each species (n1+n2)
o Eg. Cross roan and sable antelope -> chromosomes similar
enough to pair up at meiosis (fertile)
o Eg. radish and cabbage -> cross together but in hybrid
chromosomes are too different to synapse and do not
segregate -> leads to aneuploidy (sterile)
Allotetraploid
o If chromosome number is doubled (replicate but no cell
division) during mitosis (2n1+2n2)
o All the chromosomes can form bivalents at meiosis (balanced)
o Results in diploid gametes, chromosomal complement is
balanced -> fertile and viable
o Usually confined to plants
-> due to rare chromosome division without cell division occurs
in somatic tissues of the hybrid -> all descendent cells are
irreversibly allotetraploid
o Hybrid will be AB with mosaic of AB and AABB
o If AABB cells are part of germ line, gametes will arise to be AB
(diploid)
-> can fertilise each other to make fully allotetraploid
individual which is fertile
o Phenotype: resembles a blend of phenotypes of each diploid,
may have different desirable traits
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Changes in chromosome number: polyploidy and population genetics. Autotetraploidy (4n: arises due to accident during mitosis: chromosomes replicate but cell division does not occur, plants: If occurs in one cell, all descendants are autotetraploid. Individuals will then be mosaic of 2n and 4n cells. If 4n cells are part of germline -> gametes will be diploid -> can fertilise each other to give fully autotetraploid individual. Phenotype: everything is larger, slower growing, otherwise closely resembles diploid. Usually fully fertile/viable as chromosomal complement is balanced. Plants self-fertilise hence zygotes = 4n: 4n zygote is not viable in mammals, arises if diploid gamete (2n) joins with a haploid (1n) gamete. > 3n zygote: haploid gamete comes from normal diploid, diploid gamete can come from: A diploid unreduced gamete -> during meiosis when all chromosomes at either anaphase i or ii are incorporated.
