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- variations in chromosome number; when an organism gains or loses one or more chromosomes
- aneuploidy = 2n ± x chromosomes
- monosomy = 2n - 1
- disomy = 2n
- trisomy = 2n + 1
- complete haploid sets of chromosomes are present
- euploidy = multiples of n
- diploidy = 2n
- polyploidy = 3n, 4n, 5n, ...
- triploidy = 3n
more than two sets of chromosomes are present
- when chromosomes or chromatids fail to disjoin and move to opposite poles during meiosis I or II
- fertilization of abnormal gametes results in zygotes with three members (trisomy) or only one member (monosomy), rather than the normal disomy of particular chromosome
Down syndrome due to nondisjunction of which chromosome?
most occur during which Meiosis?
- 21, thus trisomy 21
- 75% occur during Meiosis I
When do tetraploids arise?
when chromosomes have replicated and the parent cell fails to divide, instead it enters interphase and chr. number duplicated
two primary ways in which the structure of chromosomes can be altered (and how)
the total amount of genetic information in the chromosome can change: deletions or duplications
the genetic material remains the same, but is rearranged: inversions or translocations (reciprocal or nonreciprocal)
- when a chromosome breaks in one or more places and a portion of it is lost
- terminal deletion: near one end
- intercalary deletion: from the interior of the chromosome
two ways duplications arise?
- unequal crossing over between synapsed chromosomes during meiosis
- replication error prior to meiosis
involves a rearrangement of the linear gene sequence rather than the loss of genetic information
inversions may or may not include the centromere; two types...
- paracentric inversion: does not include centromere
- pericentric inversion: includes the centromere
organisms with one inverted chromosome and one non inverted chromosome
synapsis of inverted chromosomes requires ....
an inversion loop
for a paracentric inversion crossover, what happens to the centromeres?
- one recombinant chromatid is dicentric (two centromeres)
- one is acentric (lacking a centromere)
- the other to do not synapse
when genes change their position in a chromosome, their expression and thus phenotype may change
the movement of a chromosomal segment to a new location in the genome
the exchange of segments between two non homologous chromosomes
REVIEW: centromere position 4 types
b/t middle and end:
close to end:
- middle: metacentric
- b/t middle and end: submetacentric
- close to end: acrocentric
- at end: telocentric
Robertsonian translocation, AKA centric fusion
- involves breaks at the extreme ends of the short arms of two non homologous acrocentric chromosomes
- -->the small segments are lost and the larger segments fuse at their acrocentric chromosomes
Example of Robertsonian translocation, aka centric fusion
- Familial Down syndrome
- one of the parents contains a 14/21, D/G translocation
more susceptible to chromosome breakage when cells are cultured in the absence of certain chemicals such as folic acid
Fragile X Syndrome (Martin-Bell Syndrome)
- most common form of inherited mental retardation; dominant trait
- genetic anticipation is a characteristic