Chapter 8

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Chromosome Mutations or Chromosome Aberrations

Changes in the total number of chromosome, such as a deletion or rearrangement

Aneuploidy

Variations in chromosome number. When an organism gains or loses one ore more chromosomes and has other than an exact multiple of the haploid set

Euploidy

Completely haploid sets of chromosomes are present

Polyploidy

Occurs when more than two sets of chromosomes are present

Nondisjunction

Chromosomal variation arises from nondisjunction, where chromosomes or chromatids fail to disjoin and move to opposite poles during Meiosis I or II

Monosomy

The loss of one chromosome to produce a 2n-1 complement

Trisomy

2n+1 Chromosomes. Often lethal for autosomes, but not sex chromosomes. Three copies of one chromosome are present so pairings are irregular.

Trivalent

When three copies of a chromosome are synapsed

Univalent

An unpaired chromosome, can be present along with a bivalent instead of a trivalent. When 3 chromosomes aren't synapsed, instead 2 are and 1 is left unpaired.

Down Syndrome

Results from trisomy of chromosome 21

Familial Down Syndrome

Runs in families, involve a translocation of chromosome 21

Amniocentesis or Chorionic Villus Sampling (CVS)

Testings for women who become pregnant late in their reproductive years

Patau Syndrome

Trisomy 13

Edwards Syndrome

Trisomy 18

Triploid

3n

Tetraploid

4n

Pentaploid

5n

Autopolyploidy

The addition of one or more sets of chromosomes identical to the haploid complement of the same species

Allopolyploidy

The combination of chromosome sets from different species as a consequence of interspecific matings

Allotetraploid

Arises from hybridization of two closely related species

Endopolyploidy

The condition in which only certain cells in an otherwise diploid organism are polyploid

Deletion (deficiency)

Chromosome breaks in one or more places and a portion if it is lost, the missing piece.

Terminal Deletion

Deletion near one end

Intercalary Deletion

Deletion from the interior of the chromosome

Cri-du-chat

Results form a segmental deletion of a small terminal portion of the short arm of chromosome 5

Duplications

Arise as the result of unequal crossing over during meiosis or through a replication error prior to meiosis

Gene Redundancy

ORganisms that have multiple copies of ribosomal RNA genes (rRNA)

Gene Amplification

A mechanism to increase the rRNA

Inversion

Involves a rearrangement of the linear gene sequence rather than the loss of genetic information. Segment turned 180 degrees in a chromosome, requires two breaks and reinsertion of the inverted segment. May arise from chromosomal looping

Paracentric inversion

Does not change the relative lengths of the two arms of a chromosome

Pericentric Inversion

Includes centromere, does change the relative lengths of the two arms of a chromosome

Paracentric inversion crossover

One recombinant chromatid is dicentric (two centromeres) and one is acentric (lacking a centromere)

Translocation

Movement of a chromosomal segment to a new location in the genome

Reciprocal Translocation

Involves the exchange of segments between two nonhomologous chromosomes. Unusual synapsis

Alternate Segregation

Leads to normal and a balanced gamete

Adjacent Segregation

Leads to gametes containing duplications and deficiencies

Fragile Sites

more susceptible to chromosome breakage when cells are cultured in the absence of certain chemicals such as folic acid

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