Chromosomal Mutations

Vocabulary:

• Short arm (p): Part of a chromosome visually, shorter end of the centromere, placed on top in a karyotype

• Long arm (q): Part of a chromosome visually, longer end of the centromere

• Metacentric: When the centromere is in the middle of the chromosome

• Submetacentric: When the centromere is more towards one side than the other

• Acrocentric: When most of the chromosome is on one side of the centromere

• Telocentric: When the centromere is on the edge of the chromosome

• Banding: Created by the staining of chromosomes, can be used to distinguish homologous pairs

• Giemsa stain: Provides dark/light banding

• Q bands: Created by fluorescent staining using quinacrine mustard

• Aneuploidy: A mutation in the number of total chromosomes, written as 2n+1 with an extra

• Polyploidy: A mutation where there are one or more extra full sets of chromosomes, written as 3n when 3 full sets

• Monosomy: Loss of a single chromosome in diploid genomes

• Trisomy: Gain of a single chromosome in the diploid genome

• Euploidy: Typical chromosomes, one complete haploid set

• Triploidy: When there are three full sets of chromosomes

• Disomy: When an organism has 2n chromosomes, can be biparental or not

• Nondisjunction: When a chromosome/chromatids fail to disjoin and move to opposite poles during meiosis I or II

• Pseudodominance: Occurs due to a monosomy of a recessive allele, one copy is enough for it to be displayed

• Haploinsufficiency: When a monosomy isn’t sufficient enough for an organism to survive, as enough genetic product isn’t being produced

• Down syndrome: Caused by trisomy of chromosome 21 in humans, has 12 phenotypes but only 6-8 are typically displayed, can have a flat face, low muscle, short height, higher risk for cataracts, heart defects/immune disorders, intellectual disabilities. More likely the older the mother is, occurs in meiosis I

• Familial down syndrome: A Robertsonian translocation of the trisomic 21 chromosome onto the 14 chromosome, can be inherited though not displayed, can “pass down syndrome” on to children

• Patau syndrome: Trisomy of chromosome 13, fatal within early years of life

• Edwards syndrome: Trisomy of chromosome 18, fatal within early years of life

• Autopolyploidy: Polyploidy due to nondisjunction within a species

• Allopolyploidy: Polyploidy coming from fertilization from different species, forming a hybrid, viable offspring are more common with closely related species, though often infertile

• Unbalanced gamete: Tends to come from having an odd number of chromosomes in polyploidy, typically nonviable due to dosage compensation not working

• Colchicine: A chemical that can induce autotetraploids to form from diploids, used often in crops

• Amphidiploid: Happens when a sterile hybrid doubles its chromosomes (spontaneously or induced by colchicine), becomes fertile. This is often done with plants, the parents are known

• Deficiency: The missing piece of a chromosome that has broken and lost a portion in at least 1 place, can lead to haploinsufficiency or pseudodominance

• Terminal deletion: A deficiency near one end

• Intercalary deletion: A deficiency on the interior of the chromosome

• Cri-du-chat: A mutation that results from a segmental deletion of a small terminal portion of the p arm of chromosome 5

• Gene redundancy: When an organism has multiple copies of the same gene, for example rRNA

• Gene amplification: Increases levels of rRNA, oocytes store nutrients such as ribosomes, redundant gene copying

• Duplication: When two copies of a gene are present that shouldn’t be, typically only one

• Gene family: A regional group or genes whose products perform the same or similar general functions

• Inversion: A rearrangement of the linear gene sequence, no loss of genetic information. Chromosome is turned around 180 degrees, requires two breaks to occur with a reinsertion after the segment flips, can occur due to looping

• Paracentric inversion: An inversion that doesn’t change the relative length of p and q arms

• Pericentric inversion: An inversion that contains the centromere, ends up changing the length of p and q arms

• Inversion loop: Required to form for chromosomes to invert, if crossing over occurs gametes will have problems

• Dicentric: Describes a chromosome that has 2 centromeres due to crossing over during an inversion

• Acentric: Describes a chromosome that has no centromere due to crossing over during an inversion

• Translocation: A movement of a chromosomal segment to a new location in the genome

• Reciprocal translocation: The exchange of segments between two nonhomologous chromosomes, has an unusual synapse configuration during meiosis

• Alternate segregation: Leads to normal and balanced gamete formation, even after translocation

• Adjacent segregation: Leads to gametes containing duplications and deficiencies after translocation

• Robertsonian translocation (centric fusion): Involves breaks at extreme ends of the p arms or 2 nonhomologous acrocentric chromosomes, leads to small segment loss and fusion of the larger segments

• Fragile site: A region of a chromosome more susceptible to chromosome breakage

• Fragile-X syndrome (FXS): Common inherited intellectual disability, dominant and involves fragile site FMR1 is lost

• Genetic anticipation: When the number of irregular trinucleotide repeats increase with generations, seen in FXS