Chapter 12: Chromosomal rearrangements and changes in chromosome number

four types of chromosomal rearrangement

- deletion

- duplication

- inversion

- translocation

fluorescent in situ hybridization can detect large chromosomal rearrangements

homozygosity for deletions is…

often harmful

deletion heterozygotes

- can have mutant phenotype due to gene dosage effects (haploinsufficiency)

- increases risk of altered phenotype due to mutation in the other copy of the gene

- may uncover existing recessive mutant alleles

what is responsible for chronic myelogenous leukemia?

a reciprocal translocation

Transposable elements (TE)

any segment of DNA that has the ability to move from place to place within a genome (studied by Marcus Rhoades and Barbara McClintock). Are unstable and disrupt gene function and alter genomes. Also have the ability to jump in and out of the gene if an autonomous TE

structure of DNA transposons

- contain inverted repeats of 10-200 base pairs at each end

- gene encoding transposase which recognizes the inverted repeats and cuts at the border between the inverted repeats and genomic DNA

aneuploidy

the loss or gain of one or more chromosomes (usually lethal or causes abnormalities). individuals whose chromosome number is not an exact multiple of the diploid number for that species are considered aneuploid.

nondisjunction

causes aneuploidy due to failure of chromosomes to cross over during meiosis 1 and/or 2.

Exceptions to diploidy

- most animals are diploid

- exceptions are ants, bees, hermaphroditic worms, and some fish

- polyploidy in humans is lethal

formation of a triploid organism

note: all triploid organisms that undergo meiosis become sterile due to imbalanced gametes

tetraploid cell generation

polyploids in agriculture

approximately one third of all known flowering plant species are polyploid.

tetraploids - alfalfa, coffee, peanuts, Macintosh apples, bartlett pear

octoploids - strawberries

Euploid

a complete set of chromosomes (usually diploid)

Polyploid

a euploid species that carries 3 or more complete sets of chromosomes

autopolyploid

a kind of polyploid that derives all its chromosomes sets from the same species

allopolyploid

hybrids in which chromosome sets come from distinct but related species. Usually infertile because chromosome sets cannot easily pair and segregate properly

amphidiploid

has two diploid genomes, each from a different parental species. Example: Raphanobrassica

colchicine

causes chromosome doubling in germ cells. Used to generate hybrids such as triticale from wheat and rye. treatment prevents spindle formation and results in doubling of chromosome numbers

Gene families

sets of related genes with slightly different functions (usually diverged in sequence and function)

example of rapid chromosomal evolution