Lecture 12: Chromosomal mutations

I. Importance
useful for genetic analysis

useful for breeding

source of some human diseases

extremely important in evolution and reconstruction of evolutionary events

II. Two types of mutations:

rearrangements- sequence of DNA is changed in 1 or more chromosomes:

deletions, duplications, inversions, translocations and movement of transposable elements

change in chromosome number: aneuploidy, polyploidy

III. Rearrangements in more detail

A. Deletions- due to breakage from high energy, also transposable elements

Deletions prevent recombination, also helpful in discovering location of loci

B. Duplication, can be in different orientations- tandem or dispersed, due to breakage, misalignment during meiosis, or transposable elements

C. Inversion- can lead to loss of gene function (if in the middle of a gene), or + position effects, but also

Very important in preventing recombination in inversion heterozygotes

D. Reciprocal translocations- due to breakage, very important in reducing fertility in translocation

Heterozygotes- thus act as partial species barriers

III. Rearrangements in greater detail

A. Deletions restrict recombination, changes gene dosage (can have harmful effects even as a heterozygote), can be used to map genes

B. Duplications, breakage of DNA or unequal crossing-over can lead to duplications

C. Inversions can lead to gain or loss of gene functions and can result in position effects leading to change in level or timing of gene expression, inversions restrict recombination. No recombination betweenwild type and inversion type within inversion sequence

D. Reciprocal translocations can lead to partial sterility in translocation hetgerozygotes

E. Transposable Genetic Elements

the movement of small segments of DNA from one position of the genome to another via entities known

as transposable genetic elements

History: Marcus Rhoades and Barbara McClintock

Two transposable genetic elements alone make up 7% of the human genome!

Can classify by how they move around:

Retroposons- transpose via reverse transcriptio of an RNA intermediate

Transposons- transpose DNA directly insertion of transposable elements can mutate a gene

unequal crossing over, or two tandem elements can cause chromosomal rearrangement and relocation of genes:

IV. Changes in Chromosome number

A. Aneuploidy- chromosome number not exact multiple of haploid number

Arises from nondisjunction of chromosomes during meiosis or mitosis

B. Polyploidy- three or more complete sets of Chromosomes, x = basic chromosome number, x = 23 for humans

Triploids are sterile

Autopolyploids- increase of same chromosomes (doubling of species A’s chromosomes)

Allopolyploids- hybridization between species, then doubling of chromosome content (species A, x = 10 and species B, x = 5, hybridize, F1 is sterile with 15 chromosomes, doubling gives 30 with x = 15 and chromosomes can pair in meiosis

once chromosomes are doubled then can behave properly during meiosis and now called amphidiploids

V. Evolutionary consequences of chromosome mutations

deletions- generally harmful, could bring locus closer to a regulatory gene, changing timing of expression

duplication- can give rise to gene families, of similar genes which take on new or related function

inversions- recombination suppressors

translocations- lead to species formation

transposition- generate novel mutations

polyploidy- instant speciation and other consequences

VI. Summary

Evolution of organisms is associated with chromosome mutations, also many diseases

Terms and Concepts to know: deletions, duplications, inversions, translocations and movement of transposable elements, change in chromosome number: aneuploidy, polyploidy, Figs and Tables (1): 1, 2, 4a, 6, 7, 9, 10, 13, 16, 18, 23, 24, 26, 28, 29, 32, 34