L11 - Chromosomal Mutations

causes/consequences, evolutionary opportunities

Chromosomal Rearrangement: Duplications

Mutation in which part of the chromosome has been doubled

Chromosomal Rearrangement: Deletions

Loss of a chromosome segment

Chromosomal rearrangements are caused by ______.

Unequal crossing over

Chromosomal rearrangements are detected by _________.

• Loops in prophase

  • Only present in individuals who are heterozygous for rearrangements

Heterozygous vs. Homozygous chromosomal mutations

• Heterozygous duplication or deletion results in loops in prophase I

Consequences of Chromosomal Rearrangements:

• Duplications

  • Unbalanced gene dosage

• Deletions

  • Unbalanced gene dosage

  • Pseudodominance

  • Haploinsufficiency

  • Loss of function due to complete deletions

Consequence: Unbalanced gene dosage (Duplication)

• Region of fly chromosome that controls eye development duplicated

  • Heterozygous Bar results in underdeveloped eyes

  • Homozygous Bar results in extremely underdeveloped eyes due to the presence of too many copies of the gene

Consequence: Pseudodominance

Expression of recessive alleles when paired with deletions

Consequence: Haploinsufficiency

• Cri-du-chat syndrome:

  • Deletion on chromosome 5

  • Affected individuals are heterozygous (hemizygous at the gene level)

  • One copy of the gene is not sufficient for normal development

Consequence: Complete deletion

• Complete loss of function due to deletion

• Ex: Colorblind individuals

Evolutionary novelty of chromosomal rearrangement

Duplicated genes are an important source of evolutionary novelty

Chromosomal Rearrangement: Inversions

• A chromosome segment is inverted 180 degrees (chromosome breaks in 2 places)

• No lost/gained genetic material, only DNA sequence has been altered

• Results in pronounced phenotypic affects

• *Loops occur in heterozygous inversion individuals

Inversions are caused by _____________.

Radiation, major breaks (causes chromosome to reattach incorrectly, altering DNA sequence)

Two Types of Inversions:

• Paracentric: does not include centromere

• Pericentric: includes centromere

Consequences of Inversions:

• Mild effects in original heterozygote

  • All genes are present

  • Some effect of gene positioning (regulatory)

• Major effects during crossing over in Prophase I

  • Formation of nonviable gametes (50%)

  • Overall effect of repressing recombination at break points of inversion

Inversions: Paracentric

• One chromosome forms without centromeres and one with 2 centromeres

• Crossing over within loop creates ½ nonviable gametes

• Dicentric = broken (nonviable due to absence of many genes)

• 1 lost chromosome due to no centromeres

Inversions: Pericentric info

• Includes centromere

• Inversions are very common

• Crossing over within loop creates ½ nonviable gametes

Inversions are important in ____________.

• Speciation (groups of same species slowly segregate due to genetic differences)

  • Inversions were important to distinguish chimpanzees from humans at the genetic level

• Recombination is suppressed around inversions

Chromosomal Rearrangement: Translocations

• The movement of genetic material between nonhomologous chromosomes or within the same chromosome (not the same as crossing over)

• Types:

  • Nonreciprocal: material moves from one chromosome to another without reciprocal exchange

  • Reciprocal: Two-way exchange of segments between the chromosomes

• Detection at Prophase 1: cross formation (KINDA SIMILAR TO WHY LOOPS FORM)

Causes of translocations include ______________.

• Radiation, transposons, and unknown

Translocation consequences:

• Mild effects in the original heterozygote

  • All genes are present

  • Loss of function if genes are interrupted

  • Some effect of gene positioning (regulatory)

• Major effects during crossing over in Prophase I

  • Formation of nonviable gametes

    • 
      

Translocation Types: Alternate Segregation

• Anaphase II: 2 translocated move to one end while 2 normal move to the other end of cell

• 4 viable gametes

• Full gene complement

Translocation Types: Adjacent-1 Segregation

• Anaphase II: 1 normal + 1 translocated move to each end of the cell

• 4 nonviable gametes

• Duplicated and deleted genes

Translocation Types: Adjacent-2 Segregation (rare)

• Anaphase II: 1 normal and 1 translocated to each end of cell

• 4 nonviable gametes

• Duplicated and deleted genes

Robertsonian Translocation

• Fuse 2 chromosomes together

• Can cause Down’s syndrome

• Results in the formation of one big chromosome and a small fragment (lost over evolutionary time)

  • ***Chromosome number is reduced

Robertsonian Translocation is important in ______________.

Evolution