Genetics: Chapter 7

Chromosomes mutations: Variation in number and arrangment

Phenotypic variations

Results from modifications at the chromosome level

• Change in total chromosome number

• Deletion or duplication of genes or segments of chromosomes

• Rearrangements of genetic material within or among chromosomes

Aneuploidy

Organism gains/loses one or more chromosomes but not a complete set

• ex: monosomy (loss of a single chromosome from a diploid genome), trisomy, tetrasomy

• 45, 47, 48, 49 chromosomes

Euploidy

Complete haploid sets of chromosomes are present

• Normal set of 46 chromosomes

Polyploidy

More than two sets of chromosomes are present (in plants, NOT humans)

• subsets of polyploidy = autopolyploidy and allopolyploidy

  • autopolyploidy: sets of chromosomes from the same species

  • allopolyploidy: sets of chromosomes from different species

Nondisjunction

Chromosomal variation caused by errors during gamete production

• Paired homologs fail to disjoin and move to opposite poles during meiosis I or II

Monosomy

The loss of one chromosome (2n − 1) => severe phenotypic effects

• Monosomy for the X chromosome occurs in humans

• Monosomy for autosomes: not tolerated in humans/animals, tolerated in plants

Haploinsufficiency

A single copy of a recessive gene is insufficient to provide a life-sustaining function for an organism

Trisomy

An extra chromosome (2n+1) produces more viable organisms than the loss of a chromosome

• Provided that the chromosome involved is relatively small

• In humans, the addition of a large autosome to a diploid genome has
  severe effects, usually lethal during development

Trisomic plants

Viable, but phenotype may be altered

• slower growth

• varied morphology of leaf

• stem

• grain

Trisomy 21: Down syndrome (47, 21+)

Trisomy of chromosome 21 occurs in 1/800 live births (4,000-5,000 births annually)

• Symptoms: prominent epicanthic fold in eyes, flat face, round head, short
  stature, protruding, furrowed tongue, short hands, cognitive disabilities, poor muscle
  tone, average life span of 50 years
  
  

DSCR: Down Syndrome Critical Region

On chromosome 21, a region containing dosage-sensitive genes

• genes responsible for many of the phenotypes associated with the syndrome

• Increased risk of leukemia

• An extra copy of DSCR1 is associated with decreased risk of some cancers

  • DSCR1 gene encodes a protein that suppresses vascular endothelial growth factor (VEGF) => blocks angiogenesis

Other human aneuploidies

Patau syndrome (47, 13+), Edwards syndrome (47, 18+)

Tetraploid Cells

• Can be produced experimentally —> applying heat or cold shock to diploid cells

  • Interferes with spindle fiber formation

Allotetraploid/Amphidiploid

A tetraploid has genomes from separate species

• Is called allotetraploid because there are four sets of chromosomes, but they are not from the same genome

Chromosome aberrations

Changes that delete, add, or rearrange portions of one or more chromosomes

• These changes are due to one or more breaks along chromosome, followed by loss or rearrangement of genetic material

• Alterations in gametes are heritable

Deletions

Portion of chromosome lost due to breaks (can be called deletions OR deficiencies)

• Terminal Deletion: deletion near one end of chromosome

• Intercalary Deletion: deletion in the interior of chromosome

Deletion Loop (compensation loop)

Synapsis occurs between normal homolog and chromosome with intercalary deletion

• The unpaired region of the normal homolog must “buckle out” into a deletion loop for synapsis to occur

Cri du Chat Syndrome

Loss/deletion of a small variable part of short arm (p arm) on chromosome 5

• (46, 5p-)

Chromosomal Duplications

Repeated segment of a chromosome

• Due to unequal crossing over between synapsed chromosomes in meiosis OR through replication error prior to meiosis

Aspects of Duplication

1. Gene redundancy:

2. Phenotypic variation:

3. Source of genetic variability during evolution:

Inversions

Segment turned 180˚ within chromosome —> no loss of genetic information

• Pericentric inversion: includes the centromere

• Paracentric inversion: does not include the centromere

• Two breaks in chromosomes followed by reinsertion of inverted segment

• Chromosomal loop forms prior to breakage and create “sticky ends” that close together and rejoin