POLYPLOIDY

Polyploidy

…more than two haploid sets of chromosomes are present,

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• 2n = diploid,
• 3n = triploid,
• 4n = tetraploid,
• etc.

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Amphidiploid

…double diploid,

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2n₁ + 2n₂

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…have balanced gametes of the type n₁ + n₂,

these gametes fuse to make fertile 2n₁ + 2n₂.

Allopolyploidy Applications

B. oleracea (cabbage, cauliflower, Brocolli, kale, etc.)

2n = 18

B. campestris (turnip, turnip rape)

2n = 20

n₁ + n₂ = 19

4n x 2n = 3n?

• The creation of triploids can be accomplished by crossing a tetraploid with a diploid,

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• Most triploid individuals are sterile.

Generation of a Triploid Cells

Meiosis in a Triploid Organism

Why Wouldn’t this work?

Environmental Applications?

grass carp

(Ctenopharyngodon idella)

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• Triploid grass carp prefer pondweeds,
• do not prefer plants such as cattail, water lily, etc.

Polyploidy Summary

• More than 2 whole sets of chromosomes,

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• Autopolyploidy,
• from the same genome,
• naturally occurring, or induced,
• often results in larger varieties,

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• Allopolyploidy,
• from different genomes,
• naturally occurring, or induced,
• often results in larger varieties,

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• Autotriploids,
• most often sterile
• can produce beneficial traits.

Monoploidy

…a haploid of a diploid is monoploid,

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…has one chromosome set.

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Monoploid

• male wasps, bees and ants have only 1 haploid genome,

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• males develop from unfertilized eggs,
• gametes are formed by mitosis.

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Monoploid Applications

• monoploid plants can be created by culturing pollen grains (n = 1),

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• the population of haploid organisms is then screened for favorable traits,

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• the plants are then treated with colchicine which generates a 2n plant homozygous for the favorable traits.

Chromosomal Mutations

• chromosome number,

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• structure,

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Chromosome Structure

• Changes in chromosome structure can come about due to,

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deletions

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duplications

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rearrangements

Chromosomal Deletions

• a deletion results in a lost portion of a chromosome,

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Deletion Causative Agents

heat,

radiation,

viruses,

chemicals,

errors in recombination.

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Terminal Deletions

Off the End

Intercalary Deletions

From the Middle

Intercalary Deletions

From the Middle

Recognizing Deletions

Homologous Pairs?

Hemizygous

Hemizygous: gene is present in a single dose.

Psuedodominance: hemizygous genes are expressed.

Deletions

…result in partial monosomy,

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• remember monosomy: 2n, -1,

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…the organism is monosomic for the portion of the chromosome that is deleted,

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…as in monosomy, most segmental deletions are deleterious.

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Cri-du-chat Syndrome�(46, -5p)

46, -5p

...terminal deletion of the small arm (petite arm) of chromosome 5,

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• Cri-du-chat Syndrome,

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• 0.002% live births,
• anatomic mutations,
• often mental retardation,
• abnormal formation of vocal mechanisms.

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Chromosomal Duplication

...an event that results in the increase in the number of copies of a particular chromosomal region,

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Duplication Cause and Effect

Causes:

• duplications often result from unequal crossing over,
• can occur via errors in replication during S-Phase.

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Effects:

• results in gene redundancy,
• produces phenotypic variation,
• may provide an important source for genetic variability during evolution.

Unequal Crossing Over

Produces both duplications and deletions!

Duplication Phenotypes

Duplication in Evolution

…essential genes do not tolerate mutation,

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…duplications of essential genes, then subsequent mutations, confers adaptive potential to the organism,

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…new gene family members are ‘recruited’ to perform new functions.

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nutrients

algae

need uptake

Arabidopsis

Chromosome Structure

• Changes in chromosome structure can come about due to,

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deletions

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duplications

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rearrangements

Chromosomal Inversions

…inversion: aberration in which a portion of the chromosome is turned around 180^o.

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Paracentric Inversion

...an inversion in which the centomere is not included,

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...a paracentric inversion does not change arm length ratio.

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B

A

C

Inversion Heterozygotes

…an organism with one wild-type and one chromosome containing an inversion,

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…not heterozygous for the genes, heterozygous for the chromosomes.

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Inversion Loop�no crossing over

Produces haploid gamete.

Paracentric

Produces gamete with inversion.

Paracentric

Produces a chromosome with two centromeres.

Nonviable gametes.

Paracentric

Dicentric

...a chromosome having two centromeres;

Non-Viable (gametes) Segregate

Dicentric/Ascentric

…results only when the crossing over occurs within the region of the paracentric inversion,

No centromeres. Deletions.

Nonviable gametes.

Paracentric

Acentric

…a chromosome having no centromeres,

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…segregates to daughter cells randomly, or is lost during cell division,

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…deletions impart partial monosomy.

Paracentric Outcomes

1 Normal Gamete, 1 Inversion Gamete, No Crossover Classes

Recombination is not inhibited, but recombinant gametes are selected against.

Pericentric Inversion

...an inversion in which the centromere is included,

...a pericentric inversion results in a change in chromosome arm length.

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Pericentric

Recombination and Inversions

• Paracentric and Pericentric;

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• 1 Normal Gamete,
• 1 Inverted Gamete,
• No Crossover Classes = No Recombination,

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Inversions select against recombinant gametes, thus preserves co-segregation of specific alleles.

Inversions and Evolution

• Inversions ‘lock’ specific alleles together,

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• all offspring get their alleles from either a wild-type, or inverted chromosome,

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• If the ‘set of alleles’ is advantageous, the set can be maintained in the population.

Translocations

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…translocation: aberration associated with the transfer of a chromosomal segment to a new location in the genome.

Terminal Translocation

Reciprocal Translocation

Translocation and Semi-Sterility

…semi-sterility; a condition in which a proportion of all gametophytes (in plants) or zygotes (in animals) are inviable.

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• Up to 50% are inviable as a result of translocations.

Robertsonian Translocations

…the fusion of long arms of acrocentric chromosomes,

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Down Syndrome

• 95% of Down Syndrome individuals are a result of Trisomy 21,

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• the probability of having a second Down Syndrome child is usually similar to the population at large,

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• However, there is second cause of Down Syndrome caused by a Robertsonian translocations that is heritable.