©2000 timothy g. standish variation in chromosome number and arrangement timothy g. standish, ph. d

©2000 Timothy G. Standish

Variation in Variation in Chromosome Number Chromosome Number

and Arrangementand ArrangementTimothy G. Standish, Ph. D.


The Modern SynthesisThe Modern Synthesis Charles Darwin recognized that variation existed in

populations and suggested natural selection as a mechanism for choosing some variants over others resulting in survival of the fittest and gradual changes in populations of organisms.

Without a mechanism for generation of new variation, populations would be selected into a corner where only one variation would survive and new species could never arise.

The Modern Synthesis combines the mechanism of mutation in DNA to generate variation with natural selection to produce new species.


MutationMutation Mutation = Change Biologists use the term “mutation” when talking

about any change in the genetic material. Not all result in a change in phenotype.

There are two major types of mutations: Macromutations - Also called macrolesions and

chromosomal aberrations. Involve changes in large amounts of DNA.

Micromutations - Commonly called point mutations and microlesions.


MacromutationsMacromutations Six major types of macromutations in two

categories are recognized:A Change within or between chromosomes

1 Deletions - Loss of chromosome sections2 Duplications - Duplication of chromosome sections3 Inversions - Flipping of parts of chromosomes4 Translocations - Movement of one part of a

chromosome to another part

B Changes in Chromosome number5 Polyploidy - Addition of whole sets of chromosomes6 Aneuploidy - Addition or subtraction of

chromosomes, but not whole sets


1 Deletion1 Deletion

ChromosomeCentromere

A B C D E F G H

Genes

E F

A B C D G H


Deletions Can Be Seen Deletions Can Be Seen During Pairing of Homologous During Pairing of Homologous

Giant ChromosomesGiant ChromosomesA B C D E F G H

A B C D G H


Deletions Can Be Seen Deletions Can Be Seen During Pairing of Homologous During Pairing of Homologous

Giant ChromosomesGiant Chromosomes

A B C D G H

E F

A B C D G H

Compensation loop


Production of DeletionsProduction of Deletions

2. Intercalary deletions

A B C D E F G H

A B C D E F

A B C D G H

Two types of deletions occur:

1. Terminal deletions

A B C D E F G H

A B C D G HF E


Impact of DeletionsImpact of Deletions Large deletions are generally lethal In a very general way, the larger the deletion, the larger the

impact Deletion of genes essential for survival is always lethal Pseudodominance, in which typically recessive alleles

behave in a dominant fashion may result from deletions due to short stretches of chromosomes which become hemizygous due to a deletion in one homolog

The Notch mutation, deletion of a short stretch of the Drosophila X chromosome, provides an example of this

When the notch deletion is on one X chromosome, white, facet and split alleles on the other X chromosome behave in a dominant fashion


Cri-du-Chat SyndromeCri-du-Chat SyndromeThe cry of the catThe cry of the cat

Cri-du-chat syndrome results from deletion of approximately 1/2 the human chromosome 5 p arm

It is one of very few human deletions known to survive to live birth

The larger the deletion the more severe the symptoms which include:– Gastrointestinal and cardiac malformation– Mental retardation– Abnormal development of the glottis and larynx resulting

in a cry resembling that of a mewing cat Incidence of about 1/50,000 live births


2 Duplication2 Duplication

A B C D E F E F G H

ChromosomeCentromere

A B C D E F G H

Genes

E F

Duplication


Duplication Also Produces Duplication Also Produces Compensation LoopsCompensation Loops

A B C D E F E F G H

A B C D E F G H


Duplication Also Produces Duplication Also Produces Compensation LoopsCompensation Loops

A B C D E F G H

E F

A B C D E F G H

Compensation loop


Production of DuplicationsProduction of Duplications Duplications are thought to be the result of

unequal crossing over

A B C D E F G H

A B C D E F G H

A B C D E F G H

A B C D E F G H

A B C D G H

A B C D E F E F G H


Impact of DuplicationsImpact of DuplicationsMore copies of a gene results in more of

the gene product of the geneThis can result in dosage problemsPosition effects may also have an

influence on the way gene duplication impacts organisms


3 Inversion3 InversionChromosome

Centromere

A B C D F E G H

Genes

A B C D E F G H

Inversion


4 Translocation4 Translocation

A B E F C D G H

ChromosomeCentromere Genes

A B C D E F G H


Does Speciation Occur?Does Speciation Occur?YesThere are a number of well -

documented examples of speciation

A good example is the London planetree:


Normal Chromosome Normal Chromosome Numbers In Selected SpeciesNumbers In Selected Species

Organism Chromosomes

Homo sapiens 46

Rat 42

Hen 78

Drosophila 8

Hydra 30

Corn 20

Peas 14

Cat 38

E. coli 1


American American Planetree Planetree

or or SycamoreSycamore

Platanus occidentalis


The London PlanetreeThe London Planetree

AtlanticAtlantic

North America

Africa

Europe

Platanus occidentalisPlatanus occidentalis Platanus orientalisPlatanus orientalis

Oxford Botanical Gardens

>5,000 km>5,000 km

50,000,000 Years50,000,000 Years

Platanus acerifoliaPlatanus acerifolia

c1670c1670

Speciation in the production of P. acerifolia was the result of hybridization, not mutation and selection

If they can interbreed, are P. occidentalis and P. orientalis really separate species?


In His Own WordsIn His Own Words

"I may, of course, be egregiously wrong; but I cannot persuade myself that a theory which explains several large classes of facts can be wholly wrong...."

Charles Darwin, November 13, 1859, in letter to L. Jenyns


Has Natural Selection Been Shown Has Natural Selection Been Shown To Produce New Species?To Produce New Species?

No Darwin emphasized the slow gradual change that is

necessary if natural selection is to work on changing a population gradually from one species to another

“Many large groups of facts are intelligible only on the principle that species have been evolved by very small steps.”– The Origin of Species Chapter VII under

“Reasons for disbelieving in great and abrupt modifications”

©2000 timothy g. standish variation in chromosome number and arrangement timothy g. standish, ph. d

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