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 Speciation Lessons

Chromosome Fusion Lesson

For additional examples and explanations of chromosomal changes, click on:


Chromosomal Changes and Speciation

Robertsonian translocation (= centric fusion translocation): A common and significant type of chromosome rearrangement that is formed by fusion of the whole long arms of two acrocentric chromosomes (chromosomes with the centromere near the very end). One in about 900 babies is born with a Robertsonian translocation making it the most common kind of chromosome rearrangement known in people. All five of the acrocentric chromosomes in people -- chromosome numbers 13, 14, 15, 21 and 22 -- have been found to engage in Robertsonian translocations. However, the formation of Robertsonian translocations was discovered by Hecht and coworkers to be highly nonrandom. Far and away the most frequent forms of Robertsonian translocations are between chromosomes 13 and 14, between 13 and 21, and between 21 and 22. From Wikipedia: Robertsonian translocation.

Scitable by nature educationIn balanced form, a Robertsonian translocation takes the place of two acrocentric chromosomes and results in no problems for the person carrying it. But in unbalanced form, Robertsonian translocations produce chromosome imbalance and cause syndrome of multiple malformations and mental retardation. Robertsonian translocations between chromosomes 13 and 14 lead to the trisomy 13 (Patau) syndrome. And the Robertsonian translocations between 14 and 21 and between 21 and 22 can and do result in (trisomy 21 (Down) syndrome.
Robertsonian translocations are named for the America insect geneticist W.R.B. Robertson who first described this form of translocation (in grasshoppers) in 1916 and are also known as whole-arm or centric-fusion translocations or rearrangements. From MedicineNet.com. Figure from SciTable by Nature Education.

Centric Fusion Translocation, also called a Robertsonian translocation, in which there is fusion of an entire long arm of one acrocentric chromosome with a similarly intact long arm of another acrocentric chromosome. The short arms of the chromosomes participating in the translocation are usually lost.

Acrocentric chromosomes have their centromere near but not at the very end of the chromosome. In humans, chromosomes 13-15, 21 and 22 are acrocentric.

Centric-fusion translocations are relatively common. In balanced form, they take the place of two acrocentric chromosomes and result in no problems. But in unbalanced form, centric-fusion translocations account for some cases of the trisomy 13 (Patau) syndrome and trisomy 21 (Down) syndrome.

Dobzhansky-Muller incompatabilities: The Bateson-Dobzhansky-Muller Model is a model of the evolution of genetic incompatibility. The theory was first described by William Bateson in 1909 , then independently described by Theodosius Dobzhansky in 1934 , and later elaborated by Herman Muller.

The model attempts to explain how incompatibilities between closely related species develop without either of them going through an adaptive valley. In its simplest form the model shows that changes in at least two loci are required to cause hybrid incompatibility, or at least a decrease in fitness between individuals from two ancestrally identical, but allopatric populations. This is based on the idea that a new allele which has arisen at one locus in one population should not cause decreased fitness when placed into the identical (except for one allele) genetic background of the second population. Therefore another allele at a second locus which is incompatible with the first must have arisen. From Wikipedia: Bateson-Dobzhansky-Muller Model

Non-Disjunction: from TutorVista.com

(In mitosis) The failure of sister chromatids to separate during and after mitosis.
(In meiosis) The failure of homologous chromosomes to segregate or to separate during and after meiosis.

From Wikipedia: Non-Disjunction

Figure from TutorVista.com

Trivalent Chromosomes
A chromosome formed by three homologous chromosomes that lie close together or appear to join completely during meiosis

Pachytene: The third stage of the prophase of meiosis during which the homologous chromosomes become short and thick and divide into four distinct chromatids.

Metacentric chromosomes: a chromosome having two equal arms because the centromere is in median position

Acrocentric chromosome: a chromosome with the centromere near one end so that one chromosomal arm is short and one is long.

Pericentric inversion: A basic type of chromosome rearrangement in which a segment that includes the centromere (and so is pericentric) has been snipped out of a chromosome, turned through 180 degrees (inverted), and inserted back into its original location in chromosome.

Epistatic Interaction: An epistatic interaction between two genes occurs when the phenotypic impact of one gene depends on another gene, often exposing a functional association between them. See article in PLoS Genetics

Aneuploidy: an abnormal number of chromosomes (usually one more or one less than normal number).

Polyploidy examples from Wikipedia Polyploidy: cells and organisms containing more than two paired (homologous) sets of chromosomes. Most eukaryotic species are diploid, meaning they have two sets of chromosomes — one set inherited from each parent. However polyploidy is found in some organisms and is especially common in plants. In addition, polyploidy also occurs in some tissues of animals who are otherwise diploid, such as human muscle tissues. [Human liver cells are mostly hexaploid (6 sets), and many skin cells are tetraploid.]. This is known as endopolyploidy. (Monoploid organisms also occur; a monoploid has only one set of chromosomes. These include the vast majority of prokaryotes.) Figure from Wikipedia Wikimedia Commons, Author: Ehamberg.


Euploidy: having an abnormal integral multiple of the monoploid number of chromosomes (e.g., 3N or 4N)


Mule: the hybrid offspring from a male donkey (jack) and a female horse (mare). Mules are usually sterile. Some female mules have given birth to viable offspring. No record of male mules siring offspring. Horses and Donkeys typically have 64 chromosomes. Their hybrid offspring typically have 63 chromosomes (an example of aneuploidy).

Hinny: the hybrid offspring from a male horse (stallion) and a female donkey (jenny). Hinnies are typically sterile. Only one known case of a female hinny bearing a viable offspring.


For Excellent Graphic Explanations of Many Causes of Speciation, see theUnderstanding Evolution Site.

Allopatric speciation: speciation that occurs when biological populations of the same species become isolated due to geographical changes such as mountain building or social changes such as emigration. The isolated populations then undergo genotypic and/or phenotypic divergence as: (a) they become subjected to different selective pressures, (b) they independently undergo genetic drift, and (c) different mutations arise in the populations' gene pools.

Parapatric speciation: no specific extrinsic barrier to gene flow. The population is contiguous, but nonetheless, the population does not mate randomly. Individuals are more likely to mate with their geographic neighbors than with individuals in a different part of the population’s range. In this mode, divergence may happen because of reduced gene flow within the population and varying selection pressures across the population’s range.

Sympatric speciation: geographically overlapping populations.


Epigenetics is the study of heritable changes in gene expression or cellular phenotype caused by mechanisms other than changes in the underlying DNA sequence.