The Ac-Ds System

Chromosome Aberrations

Chromosome Aberrations Variations in the number and/or arrangement. Chromosome abnormalities are passed on to offspring in a predictable manner, resulting in unique genetic outcomes.

Definitions Euploid: An organism or cell with the correct number of chromosomes. Aneuploid: An organism that has lost or gained an individual chromosome, but not an entire set. Monosomy: The loss of a single chromosome from an otherwise diploid genome. Trisomy: The gain of a single chromosome from an otherwise diploid genome.

Note: polyploidy can be the normal condition in plants. Definitions Polyploid: An organism with more than two entire sets of chromosomes. Triploid: An organism with three sets of chromosomes. Tetraploid: An organism with four sets of chromosomes. Note: polyploidy can be the normal condition in plants.

Notes on Chromosome Abnormalities Most of the examples will be conditions observed in humans. Often the abnormalities produce a negative effect. In animals, the loss of function usually prevents the animal from being vigorous such that it does not become a member of the breeding population, and the conditions are not preserved. (Selection pressure)

Non-Disjunction Non-disjunction occurs when chromosomes do not assort evenly during meiosis, such that the resulting gametes have either extra chromosomes or not enough chromosomes. Non-disjunction may occur during either meiosis I or meiosis II.

Non-Disjunction: Anaphase I

Non-Disjunction: Anaphase II

Non-Disjunction Klinefelter Syndrome: (47:XXY) An extra X chromosome; males, may be unusually tall (>6 ft), underdeveloped testes, usually infertile, some feminization. ( to XXX, XXXX, more severe) Turner Syndrome: (45:X) Loss of one X chromosome; females characterized by short stature (<5 ft), subfertile ovaries and inadequate secondary sex characteristics.

Klinefelter Syndrome

Monosomy Turner Syndrome is an example of monosomy, or loss of a chromosome. Loss of autosomes (non-sex chromosomes) are not usually tolerated in either humans or animals (organism dies during development).

As usual, there is something of an exception.

Partial Monosomy Cri-du-Chat (“Cry of the cat”) syndrome: (46:-5p) Due to a partial deletion of chromosome 5; infants have a high-pitched, mewling cry, due to abnormal development of the larynx and glottis. Infants are also characterized by abnormal gastrointestinal tract, mental and cardiac development. The longer the deletion, the more severe the complications.

Trisomy Trisomy 21: Down Syndrome; impaired mental and cardiac development. Affected individuals usually live to the mid30s. Trisomy 13: Patau Syndrome; affected individuals are not mentally alert, have a cleft palate, may exhibit polydactyly (extra digit on hands or feet),

Chromosome Rearrangements These are variations in the structure and arrangement of chromosomes Note that rearrangement of chromosomes may exert a positional effect, such that genes may be suppressed or expressed by moving from one place in the genome to another.

Animation Credits The animation clips were created by Dr. Lester Newman, Professor of Biology at Portland State University http://www.irn.pdx.edu/~newmanl/moviepage.html

Duplication

Deletion May involve loss of a whole chromosome (Turner syndrome) or part of one chromosome. A partial deletion of chromosome 5 is responsible for the partial monosomy of Cri-du-Chat syndrome.

46:-5P Deletion in Cri-du-Chat

Portions of chromosomes trade places. Translocations Portions of chromosomes trade places.

Deletion

Reciprocal Translocation

Robertsonian Translocations

Familial Down Syndrome A Robertsonian translocation is responsible for about 5% of Down syndrome incidence. In these cases, one of the parents will have a translocation of chromosomes 14 and 21. As a result, 25% of that person’s gametes will have three copies of chromosome 21 (two normal, plus most of another one attached to chromosome 14). Note that in this case, the affected individual has the normal complete number of 46, but one of the copies of 14 possesses the translocation.

Inversions Portions of a chromosome become reversed. Inversions may or may not involve the centromere

Inversions Paricentric: The centromere IS part of the inversion. (To help you remember: Paricentric IS) Peracentric: The centromere is NOT part of the inversion.

Consequences of Inversions If only one homologue of a pair experiences an inversion, normal linear synapsis cannot occur during meiosis. Recombination may not occur at all, or if it does, will produce gametes with duplications and deletions. Often, gametes with broken chromosomes will form.

Fragile Sites—Fragile X Syndrome Site along X-chromosome that is prone to breakage. Occur in fetuses of women who are deficient for the B-vitamin folic acid. Associated with a particular form of mental retardation and/or attention deficit disorder.

Fragile Sites and Cancer A number of fragile sites have been associated with cancer, particularly one site on chromosome 3. The gene inactivated by the break has been shown to be involved in cancer development in a number of tissues. It is not known if breakage at the fragile site causes cancer or if activity of cancerous cells induces the break.