Chapter 6 number changes in chromosomes.docx

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University of Guelph
Molecular Biology and Genetics
MBG 2040
Dennis Baker

MBG*2040 – Miriam Bergeret Chapter 6 – Variation in Chromosome number and structure Cytological Techniques: geneticists study chromosome number and structure by staining dividing cells with certain dyes and then examining them with a microscope. Euploid - normal Aneuploidy : refers to a numerical change in part of the genome pertaining to just a single chromosome. This implies a genetic unbalance. Hypoploid: an organism in which a chromosome or a piece of a chromosome us under-represented Hyperploid : an organism in which a chromosome or a chromosome segment is overrepresented Trisomie: when u have an extra chromosome, so 3 identical ones  Trisomy in Humans: o Down syndrome, trisomy of chromosome 21: written as 47, XX, +21, can be caused by chromosome nondisjunction in one of the meiotic cell divisions. The nondisjunction event can occur in either parent, but it seems to be more likely in females. The frequency of nondisjunction increases with maternal age. This is due to factors that adversely affect meiotic chromosome behaviour as a woman ages. In human females, meiosis begins in the fetus bit it is not completed until after the egg is fertilized. During the long time prior to fertilization, the meiotic cells are arrested in the prophase of the first division. In this suspended state, the chromosome may become unpaired. The longer the time in prophase, the greater the chance of unpairing and subsequent chromosome nondisjunction. o Patau Syndrome, trisomy 13 o Edward Syndrome, trisomy 18 o Triple-X, 47, XXX. They individuals survive because two of the three x chromosome are inactivated, reducing the dosage of the X chromosome so that it approximates the normal level of one. They are female and are phenotypically normal, but sometimes exhibit a slight mental impairment and reduced fertility. o Klinefelter syndrome, 47, XXY: phenotypically they are male, but can show some female secondary sexual characteristics and are usually strile. Small testes, enlarged breasts, long limbs, nock-knees, underdeveloped body hair. Can occur from XX egg with Y sperm, or XY sperm with X egg. o 47, XYY: individuals are male, tendency to be taller.  Monosomy: occurs when one chromosome is missing in an otherwise diploid individual. In humans there is only ONE viable monosomic, the 45, X karyotype. o Turner Syndrome, 45, X: Individuals have a single X chromosome, and diploid autosomes. Phenotypically female, but sterile. Usually short in stature, webbed necks, hearing deficiencies, significant cardiovascular abnormalities. Can originate from egg or sperm lacking a sex chromosome or from the loss of a sex chromosome in mitosis sometime after fertilization. They are often somatic mosaics: have 2 types of cells in their bodies, some cells are 45 X and some 46, XX. This arises when one X is lost during development of a 46, XX zygote. Need both X’s for normal development in human females. MBG*2040 – Miriam Bergeret Deletions and Duplications of Chromosome Segments: a missing chromosome segment is referred to either as a deletion or as a deficiency. Large deletions can be detected cytologically by studying the banding patterns in stained chromosomes, but small ones can’t. in a diploid organism, the deletion of a chromosome segment makes part of the genome hypoploid, which may be associated with a phenotypic effect, especially if the deletion is large.  Cri-du-chat syndrome: this condition is caused by a deletion in the short arm of chromosome 5, the size of the deletion varies. Individuals heterozygous for the deletion and a normal chromosome have the karyotype 46 del(5)(p14), where the terms in parentheses indicate that bands in region 14 of the short arm 9p0 of one of the chromosomes 5 is missing. These individuals may be severely impaired, mentally as well as physically; their plaintive catlike crying during infancy gives the syndrome its name.  Duplication: an extra chromosome segment. The extra segment can be attached to one of the chromosomes, or it can exist as a new and separate chromosome, that is, as a “free duplication”. The organism is hyperploid for part of its genome. As with deletions, this hyperploidy can be associated with a phenotypic effect. Deletions and duplications are two types of aberrations in chromosome structure. Large aberrations can be detected by examination of mitotic chromosomes that have been stained with banding agents such as quinacrine Giemsa. However, small aberrations are difficult to detect in this way and are usually identified by other genetic and molecular techniques. Polyploidy: refers to a numerical change in a whole set of chromosomes. Common in plants, rare in animals. 50% of all known plant genera contain polyploidy species, and 2/3 of all grasses are polyploids. Many of these species reproduce asexually. General effect: cell size is increased, probably because there are more chromosomes in the nucleus. Sometimes this is correlated with an overall size increase of the organism. They tend to produce large seeds or fruits, and therefore provide greater yields in agriculture. Wheat, coffee, bananas, strawberries and cotton are all polyploidy crop plants. Sterile Polyploids: extra sets of chromosomes segregate irregularly in meiosis, leading to grossly unbalanced (aneuploidy) gametes, resulting in zygotes that die. Ex: 3N organism In metaphase, each chromosome will try to pair with its homologues. One possibility is that 2 homologous will pair completely along their length, leaving the third without a partner (univalent). Another possibility is that all 3 homologues will synapse, forming a trivalent in which each member is partially paired with each of the others. In both cases, it is difficult to predict how the chromosomes will move during anaphase fo the first meiotic division. The more likely event is that 2 of the homologues will move to one pole and one homologue will move to the other, yielding gametes with one or two copies of the chromosome. However, all 3 might move to one pole producing gametes with 0 or 3 copies of the chromosome. So the total number of chromosomes in a gamete can vary from 0 to 3n. zygotes formed by fertilization with such gametes are almost certain to die. The many methods of asexual propagation include cultivation from cuttings (bananas), grafts (winesap, gravenstein, Baldwin apples)
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