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Chapter 6 Notes.docx

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Wilfrid Laurier University
Robert Boulianne

Chapter 6 – Chromosomal Mutations • An organism or cell is euploid when it has one complete set of chromosomes, or exact multiples of complete sets. – Eukaryotes that are haploid or diploid are euploid, as are other organisms with variable numbers of chromosome sets. • Aneuploidy results from variations in the number of individual chromosomes (not sets), so that the chromosome number is not an exact multiple of the haploid set of chromosomes. • Aneuploidy can occur due to nondisjunction during meiosis. – Nondisjunction in meiosis I will produce four gametes, two with a chromosome duplicated, and two that are missing a chromosome. • Fusion of a normal gamete with one containing a chromosomal duplication will produce a zygote with three copies of that chromosome, and two of all the others. • Fusion of a normal gamete with one missing a chromosome will result in a zygote with only one copy of that chromosome and two of all others. • Nondisjunction during meiosis II produces two normal gametes and two that are abnormal (one with two sibling chromosomes and one with that chromosome missing). – Fusion of abnormal gametes with normal ones will produce the genotypes discussed previously. – Normal gametes are also produced and when fertilized will produce normal zygotes. • More complex gametic chromosome composition can result when: – More than one chromosome is involved. – Nondisjunction occurs in both meiotic divisions. – Nondisjunction occurs in mitosis (result is somatic cells with unusual chromosome complements). Aneuploidy • Autosomal aneuploidy is not well tolerated in animals, and in mammals is detected mainly after spontaneous abortion. Aneuploidy is much better tolerated in plants. • There are four main types of aneuploidy: – Monosomy involves loss of a single chromosome (2N-1) – Nullisomy involves loss of one homologous chromosome pair (cell is 2N-2) – Trisomy involves one extra chromosome, so the cell has three copies of one, and two of all the others (2N + 1) – Tetrasomy involves an extra chromosome pair, so the cell has four copies of one and two of all the others Meiotic Consequences • Some types of aneuploidy have serious meioitic consequences. – A monosomic cell (2N-1) • May produce gametes that are N (normal) and N-1 (monosomic). • Or, the unpaired chromosome may be lost completely, producing gametes that are all N-1. Trisomy-21 (Down Syndrome) • Occurs in an estimated 3,510 per 1 million conceptions and 1,430 per 1 million live births. • Down syndrome individuals are characterized by: – Low IQ – Epicanthal folds over eyes – Short and broad hands – Below-average height – Simian Crease Trisomy-13 (Patau sydrome) • Occurs in 2/10,000 live births, most die within the first three months. – Cleft lip and palate. – Small eyes – Polydactyly (extra fingers and toes). – Mental and developmental retardation. – Cardiac and other abnormalities. Trisomy-18 (Edwards syndrome) • Occurs in 2.5/10,000 live births. • 90% die within 6 months. • 80% are female. • Characteristics: – Small size with multiple congenital malformations throughout the body. – Clenched fists. – Elongated skull. – Low-set ears. – Mental and developmental retardation. Monoploidy and Polyploidy • Can result when either round of meiotic division lacks cytokinesis, or when meiotic nondisjunction occurs for all chromosomes. – Complete nondisjunction in meiosis I will produce 1/2 gametes with normal chromosomes, 1/4 with two sets and 1/4 with no sets of chromosomes. – Gametes with two sets of chromosomes fused with a normal gamete produces a triploid (3N) zygote. – Fusion of two gametes that each have two sets of chromosomes produces a tetraploid (4N) zygote. • Polyploidy of somatic cells can result from mitotic nondisjunction of complete chromosome sets. Polyploidy • Two clases based on number of chromosome sets: – Even-number polyploids are more likely to be at least partially fertile, because the potential exists for equal segregation of homologs during meiosis. – Odd-number polyploids will always have unpaired chromosomes. Balanced gametes are rare and these organisms are usually sterile or have increased zygote death. Triploids • Unstable in meiosis because random segregation means that balanced gametes (either N or 2N) are rare. n – The probability of a triploid organism producing a haploid gamete is (1/2) where n is the number of chromosomes. – Triploidy is always lethal in humans, accounting for 15-20% of spontaneous abortions and 1/10,000 live births, with most dying in the first month. – Tetraploidy in humans is also lethal, usually before birth, accounting for 5% of spontaneous abortions Polyploidy in Plants • Polyploidy is more common in plants, probably due to self fertilization. – Allows even-number polyploids to produce fertile gametes and reproduce. • Two types: – Autopolyploidy and Allopolyploidy. Autopolyploidy • Results when all sets of the chromosomes are from the same species. – Usually due to meiotic error. – Fusion of a diploid gamete with a haploid one produces a triploid organism. • Eg. Grasses, garden flowers, crop plants and forest trees. • Eg. “Seedless” fruits such as bananas, grapes and watermelons. Allopolyploidy • Results when the chromosomes are from two different organisms. – Fusion of haploid gametes followed by chromosome doubling. • Eg. Fusion of haploid gametes from plant 1 and plant 2 produces and N + 1 2 hybrid plant. No chromosomal pairing occurs in meiosis, viable gametes are not produced and the plants are sterile. • Rarely, division error doubles the chromosome sets (2N + 2N ). The diploid sets 1 2 function normally in meiosis and fertile allotetraploid plants result. Types
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