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Unformatted text preview: Midterm Test #6 Review Chapter 16 Chromosome mutation groups: changes in number and changes in structure Changes in chromosome number, 2 types: o Changes in whole chromosome sets, aberrant euploidy o Changes in parts of chromosome sets, aneuploidy. Monoploid is a normal diploid with only 1 chromosome set; a haploid has only one chromosome set normally (both represented as n ). Monoploids o Males develop by parthenogenesis (development of specialized type of egg into an embryo without the need for fertilization) o Most fail to develop; those that do are characteristically sterile due to deleterious recessive mutations (genetic load) that are masked by wild type in diploid state but defective in the monoploid state. Polyploids o Common in plants o Chromosome number normally an even number of sets o Autopolyploids: multiple chromosome sets originating from within one species o Allopolyploids: sets from 2 or more different (but closely related) species and are only homeologous (partly homologous). Aberrant euploids: correlation between copies of chromosome set and size of the organism; Autopolyploids o Triploids are usually sterile autopolyploids formed from cross of 4n and 2n Sterile because of unpaired chromosomes at meiosis. Therefore, gametes have either haploid (1 chromosome) or diploid (2) number (genomes of this type are known as aneuploid (not euploid) o Aneuploid do not give rise to viable offspring even if mated with a normal haploid (due to gene balance). o Triploid formation both natural and chemically induced (disrupting microtubule polymerization blocking chromosome replication due to spindle fibers) o Colchicine (antitubulin agent) allows S phase to take place but not segregation or cell division; thus, diploid cells lead to tetraploid cells. A/a ; B/b becomes A/A/a/a ; B/B/b/b . o Even in autotetraploid, pairing possibilities are two bivalents, one quadrivalent, or a univalent and a trivalent (sterile aneuploidy) o In the cross of A/A/a/a selfed, the probability of an a/a/a/a zygote is 1/36. First deduce frequency of a/a gametes; can only arise if both pairings are A with a and then both a alleles segregate to same pole. There is a 2/3 chance that the a will pair with an A. With these two A/a pairings, there are two equally likely segregations and therefore of the products will contain both a alleles. Thus the probability of an a/a gamete is 2/3 x = 1/6. To get a/a/a/a, 1/6 x 1/6 = 1/36 Allopolyploids: plant that is a hybrid of two or more species, containing two or more copies of each of the input genomes o Radish and cabbage crossed to produce viable hybrid, yet it was functionally sterile because the chromosomes were different enough from each other that the pairs did not synapse and segregate normally at meiosis, thus unable to produce functional gametes o However, spontaneous chromosome doubling occurred to produce fertile, viable progeny. n1 + n2 fuse with other gametes to form 2n1 + 2n2; sometimes called progeny....
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- Fall '07