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Transcript
Chapter 11 Chromosome Mutations • Changes in chromosome number • Chromosomal rearrangements • Evolution of genomes Aberrant chromosome constitutions of a normally diploid organism Name Monoploid Designation Constitution Chromosome number n ABC 3 Diploid 2n AABBCC 6 Triploid 3n AAABBBCCC 9 Tetraploid 4n AAAABBBBCCCC 12 Monosomic 2n-1 ABBCC 5 Trisomic 2n+1 AABBCCC 7 Changes in chromosome number About 70% of the plant species can be considered as polyploids. Polyploidization is often used in plant breeding. Euploidy Organisms with multiples of the basic chromosome set are called euploid. The size of epidermal leaf cells of tobacco increases with the ploidy level. Generating monoploid plants by tissue culture Pollen (haploid) are grown in tissue culture in the presence of certain plant hormones. Haploid embryoids develop that can be grown into mature monoploid plants. Advantage for plant breeding? The use of colchicine to generate a diploid from a monoploid. Colchicine disrupts spindle fibre formation during mitosis, preventing the migration of chromatids after the centromere is split. A single cell results that contains pair of identical chromosomes that are homozygous at all loci. The use of colchicine to generate a diploid from a monoploid. Why are monoploid plants produced? Is there any genetic advantage? Monoploids Male bees, wasps and ants are monoploid. These males develop parthogenetically from unfertilized eggs. However, in most species, monoploids are abnormal individuals. Polyploids are distiguished in autopolyploids, that contain multiple chromosome sets from one species and allopolyploids that are composed of chromosome sets from two or more different species. Allopolyploids form only between closely related species. Triploids are usually autopolyploids. They can result from a cross of a tetraploid with a diploid. Polyploids with odd numbers of chromosome sets are sterile, because their gametes are aneuploid. (Note that each chromosome is really a pair of chromatids). Meiotic pairing possibilities in tetraploids. (Note that each chromosome is really a pair of chromatids). Functional gametes Nonfunctional gametes Complex segregation ratios result from tetraploids The amphidiploid (or allotetraploid) Raphanobrassica originates from cabbage (Brassica) and radish (Raphanus) The amphidiploid Triticale originates from rye (Secale) and wheat (Triticum) The evolution of modern hexaploid wheat, in which amphidiploids are produced at two points. A, B, D are different chromosome sets. Aneuploidy Chromosome number differs from wild type by part of the chromosome set. Aneuploid organisms are produced mainly by nondisjunction at meiosis. Monosomics (2n-1) Turner syndrome (X0) Trisomics (2n+1) Klinefelter syndrome (XXY) Trisomics (2n+1) Down syndrome (trisomy 21) Maternal age and the incidence of trisomy 21 What is the reason that the sex chromosomes or chromosome 21 are often observed in human aneuploids? The concept of gene balance. Dosage compensation X-chromosome inactivation Chromosomal rearrangements Origins of chromosomal rearrangements Origins of chromosomal rearrangements Inversions A B C D E F paracentric A B C E D F pericentric A D B E F normal sequence C Inversion heterozygotes pair in a loop at meiosis Synaptonemal complexes in mouse heterozygous for a paracentric inversion Meiotic products resulting from a single crossover within a paracentric inversion loop. Two nonsister chromatids cross over within the loop. Meiotic products resulting from a meiosis with a single crossover within a pericentric inversion loop. Message Heterozygous inversions are characterized by reduced fertility from unbalanced or deleted meiotic products. Reciprocal translocations The meiotic products resulting from two most commonly encountered chromosome segregation patterns in a reciprocal translocation heterozygote. A semisterile maize plant producing normal and aborted pollen. The clear pollen grains contain chromosomally unballanced meiotic products of a reciprocal translocation heterozygote. The opaque grains are normal. Message Heterozygous reciprocal translocations are characterized by semisterility. Message Chromosomal rearrangements lead to reduced fertility and contribute to speciation. Position-effect variegation Evolution of genomes and speciation 1. The genomes are made up of many duplicated segments 2. The genomes of two related species are characterized by many structural changes. Chromosomal synteny Chromosomal segments of two species that contain similar genes in an identical order are said to be synthenic. Syntheny of human and mouse chromosomes