Chapter 6Variation in ChromosomeStructure and Number José A. Cardé, PhD Universidad Adventista de las Antillas Agosto 2013
Chapter Outline • Cytological Techniques • Polyploidy • Aneuploidy
Cytological Analysis Celulas en division celular /metafase Danar el huso
Cytological Variation: An Overview • Changes in ploidy afecta el fenotipo • Euploid organisms with complete or normal sets of chromosomes (diploid = 2n; triploid = 3n; tetraploid = 4n) • Aneuploid organisms have particular chromosomes or parts of chromosomes under- or over-represented, (non euploid) • Aneuploidy implies a genetic imbalance; polyploidy does not. • Rearrangements are changes in chromosome structure.
Key Points • Cytogenetic analysis usually focuses on chromosomes in dividing cells. • Dyes such as quinacrine and Giemsa create banding patterns that are useful in identifying individual chromosomes within a cell. • A karyotype shows the duplicated chromosomes (metafase) of a cell arranged for cytogenetic analysis.
Extra sets of chromosomes in an organism can affect the organism’s appearance and fertility. La presencia de sets extra de cromosomas. Polyploidy
Polyploidy is Common in Plants Crisantemos 4n, Strawberry 8n, algodon 4n y guineo 3n
Sterile Polyploids • - Meiosis en un triploide • - Hay sinapsis entre 2 de los 3 homologos • Queda un univalente para moverse solo en anafase • o… • Se forma un trivalente que se puede mover como una unidad • en la anafase 2 puede que se separe otra vez
Fertile Polyploids • Diploides se cruzan y forman un hibrido • esterilidad por meiosis irregular • si hay duplicacion meiosis puede ocurrir • cada cual encuentra su pareja y se segregan normal, • habra gametos y fertilidad
Allopolyploids vs. Autopolyploids • Allopolyploids are created by hybridization between different species. • Autopolyploids are created by chromosome duplication within a species. • Chromosome doubling is a key event in the formation of polyploids. • Mitosis sin Citoquinesis • Produccion de gametos no reducidos
Tissue-specific Polyploidy and Polyteny • Endomitosis involves chromosome replication and separation of sister chromatids without cell division. This produces polyploid tissues. • If sister chromosomes do not separate, the resulting chromosomes are polytene.
The Polytene Chromosomes of Drosophila • Drosophila polytene chromosomes are produced by 9 rounds of replication. • 1) Homologous polytene chromosomes pair. • 2) All of the centromeres congeal into a chromocenter.
Key Points • Polyploids contain extra sets of chromosomes. • Many polyploids are sterile because their multiple sets of chromosomes segregate irregularly in meiosis. • Polyploids produced by chromosome doubling in interspecific hybrids may be fertile if their constituent genomes segregate independently. • In some somatic tissues—for example, the salivary glands of Drosophila larvae—successive rounds of chromosome replication occur without intervening cell divisions and produce large polytene chromosomes that are ideal for cytogenetic analysis.
The under- or overrepresentation of a chromosome or a chromosome segment can affect a phenotype. Aneuploidy
Vocabulary • Aneuploidy—a numerical change in part of the genome • Trisomy—triplication of one chromosome • Hypoploid—an organism in which a chromosome or chromosome segment is underrepresented • Hyperploid—an organism in which a chromosome or chromosome segment is overrepresented • Monosomy—the absence of one chromosome in an otherwise diploid individual
Down Syndrome:AHuman Trisomy 47, XX, +21
Chromosome Mosaics • Turner Syndrome Somatic Mosaics (two types of cells: 45, X and 46, XX cells) - loss of an X chromosome during development • Drosophila gynandromorphs (XX/XO mosaics); sex determined by the ratio of X chromosomes to autosomes: are part female and part male (XX females, XO males) • 45X no have Barr bodies, the only X is active
Key Points • In a trisomy, such as Down Syndrome in humans, three copies of a chromosome are present; in a monosomy, such as Turner Syndrome in humans, only one copy of a chromosome is present. • Aneuploidy may involve the deletion of duplication of a chromosome segment. • Asignned: deletions, duplications, inversions, translocations, robertsonian translocations