Transcript Chapter 9
Chapter 8 Human Karyotypes and Chromosome Behavior The banding patterns of human chromosomes The short arm of a chromosome is called “p” and the long arm “q”. A, B, C, etc. refer to grouping of chromosomes by size. The 3 shapes of chromosomes based on the location of the centromere The proposed mechanism for the creation of the long human chromosome 2 by fusion of two shorter chromosomes In human females, one of the 2 X-chromosomes is inactivated early in development Proposed steps in the creation of the modern Y-chromosome by rearrangement of segments in an ancient Y-chromosome Distribution of Y-chromosome haplotypes, presumed to have descended from Genghis Khan The probability of a child with Down’s syndrome increases with Mother’s age Pairing and segregation of 3 homologous chromosomes in meiosis I This kind of chromosome pairing occurs in trisomics (2n+1) or triploids (3n) Mapping the deletion of part of a chromosome segment by testcrosses and uncovering of recessive genes Mapping of genes in Drosophila using overlapping deletions and polytene chromosomes Unequal crossing over of misaligned repeat sequences leads to gain or loss of repeats Unequal crossing over involving eye pigment genes Mechanism of creation of a chromosomes with an inverted segment Pairing of homologous chromosomes in an inversion heterozygote An inversion which does not involve the centromere is called a paracentric inversion Absence of recombination within an inversion loop does not create deletions or duplications A crossover within an inversion loop of a paracentric inversion creates dicentric and acentric chromosomes A crossover within an inversion loop of a pericentric inversion creates chromosomes with deletion and duplication When an inversion involves the centromere, it is called a pericentric inversion. Crossing over in a pericentric inversion does not create dicentric and acentric chromosomes Structure of chromosomes with a reciprocal translocation Pairing and segregation of chromosomes with a reciprocal translocation during meiosis I Mechanism of creation of a Robertsonian translocation Pairing and segregation with a Robertsonian translocation involving human chromosomes 14 and 21 Such a translocation results in a high probability of having a child with Down’s syndrome. Variegation (mottling) of eye color due to positioning of the eye color gene near centromeric heterochromatin When the expression of a gene is affected by its location on a chromosome (even though the gene itself is not changed), such a variation is called “position effect” Two kinds of polyploidy Multiplication of the entire chromosome complement is called polyploidy. When all the genomes are the same, it is called autopolyploidy. When two (or more) different genomes are duplicated, it is called allopolyploidy. Formation of a tetraploid organism Creation of a totally homozygous diploid cell by doubling of chromosome number in a monoploid cell by colchicine Monoploid cells can only be grown in plants. In humans, the only viable monoploid cells are the egg and the sperm. Monoploidy in somatic cells is lethal. Evolution of wheat genome through allopolyploidy Evolution of wheat genome