Human Chromosomes and Chromosome Behavior 5 Human Chromosomes and Chromosome Behavior

Karyotype = stained and photographed preparation of metaphase chromosomes arranged according to their size and position of centromeres

Human Chromosomes Each chromosome in karyotype is divided into two regions (arms) separated by the centromere p = short arm (petit); q = long arm p and q arms are divided into numbered bands and interband regions based on pattern of staining Within each arm the regions are numbered.

Centromeres • Chromosomes are classified according to the relative position of their centromeres In metacentric it is located in middle of chromosome In submetacentric—closer to one end of chromosome In acrocentric—near one end of chromosome Chromosomes with no centromere, or with two centromeres, are genetically unstable

Abnormal Chromosome Numbers Aneuploid = unbalanced set of chromosomes = relative gene dosage is upset (example: trisomy of chromosome 21) Monosomic = loss of a single chromosome copy Polysomic = extra copies of single chromosomes Most chromosome abnormalities lethal, frequently in spontaneous abortions. Exceptions are trisomy 13, trisomy 18, and trisomy 21 (Down syndrome), and the Sex chromosomes

An extra X or Y chromosome usually has a relatively mild effect. Why? 1) X chromosme inactivation/Dosage Compensation 2) Not much (essential) on the Y EXAMPLES: Trisomy-X = 47, XXX (female) Double-Y = 47, XYY (male) Klinefelter Syndrome = 47, XXY (male, sterile) Turner Syndrome = 45, X (female, sterile)

Chromosome Abnormalities Deletions/Duplications Inversions Translocations

Deletions Duplications

Chromosome Inversions Inversions = genetic rearrangements in which the order of genes in a chromosome segment is reversed Inversions do not alter the genetic content but change the linear sequence of genetic information In an inversion heterozygote, chromosomes twist into a loop in the region in which the gene order is inverted Chromosome Inversions A B C C B A B A C

Paracentric inversion Does not include centromere Crossing-over produces one acentric (no centromere) and one dicentric (two centromeres) chromosome Pericentric inversion Includes centromere Crossing-over results in duplications and deletions of genetic information

Reciprocal Translocations Adjacent-2 segregation: homologous centromeres stay together at anaphase I; gametes have a segment duplication and deletion Alternate segregation: half the gametes receive both parts of the reciprocal translocation and the other half receive both normal chromosomes; all gametes are euploid, i.e have normal genetic content, but half are translocation carriers

Polyploidy Polyploid species have multiple complete sets of chromosomes The basic chromosome set, from which all the other genomes are formed, is called the monoploid set The haploid chromosome set is the set of chromosomes present in a gamete, irrespective of the chromosome number in the species. Polyploids can arise from genome duplications occurring before or after fertilization Through the formation of unreduced gametes that have double the normal complement of chromosomes or Through abortive mitotic division, called endoreduplication.

Polyploids can generate new species

A seedless watermelon is a sterile hybrid which is created by crossing male pollen for a watermelon, containing 22 chromosomes per cell, with a female watermelon flower with 44 chromosomes per cell.

The karyotype of the Chinook salmon has been characterized as 2N = 68, with 16 pairs of metacentric chromosomes and 18 pairs of acrocentric chromosomes (Simon 1963)