Chapter 8 Human Karyotypes and Chromosome Behavior Jones and Bartlett Publishers © 2005

Changes in chromosome structure - deletions - duplications - inversions - translocations

Deletions/Deficiencies Interstitial deficiency Terminal deficiency

Terminal Deficiencies  unstable chromosomes

Haplo-insufficient genes Heterozygotes for a deficiency may have a mutant phenotype. If two working copies of a gene are required for normal gene function, only havinog one is a problem. ‘Cri du Chat’ syndrome in humans: Deficiency of a part of chromosome 5

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

Duplications Mutations can produce an extra copy of a part of a chromosome called a duplication. Tandem duplication – same sequence, adjacent to the original copy. Reverse tandem duplication – opposite orientation. Displaced duplication Small free chromosome

Unequal crossing over of misaligned repeat sequences leads to gain or loss of repeats

Unequal crossing over involving eye pigment genes

Inversions

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

Synapsis between chromosomes bearing inversions requires the formation of an Inversion Loop

If there is no crossing over inside the loop  chromosomes disjoin normally

Crossing over in an inversion loop for a pericentric inversion  duplications and deletions

Crossing over in an inversion loop for a paracentric inversion  messed-up chromosomes

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

Translocations Broken chromosomes can reattach to different chromosomes that also were fragmented. Exchanges between nonhomologous chromosomes are called translocations. Reciprocal translocations – distal portions of two chromosomes are exchanged. Transposition – insertion into another chromosome.

Structure of chromosomes with a reciprocal translocation

Pairing and segregation of chromosomes with a reciprocal translocation during meiosis I

Robertsonian translocation Exchange of entire arms between chromosomes. This can result in change of chromosome number in a monoploid set. Can occur by fusion of two acrocentric chromosomes, or a split in a metacentric into two. Robertsonian translocations can result in barriers, speciation.

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 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.