Types of Chromosome Mutations Refer to Figure 17-2, Griffiths et al., 2015.

Chromosome Mutations Deletion/Deficiency Duplication Inversion A B C D E F A C D E F Deletion/Deficiency A B C D E F A B B C D E F Duplication A B C D E F A E D C B F Inversion A B C D E F A B C D J K Translocation G H I J K G H I E F

Pairing in pericentric inversion heterozygotes and resulting meiotic products.

Pairing in paracentric inversion heterozygotes and resulting meiotic products. Anaphase bridge results in random breakage of chromosomal material. 2 of 4 meiotic products are not genetically balanced and will not produce viable gametes.

Possible effects of inversion at the molecular level No disruption of any gene. Chromosomal rearrangement is the only result. Disruption of one gene by chromosomal breakage. Disruption of two genes and fusion of those two genes.

Chromosome Mutations Deletion/ Deficiency Duplication Inversion A B C D E F A C D E F Deletion/ Deficiency A B C D E F A B B C D E F Duplication A B C D E F A E D C B F Inversion A B C D E F A B C D J K Translocation G H I J K G H I E F

Translocation In reciprocal translocation, exchange of chromosomal segments between two nonhomologous chromosomes establishes new linkage groups. A B C D E F G H G C D E F A B H In Robertsonian translocation, long arms of two acrocentric chromosomes are combined to form one large chromosome and one small chromosome. If the short metacentric chromosome does not contain essential genetic information, it could be lost without any consequence to viability.

Reciprocal Translocation heterozygotes are semisterile. 50% of gametes are genetically unbalanced. In plants, these gametes are not viable. In animals, zygotes that are formed by these gametes are not viable. Adjacent segregation produces genetically unbalanced gametes. Alternate segregation produces genetically balanced gametes.

Down Syndrome and Translocation Heterozygote Down syndrome is caused by trisomy 21 (3 copies of chromosome 21). 95% of Down syndrome cases are associated with nondisjunction and shows no familial recurrence. The other 5% (familial Down syndrome) is attributed to Robertsonian translocation between chromosome 21 and chromosome 14.

Chromatin and Gene Expression Heterochromatin Contains methylated histones (H3) Associated with heterochromatin protein-1 (HP-1) Transcriptionally Active Euchromatin Contains hyperacetylated histones Prevention of Heterochromatin Formation DNA elements (barrier insulators) promote binding of histone acteyltransferase

Gene Silencing is Caused by the Spread of Heterochromatin When a chromosome mutation places a gene next to heterochromatin, the gene can become inactivated. Inversion, deletion, duplication, and translocation can place a gene next to heterochromatin. Refer to Figure 12-25, Griffiths et al., 2015. Heterochromatin May Spread Farther in Some Cells Than in Others

Position-effect Variegation A heterozygote for a gene and a translocation can show variegated phenotype for that gene. Position-effect variegation is exhibited by this w+/w heterozygote. Wild-type allele is no longer wild-type in its expression in some of the eye facets. Any chromosomal change that places a locus next to heterochromatin can result in inactivation of that gene. A tissue or organ that is comprised of a mixture of cells that express one or the other phenotype exhibit this variegation.

Chromosome Mutations Deletion/Deficiency Duplication Inversion A B C D E F A C D E F Deletion/Deficiency A B C D E F A B B C D E F Duplication A B C D E F A E D C B F Inversion A B C D E F A B C D J K Translocation G H I J K G H I E F

Deletions a a Deletions can be terminal or interstitial. An intragenic deletion inactivates a gene; multigenic deletions affect several genes. Homozygous condition for chromosomal deletion is often lethal. Chromosome deletion cannot revert. Synapsis of a normal chromosome and a deletion chromosome produces a deletion loop during meiosis. In heterozygous deletions, recombination frequencies between genes outside the deletion are lower than normal. a a normal chromosome deletion chromosome Recessive alleles covered by a deletion are unmasked and are expressed phenotypically. This effect is known as pseudodominance.

Deletions Cancer cells often harbor chromosome mutations. Cells of a tumor do not always show the same chromosome mutation. Chromosome-specific deletions are associated with certain tumors.

Chromosome Mutations Deletion/Deficiency Duplication Inversion A B C D E F A C D E F Deletion/Deficiency A B C D E F A B B C D E F Duplication A B C D E F A E D C B F Inversion A B C D E F A B C D J K Translocation G H I J K G H I E F

Duplications Arrangement of a duplication can be in tandem or reverse order. Several pairing possibilities exist in heterozygotes of a normal chromosome and a side-by-side duplication. Heterozygous condition results in a loop structure during meiosis. Duplications (and higher order duplications) can occur by unequal crossing-over after asymmetric synapsis of chromosomes.

Types of Chromosome Mutations Refer to Figure 17-2, Griffiths et al., 2015.

Changes in Chromosome Number I. Classes A. Aberrant Euploidy B. Aneuploidy II. Euploidy A. Monoploid (x): chromosome number in a basic set B. Euploidy: multiples of monoploid number C. Polyploid: euploid with > two sets of chromosomes III. Examples of Polyploidy A. Autopolyploidy : multiple chromosome sets from one species B. Allopolyploidy: chromosome sets from different species

Refer to Table17-1, Griffiths et al., 2015.

Colchicine Induces Polyploidy Refer to Figure 17-5, Griffiths et al., 2015.

Origin of the Amphidiploid Raphanobrassica Refer to Figure 17-7, Griffiths et al., 2015.

Origin of Three Allopolyploid Species of Brassica Refer to Figure 17-8, Griffiths et al., 2015.

Polyploidy in Animals Parthenogenesis - development of unfertilized egg into embryo polyploidy in leeches, flatworms, brine shrimp polyploidy in salamanders, lizards Polyploid frogs and toads undergo sexual reproduction. Polyploid fish (such as salmon, trout) are not unusual. Triploid oysters are of economic value. In general, polyploid mammals are not viable.