2Aneuploidy (2n +/- m)Aneuploids differ from wild-type individuals by part of a chromosome set.Nondisjunction is a common mechanism for aneuploidy.Aneuploidy can be the cause of disorders such as Down Syndrome.
3Examples of Aneuploidy Nullisomy (2n -2)Usually lethal in diploidsTolerated in some polyploidsMonosomy (2n - 1)Gene imbalance results (missing chromosome)Expression of genes on lone chromosome (hemizygosity)Trisomy (2n + 1)Gene imbalance results (extra chromosome)Viability depends on chromosome and speciesDisomy (n + 1)Phenomenon in haploid organisms
9Trisomy 13: Patau Syndrome Rarely do fetuses go to term. Rarely do babies survive.Symptoms include:* cleft palate* atrial septal defect* inguinal herniaPolydactyly is common.
10Trisomy 13: Patau Syndrome Severity of symptoms is variable.7-year old survivor (deaf and legally blind).Others are not as fortunate.CYCLOPIA
11Trisomy 18: Edward’s Syndrome Rarely do fetuses go to term. Rarely do babies survive.Symptoms include:* cleft palate* small face* short sternum* low-set ears* overlapping fingers* clenched fist* clubfeet
12Trisomy 18: Edward’s Syndrome These two girls are rare survivors.
14Heterochromatin Features of Constitutive Heterochromatin Present at homologous sites on pairs of chromosomesAlways genetically inertDNA sequences that are not organized into genes4. Telomeric and centromeric regionsFeatures of Facultative HeterochromatinReferred to as silent chromatinPotential to become heterochromatic (Barr body)3. Genetic information that is not expressed once chromatin becomes condensed
15Mammalian X Chromosome Inactivation Calico cats are usually females heterozygous for orange allele and black allele at an X-linked locus.Male calico cats are very rare. Why is this the case?
16X Chromosome Inactivation Most genes on the inactivated X chromosome are silenced (turned off, not expressed).Genes on the inactivated X chromosome remain silenced in descendant cells.Heritable alteration of this type is an example of epigenetic inheritance.Chromosomal DNA sequence remains unchanged.
17Mammalian X Chromosome Inactivation Inactivated X chromosome becomes highly condensed, darkly staining structure called Barr body. State of inactivation is transmitted down the cell lineage.