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

Aneuploidy (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.

Examples of Aneuploidy Nullisomy (2n -2) Usually lethal in diploids Tolerated in some polyploids Monosomy (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 species Disomy (n + 1) Phenomenon in haploid organisms

Meiotic Nondisjunction Generates Aneuploid Products Refer to Figure 17-12, Griffiths et al., 2015.

Examples of Aneuploidy Sex Chromosomes (Human X and Y) XO : Turner Syndrome XXY: Klinefelter Syndrome XXX: Triplo-X Syndrome XYY: XYY Syndrome Autosomes Trisomy 13: Patau Syndrome Trisomy 18: Edward’s Syndrome Trisomy 21: Down Syndrome

Heterochromatin Features of Constitutive Heterochromatin Present at homologous sites on pairs of chromosomes Always genetically inert DNA sequences that are not organized into genes 4. Telomeric and centromeric regions Features of Facultative Heterochromatin Referred to as silent chromatin Potential to become heterochromatic (Barr body) 3. Genetic information that is not expressed once chromatin becomes condensed

Mammalian 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?

X 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.

Mammalian X Chromosome Inactivation Inactivated X chromosome becomes highly condensed, darkly staining structure called Barr body. State of inactivation is transmitted down the cell lineage.

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

The fates of a million implanted human zygotes 50% of spontaneous abortions are associated with chromosome abnormalities. Over half of these are trisomics, with trisomy 21 accounting for 9% of them. 18% of abnormalities are the Turner condition. ~ 0.6% of birthed infants have chromosome abnormalities. ~ 36% of birthed infants with chromosome abnormalities are aneuploidy of sex chromosome. ~ 23% of birthed infants with chromosome abnormalities are trisomics. Refer to Figure 17-37, Griffiths et al., 2015.

Relationship between Genotype and Phenotype Molecular Basis for Relationship between Genotype and Phenotype genotype DNA DNA sequence transcription RNA translation amino acid sequence protein function phenotype organism