Types of Chromosome Mutations

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 diploidsTolerated 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

Turner SyndromeXO

Klinefelter SyndromeXXY

Meiotic Products of a Trisomic

Characteritstics of Down Syndrome (Trisomy 21)

The Maternal-Age Effect in Down Syndrome

Trisomy 13: Patau Syndrome

Rarely do fetuses go to term. Rarely do babies survive.

Symptoms include:

* cleft palate* atrial septal defect* inguinal hernia

Polydactyly is common.

Trisomy 13: Patau Syndrome

Severity of symptoms is variable.

7-year old survivor (deaf and legally blind).

Others are not as fortunate.

CYCLOPIA

Trisomy 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

Trisomy 18: Edward’s Syndrome

These two girls are rare survivors.

Heterochromatin

Features of Constitutive Heterochromatin

1. Present at homologous sites on pairs of chromosomes

2. Always genetically inert

3. DNA sequences that are not organized into genes

4. Telomeric and centromeric regions

Features of Facultative Heterochromatin

1. Referred to as silent chromatin

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