Genetic Chromosome Abnormalities: Aneuploidy, Polyploidy, and Syndromes

Aneuploidy

Change in the number of individual chromosomes.

Polyploidy

Gain of full extra sets of chromosomes.

Nullisomy (2n-2)

Loss of both members of a homologous chromosome pair.

Monosomy (2n-1) L

oss of one chromosome; only viable human monosomy is XO (Turner Syndrome).

Trisomy (2n+1)

Gain of one chromosome (e.g., trisomy 21).

Tetrasomy (2n+2)

Gain of an extra homologous pair of chromosomes.

Turner Syndrome (XO)

Only viable human monosomy; short height, wide chest, narrowed aorta, infertility.

Down Syndrome (Trisomy 21)

47,XX,+21 or 47,XY,+21; three copies of chromosome 21.

Human Trisomies 13, 18, 8

Usually lethal by age 3; rare survival.

Triplo-X (XXX)

Viable due to X-inactivation; extra Xs become Barr bodies.

Klinefelter Syndrome (XXY)

Extra X inactivated; tall stature, infertility, reduced masculine traits.

Nondisjunction

Failure of chromosomes to separate during meiosis or mitosis.

Meiosis I nondisjunction

Homologous chromosomes fail to separate; 0 normal gametes.

Meiosis II nondisjunction

Sister chromatids fail to separate; 2 normal and 2 abnormal gametes.

Mitotic nondisjunction

Occurs after fertilization; produces mosaic individuals.

Primary Down Syndrome

92% of cases; three full copies of chromosome 21; usually maternal meiosis I nondisjunction.

Familial Down Syndrome

Caused by Robertsonian translocation between chromosomes 14 and 21.

Robertsonian translocation

Fusion of two acrocentric chromosomes; can cause inherited Down syndrome.

Aneuploidy and miscarriage Aneuploidy

is a leading cause of spontaneous miscarriages.

Autopolyploidy

Extra chromosome sets from the same species; often caused by nondisjunction.

Allopolyploidy

Hybridization between two species followed by chromosome doubling.

Polyploidy in animals

Occurs in some fish and amphibians; in humans seen in liver/heart cells and cancer cells.

Bread wheat (Triticum aestivum)

Hexaploid species formed by hybridization and chromosome doubling events.

Allotriploidy (Species I + II)

Three chromosome sets from two species; may have 15 or 18 chromosomes depending on combination.

Autotetraploidy (Species II)

Four chromosome sets from one species; 2n=14 becomes 28.

Trisomy in Species I

2n=8 becomes 9 chromosomes.

Monosomy in Species II

2n=14 becomes 13 chromosomes