Inheritance Patterns• Things that cause alterations in chromosome

number or pattern lead to genetic disorders.– Alterations in chromosome number - aneuploidy

• caused by nondisjunction – failure of a chromosome to separate during meiosis

» if nondisjunction is of a single chromosome the zygote will either be monosomic or trisomic

» if nondisjunction is of a entire genome the zygote will exhibit popolyploidy• Triploidy, tetraploidy, …• Very common in plants but not viable in animals

monosomic trisomic

Examples of Syndromes Associated with Non-disjunction

– monosomic - Turner Syndrome (XO) • only viable monosomic disorder in humans • 1/500 births • sterile, less developed secondary sex characteristics (better with estrogen therapy)

– trisomic • Down Syndrome (trisomy 21)

– 1/700 births – part of the pre-birth screening panel – short stature, heart defects, prone to respiratory infections, mental retardation – risk increases dramatically for childbearing women over the age of 30

• XXX – Normal outwardly – 1/1000 births

• XXY – Klinefelter syndrome – under developed genitalia – sterile – some female sex characteristics - fleshy breasts – 1/2000 live births

• XYY • may be taller but otherwise indistinguishable

Alteration in Chromosome Structure

• homologous alterations - often lethal – deletion

• can cause a frame-shift affecting all genes down stream

• cri du chat syndrome – deletion in chromosome 5 – cry sounds like a mewing cat,

microcephaly, retardation, death in early childhood

– duplication • Caused by the attachment of

a deleted region during meiosis

• Causes gene dosing errors

– inversion • Reverse attachment of a

deleted region during meiosis • Causes reading frame errors

in genes

– non-homologous alterations • translocation

– attachment of a deleted region from a non-homologous chromosome

– reciprocal translocations occur when neighboring chromosomes cross over » gene number is conserved although gene dosing

secondary to gene position may be affected – implicated in many cancers and leukemia

Exceptions to Mendelian genetics• Genomic imprinting

– effect of the depends on the sex of the person inherited from – occurs during the formation of gametes – certain genes are turned off in the sperm & the ova

• insulin-like growth factor (used in prenatal growth) – only the paternal version is expressed

• Inherited organelle genes (extranuclear genes) – circular DNA found in the mitochondria and chloroplasts – inherited in the cytoplasm of the ova (maternal) – mitochondrial myopathy

• weakness, intolerance of activity, muscle deterioration