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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 • 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 Alteration in Chromosome Structure – 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