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Modes of Inheritance • Genetic Disorders: A disease or debilitating condition that has a genetic basis (carried by genes on chromosomes) • Genetic Disorders are classified in 4 categories • • • • 1. Dominant Allele Disorders 2. Recessive Allele Disorders 3. Sex-linked Disorders 4. Non-disjunction Disorders. 1. Single Allele Traits (Dominant Allele Disorders) • Controlled by a single allele of a gene. • Genotypes: –AA = Lethal (never born) –Aa = Have Disorder; –aa = Normal (No Disease) Examples of Dom. Allele Disorders 1. Huntington’s Disease - Neurological Disorder – Phenotypically expressed between age 35 -45 – Carried on Chromosome #4. - Dr. Nancy Wexler discovered a genetic marker for HD allele – short segment of DNA inherited by family members who carry the harmful allele but not by those who do not have the disease. This marker is a strong indicator of presence: 96% that have marker develop HD. 2. Alzheimer’s Disease – Neurological Disorder – Pheno expressed after 65 3. Cataracts – Pheno expressed after 60 4. Achondroplasia (dwarfism) – Pheno expressed after at birth (1/25,000 births) • All have a genotype of Aa or aa • No carriers for D.A. Disorders 2.Recessive Allele Disorders • Genotypes: –AA = Normal (No Disease) –Aa = Carriers (No Disease) –aa = Have disease Examples of Recessive Allele Disorders 1. Cystic Fibrosis (Caucasian Disorder) – Excess Mucus secretion – Phenotypically expressed – at birth; untreated = death by age 5 2. Tay – Sachs Disease (Jewish Disorder - 1 in 3,500 births) – Lipid accumulation in brain cells -- lysosomes lack a lipid digesting enzyme – Phenotypically expressed – at birth 3. Sickle Cell Anemia (African American Disorder) – RBC’s defective hemoglobin (substitution point mutation) – Phenotypically expressed – at birth 3. X-Linked Traits (all recessive X-linked) (Not only disorders carried on X chromosome) 1. Colorblindness • • Symptoms: Inability to distinguish colors Phenotypically expressed – at birth 2. Hemophilia • Symptoms: Blood lacks clotting • Phenotypically expressed – at birth 3. Muscular Dystrophy • Muscle deterioration – muscle tissue is destroyed. Leads to handicapped life. • Phenotypically expressed – at birth Males more likely to have X-Linked disorders 4. Non Disjunction Disorders • All non disjunction disorders is due to the inability of the homologous chromosomes to separate during Meiosis II (Anaphase II). • A gamete receives 2 copies of a chromosome • At fertilization – 3 copies of a chromosome (or only 1 copy). – Example: Trisomy 21 (Down’s Syndrome) Genetic Screening (if family history of disorder). • 2 Methods of checking karyotype of fetus: 1. Amniocentesis – Removal of amniotic fluid around the amniotic sac surrounding the fetus (14 -16 weeks). Analyze fetal cells by making a karyotype or identifying proteins in the fluid 2. Chorionic Villi Sampling – Removal of a small patch of embryonic tissue that grows between the mother’s placenta and uterus (8 -10 weeks). Analyze fetal cells by making a karyotype or indentifying proteins in the tissue.