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 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.

All have a genotype of Aa or aa No carriers for D.A. Disorders 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 Cystic Fibrosis (Caucasian Disorder) Excess Mucus secretion Phenotypically expressed – at birth; untreated = death by age 5 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) Colorblindness Symptoms: Inability to distinguish colors Phenotypically expressed – at birth Hemophilia Symptoms: Blood lacks clotting Muscular Dystrophy Muscle deterioration – muscle tissue is destroyed. Leads to handicapped life. 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: 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 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.