1. 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.

2. 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)

3. Examples of Dom. Allele Disorders
Huntington’s Disease - Neurological Disorder
Phenotypically expressed between age
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.

4. 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

5. 2.Recessive Allele Disorders
Genotypes:
AA = Normal (No Disease)
Aa = Carriers (No Disease)
aa = Have disease

6. 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

7. 3. Sickle Cell Anemia (African American Disorder)
RBC’s defective hemoglobin (substitution point mutation)
Phenotypically expressed – at birth

8. 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

9. 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)

10. 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 ( 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.