Download Modes of Inheritance

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
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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
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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.