Law: Pedigrees and Inheritance

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Modes of Inheritance, recessive disorders, penetrance, and expressivity

Modes of Inheritance for Single Gene Characteristics

1. Autosomal Recessive
2. Autosomal Dominant
3. X-Linked Recessive
4. X-Linked Dominant
5. Mitochondrial Inheritance

Autosomal Recessive Mode of Inheritance

1. Heterozygote is carrier
2. sexes equally affected
3. affected phenotype skips generations

Cystic Fibrosis

1. autosomal Recessive Disorder that affects the q arm of chromosome 7 which encodes for Cystic Fibrosis Transmembrane Conductance Regulator
2. mutation of gene causes an absence of the protein changes the transport of Cl ions which reduces fluid in glandular secretions
3. several mutations in gen are possible
4. most common mutation is on Delta 508 (Deletion of codon 508)

Effect of Cystic Fibrosis on the Pancreas

1. causes thick mucus clogs ducts of the pancreas preventing enzymes from traveling to the small intestine, impairs digestion.
2. results in malnutrition, pancreatic cysts, organ degeneration

Effects of Cystic Fibrosis on the Lungs

1. causes production of thick mucus in the lungs which block airways.
2. results in obstructive lung diseases and infections.

Symptoms related to Cystic Fibrosis

1. delayed growth
2. failure to gain weight
3. constipation
4. salty skin
5. ↑ gas and bloating
6. extended belly
7. nausea or loss of appetite
8. pale or clay colored stools

Symptoms of Cystic Fibrosis related to Lungs and Sinus

1. ↑ mucus in lungs and sinus
2. fatigue
3. nasal congestion
4. recurrent episodes of pneumonia
5. ↑ Dyspnea

Sickle Cell Anemia

1. mutation of #6 amino acid position in hemoglobin B
2. GAG is replaced with GTG causing glutamic acid to be replaced with valine in 146 amino acid chain
3. causes RBC to sickle in ↓ oxygen concentrations
4. causes accumulations of RBC and ↓ capacity to hold O₂
5. susceptible to anemia, infarctions, vascular occlusions, embolisms, splenomegaly

Autosomal Dominant Mode of Inheritance

1. no carrier state ( Heterozygous individuals are affected)
2. sexes equally affected
3. every generation is affected.

Marfan Syndrome

1. Autosomal Dominant Disorder that affects skeletal system, eyes, and cardiovascular system.
2. Affects gene FBNI, on Chromosome 15, which encodes Fibrillin, component of CT
3. mutation causes defective fibrillin that results in excess Transforming GF - B
4. 25% of affected individuals appear in families with no history of the disease (high mutation rate)

Typical Symptoms of Marfan Syndrome1

1. tall
2. thin
3. long limbs
4. thin fingers

Effects of Marfan Syndrome on the Heart

1. weakens CT at base of Aorta, causing hypertrophy and eventual split - aortic aneurysms

X Linked Recessive Mode of Inheritance

1. genes located on X-chromosome of Pair 23
2. primarily affects males and homozygous recessive females
3. females are carriers
4. males receive mutant allele from mother

Hemophilia A

1. X-linked recessive disorder
2. Defective for Clotting Factor VIII
3. Symptoms: excessive Bleeding

Hemophilia B

1. X-Linked Recessive Trait
2. "Christmas Disease"
3. Defective Clotting Factor IX
4. Symptoms: Excessive Bleeding

Color Blindness

1. X-Linked Recessive Trait
2. blindness to Red and Green light
3. affects the opsin proteins in the cones of the retina

Duchenne Muscular Dystrophy

1. X-Linked Recessive Trait
2. Progressive weakness or loss of muscle tissue
3. Mutation in gene that codes for Dystrophin causes an absence of the protein
4. Symptoms appear between 1-6 years
5. Lifespan 20-30 years

Dystrophin

1. protein that is defective or absent in Duchenne Muscular Dystrophy
2. responsible for stabilizing muscle cells so that they don't wear out from constant contraction

Beck Muscular Dystrophy

1. Milder variant of Muscular Dystrophy
2. results in shorter dystrophin proteins

X-Linked Dominant

1. more females affected (Homozygous or Heterozygous)
2. no carriers
3. affected males transfer to daughters only
3. Affected females have 50% chance of transferring the disorder to male and female offspring
4. very rare and of no real significance

Common X-Linked Dominant Disorder

1. Phosphatemia - Vitamin D resistant Rickets, poor absorption of vitamin D leads to bone deformities, short stature,
2. insensitive to Vitamin D treatments

Mitochondrial Mode of Inheritance

1. affected mother will give trait to all children and will be affected
2. affected father will give trait to no one.
3. no carriers
4. disorders reduce the amount of energy for cellular function
5. affects muscles, NS

Most common type of colorblindness

1. Green colorblindness - insensitivity to green light

Symptoms of mitochondrial myopathies

1. muscle weakness
2. necrosis of muscle tissue
3. movement of eyes
4. droopy eyelids
5. swallowing and speech

Penetrance

1. Probability that a genotype will be expressed in population
2. probability that a disease will be present when disease genotype is present.
3. "What % of the population with the genotype will actually express it?" - usually 100%

Variable/ Incomplete Penetrance

1. 50% Penetrance - half of the population with the genotype will express the disorder.
2. 30% Penetrance - 30% of the population with the genotype will have the phenotype.

Expressivity

1. degree of a gene's phenotype expression
2. degree of phenotypic expression in that portion of the population with the genotype.

Complete Expressivity

everyone that has the genotype will express all (100%) of the manifestations of the phenotype

Reduced Expressivity

everyone that has the genotype will express it, but they will not all show 100% of manifestations of the phenotype.

Albinism

1. autosomal recessive trait
2. absence of pigment in skin, eyes, and hair

Phenylketonuria (PKU)

1. autosomal recessive disorder
2. mutation of the gene that synthesizes the enzyme to catabolize phenylalanine

Huntingdon's Disease

1. Autosomal recessive disorder
2. progressive degeneration of NS; dementia, and death
3. symptoms appear in middle age

Achondroplasia

1. autosomal recessive
2. dwarfism associated with defects in growth regions of the long bones

Tay-Sachs Disease

1. autosomal recessive
2. improper metabolism of gangliosides in nerve cells; results in early death

Familial Hypercholesterolemia

1. autosomal recessive
2. elevated levels of cholesterol; predisposes to plaque formation, cardiac disease; may be most prevalent genetic disease

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