Genetics: Autosomal and X-linked Inheritance

• Here are some genetics study notes

Achondroplasia

• Autosomal dominant inheritance
• Results in short stature, normal intelligence
• Homozygous AA genotype is lethal

ADA/SCID (Adenosine Deaminase Deficiency / Severe Combined Immunodeficiency)

• Autosomal recessive inheritance
• Causes severe immune deficiency due to lack of adenosine deaminase
• Leads to recurrent infections that are fatal without treatment

Albinism

• Autosomal recessive inheritance
• Absence of melanin pigment
• Causes vision problems and sensitivity to light

Alcoholism

• Multifactorial inheritance (polygenic + environmental)
• Increases risk of alcohol dependence
• Influence by both genetic variants and environment

Androgen Insensitivity Syndrome (AIS)

• X-linked recessive inheritance
• Genetically male (XY) individuals develop female external characteristics
• Due to unresponsiveness to androgens

Bombay Phenotype

• Autosomal recessive inheritance
• Absence of H antigen prevents expression of A or B antigens
• Individuals appear to have O blood type regardless of genotype

Congenital Adrenal Hyperplasia

• Autosomal recessive inheritance
• Excess androgen production
• Can cause ambiguous genitalia in females and salt imbalance

Congenital Generalized Hypertrichosis

• X-linked dominant inheritance
• Excessive hair growth over the body
• Appears at birth

Cystic Fibrosis

• Autosomal recessive inheritance
• Thick, sticky mucus in lungs and digestive system
• Caused by CFTR gene mutation
• Common in Caucasians and may confer heterozygote advantage

DMD (Duchenne Muscular Dystrophy)

• X-linked recessive inheritance
• Progressive muscle weakness in boys
• Mutation in dystrophin gene, early onset and shortened lifespan

Down Syndrome

• Chromosomal (non-disjunctive) inheritance
• Intellectual disability, characteristic facial features, heart defects, hypotonia
• Caused by trisomy 21

Familial Hypercholesterolemia

• Autosomal dominant inheritance
• Very high LDL cholesterol levels from a young age
• Increases risk of early heart attacks

Huntington's Disease (HD)

• Autosomal dominant inheritance
• Neurodegeneration, mood changes, chorea (involuntary movements)
• Shows anticipation (earlier onset in successive generations)

Hemophilia (A or B)

• X-linked recessive inheritance
• Impaired blood clotting
• Caused by mutations in factor VIII (Hemophilia A) or IX (Hemophilia B), mainly affects males

HNPCC / Lynch Syndrome

• Autosomal dominant inheritance
• Hereditary nonpolyposis colorectal cancer, high risk of colon and other cancers
• Caused by DNA mismatch repair gene mutations

Klinefelter's Syndrome

• Chromosomal (non-disjunctive) inheritance
• Males with an extra X chromosome (XXY)
• Results in tall stature, low testosterone, infertility, and learning difficulties

Prader-Willi Syndrome

• Chromosomal (non-disjunctive imprinting error on paternal chromosome 15)
• Obesity, intellectual disability, insatiable appetite, short stature

Angelman's Syndrome

• Chromosomal (non-disjunctive imprinting error on maternal chromosome 15)
• Severe intellectual disability, minimal speech, frequent smiling/laughter, balance problems

PKU (Phenylketonuria)

• Autosomal recessive inheritance
• Inability to break down phenylalanine
• Untreated leads to intellectual disability, controlled with strict diet

Red/Green Colorblindness

• X-linked recessive inheritance
• Inability to distinguish red from green shades
• Mutation in opsin genes

Rh Incompatibility

• Not Mendelian inheritance, blood type mismatch between mother (Rh–) and fetus (Rh+)
• Mother's immune system attacks fetal red blood cells
• Can lead to hemolytic disease of the newborn

Sickle Cell Anemia

• Autosomal recessive inheritance (with incomplete dominance)
• Red blood cells form sickle shapes
• Block circulation and cause pain
• Heterozygotes have malaria resistance

Tay-Sachs Disease

• Autosomal recessive inheritance
• Neurodegeneration in infants, no cure
• Cherry-red spot on retina, high frequency in Ashkenazi Jews

Turner's Syndrome

• Chromosomal (non-disjunctive) inheritance
• Short stature, webbed neck, infertility
• Affects females with only one X chromosome (XO)

Xeroderma Pigmentosum

• Autosomal recessive inheritance
• Extreme sensitivity to UV light
• Defects in nucleotide excision repair (NER), high skin cancer risk

Alzheimer's Disease (Early Onset)

• Autosomal dominant inheritance
• Genes Involved: APP, PSEN1, PSEN2
• Memory loss and cognitive decline before age 65
• Caused by increased amyloid-beta accumulation

Alzheimer's Disease (Late Onset)

• Multifactorial inheritance
• Genetic Risk Factor: APOE4 allele
• Gradual memory loss and confusion after age 65
• APOE4 increases risk but is not directly inherited

CCR5 Gene Mutation (A32 variant)

• Autosomal recessive inheritance
• Provides resistance to HIV infection
• Homozygous individuals are highly resistant, heterozygotes partially