Inheritance and Genetic Diseases

Autosomal Recessive Diseases

Cystic fibrosis (Chromosome 7)

Mucus in the lungs and digestive tract is thick and viscous, making breathing and digestion difficult.

Tay-Sachs disease (Chromosome 15)

Neurological impairment and psychomotor difficulties develop early, followed by blindness and uncontrollable seizures before death occurs, usually before age 5.

Phenylketonuria (PKU) (Chromosome 12)

Inability to metabolize the amino acid phenylalanine and mental retardation will develop, unlessa special diet is followed.

Autosomal Dominant Diseases

Neurofibromatosis (Chromosome 17)

Benign tumors occur under the skin or deeper.

Huntington disease (Chromosome 4)

Minor disturbances in balance and coordination develop in middle age and progress towards severe neurological disturbances, leading to death.

Incomplete Dominance Diseases

Sickle-cell anemia and Sickle-cell trait (Chromosome 11)

Poor circulation, anemia, internal hemorrhaging, due to sickle-shaped red blood cells.

Multiple Allele Inheritance

ABO Blood groups. The human population contains three different alleles at the gene locus for blood type: A, B and O. The A and B blood types are codominant, and the O blood type is recessive.

Polygenic Inheritance

Hair color, eye color, or height. The phenotype depends on several different gene pairs at different loci, which act in tandem. Alleles exhibit incomplete dominance, and a continuous variation in the phenotype may be expressed between two extremes.

X-Linked Recessive Diseases

Duchenne muscular dystrophy (Chromosome X)

Muscle weakness develops early and progressively intensifies until death occurs, usually before age 20.

Hemophilia A (Chromosome X)

Propensity for bleeding, often internally, due to the lack of a blood clotting factor.

Nondisjunction Syndromes

Down (Trisomy 21)

Mental retardation; wide, flat face with upper eyelid fold; short stature.

Edward (Trisomy 18)

Malformed internal organs, face, and head; extreme muscle tone.

Turner (Monosomy X, or X0)

No sexual maturity; short stature; webbed neck; broad chest.

Klinefelter (Trisomy XXY)

Testes underdeveloped; breast development; no facial hair.

Triplo-X (Trisomy XXX)

Tall and thin with menstrual irregularities.

Jacob (Trisomy XYY)

Taller than average; speech and learning problems; persistent acne; prone to violence.