Biology (Holt)

Hemophilia
Another recessive genetic disorder is hemophilia (hee moh FIHL
ee uh), a condition that impairs the blood’s ability to clot.
Hemophilia is a sex-linked trait. More than a dozen genes code for
the proteins involved in blood clotting. A mutation on one of these
genes on the X chromosome causes the form of hemophilia called
hemophilia A. If the mutation appears on the X chromosome,
which a male receives from his mother, he does not have a normal
gene on the Y chromosome to compensate. Therefore, he will
develop hemophilia.

Huntington’s Disease (HD)
Huntington’s disease is a genetic disorder caused by a dominant
allele located on an autosome. The first symptoms of HD—mild
forgetfulness and irritability—appear in victims in their thirties or
forties. In time, HD causes loss of muscle control, uncontrollable
physical spasms, severe mental illness, and eventually death.
Unfortunately, most people who have the HD allele do not know
they have the disease until after they have had children. Thus, the
disease is unknowingly passed on from one generation to the next.

SECTION 4Complex Patterns of Heredity 181

Table 2 Some Human Genetic Disorders

Disorder Dominant or Symptom Defect Frequency Among

Recessive Human Births

Sickle Cell Recessive Poor blood circulation Abnormal hemoglobin 1:500

Anemia molecules (African Americans)

Hypercholes- Dominant Excessive cholesterol Abnormal form of cell 1:500

terolemia levels in blood, leading surface receptor for

to heart disease cholesterol

Tay-Sachs Recessive Deterioration of central Defective form of a 1:3,500

Disease in early childhood nervous system; death brain enzyme (Ashkenazi Jews)

in early childhood

Cystic Recessive Mucus clogs organs Defective chloride-ion 1:2,500

Fibrosis including the lungs, transport protein (Caucasians)

liver, and pancreas;

affected individuals

usually do not

survive to adulthood

Hemophilia A Sex-linked Failure of blood Defective form of a 1:10,000

(Classical) recessive to clot blood-clotting factor (males)

Huntington’s Dominant Gradual deterioration Inhibitor of brain-cell 1:10,000

Disease of brain tissue in middle metabolism is made

age; shortened life

expectancy

http://www.scilinks.org

Topic: Genetic Disorders

Keyword: HX4091