(b) Punnett Square

Figure 24.6

Inheritance of Huntington disease from a parent who will be affected in middle age illustrates autosomal dominant inheritance. (a). A person with just one HD allele develops the disease. A Punnett square (b) and pedigree (c) depict the inheritance of HD. The pedigree symbols for HD are completely filled in to indicate that the person is affected. Autosomal dominant conditions affect both sexes.

personality changes, such as anger and irritability. Figure 24.6 shows the inheritance pattern for HD. If one parent has the mutant allele, half of his or her gametes will have it. Assuming the other parent does not have a mutant allele, each child conceived has a 1 in 2 chance of inheriting the gene and, eventually, developing the condition.

Most of the 3,000 or so known human inherited disorders are autosomal recessive. These conditions tend to produce symptoms very early, sometimes before birth. Autosomal dominant conditions often have an adult onset. They remain in populations because people have children before they know that they have inherited the illness. Tests can detect certain genetic disorders before symptoms arise. The number of such tests will increase dramatically as researchers continue to analyze human genome information.

Certain recessive alleles that cause illness may remain in a population, even if they endanger health, because carrying a genetic disease can protect against an infectious disease. The basis of this phenomenon lies in anatomy and physiology. For example, carriers of sickle cell disease, an inherited anemia, do not contract malaria. In sickle cell disease, a tiny genetic alteration causes the gene's product — the beta globin chain of hemoglobin — to aggregate, bending the red blood cell containing it into a sickle shape that blocks blood flow when oxygen level is low. Carriers have only a few sickled cells, but these apparently are enough to make red blood cells inhospitable to malaria parasites.

Carriers of cystic fibrosis resist diarrheal disorders, in which bacterial toxins open chloride channels in the small intestine. Carriers of CF have some abnormal chloride channels, which renders these toxins ineffective.

Carriers of Tay-Sachs disease may resist tuberculosis. This association was first noted in the Jewish ghettos of World War II, where some healthy relatives of children who died of Tay-Sachs disease did not contract tuberculosis, when many people around them did. Tay-Sachs disease causes excess myelin to accumulate on nerve cells. How this protects against tuberculosis is not known. This protective mechanism of one disease against another is called balanced polymorphism.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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