Inbreeding jeopardizes the survival of the population in a number of ways. All living things inherit characteristics from their ancestors. These characteristics are inherited through genes. Inbred populations are less variable genetically than other populations. This means that individuals in the population are more alike. Less variability means that a population may be less able to adapt to environmental change or to survive a particular disease. Biologists refer to the ability to adapt as fitness. Inbreeding makes a population of animals less fit.

    Being more alike also means that individuals in small populations are more likely to share the same recessive genes and to pass these on to their offspring. Traits that are recessive only occur if the individual receives a recessive gene from both parents. Some recessive traits are harmless and don't affect the health of the individual. It appears that the kinked tail among the Florida panthers, a harmless trait, is recessive and that the high incidence of this trait is the result of inbreeding over the generations of a small isolated population.

    Other recessive traits are harmful and even lethal. Among humans a number of genetic diseases are recessive and appear more frequently in some groups than others: Tay Sachs among Jewish people of central and eastern European descent, cystic fibrosis among Caucasians in the United States, and sickle-cell anemia among African Americans. In some cases having a single recessive trait may be advantageous. For example, having a single recessive cell for sickle cell anemia provides some protection against malaria, an advantage in parts of Africa where the disease is present, whereas inheriting two recessive genes results in a potentially fatal disease.

    All living things are likely to carry some harmful recessive genes. Among humans, for example, it is estimated that every individual carries 6 to 8 harmful recessive genes. Closely related individuals are far more likely than unrelated individuals to carry the SAME harmful recessive genes and to pass these on to their offspring.

    Harmful dominant genes may be even more of a problem, particularly in small, isolated populations such as the Florida panther in which only a few males are breeding with several females. Among quarter horses the potentially fatal disease HYPP (hyperkalemic periodic paralysis) is caused by a dominant gene. The gene has been identified and all cases have been traced to the very popular stallion Impressive. In humans, the sex-linked dominant gene for hemophilia spread through the royal families of Europe.

    Many traits, including human height, eye color, and skin color, are influenced by a number of different genes. The mode of inheritance of heart defects in Florida panthers is at present unknown and is the subject of ongoing research. Among humans the type of defect found in panthers (atrial septal defect) accounts for 10 percent of all heart defects present at birth. Heart defects in humans may be caused by chromosomal abnormalities (which cause a host of other defects), a single mutant gene, or by environmental factors such as drugs, alcohol, or infections (Magid 1995). Children of parents with heart defects are at higher risk (Merck 1992). The most likely cause for most heart defects in humans is a "genetic predisposition acted upon by an environmental trigger." (Magid 1995)

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