![]() Other chapters in Help Me Understand Genetics How are genetic conditions and genes named?.What information can statistics provide about a genetic condition?.How are gene variants involved in evolution?.What does it mean to have a genetic predisposition to a disease?.What are complex or multifactorial disorders?.Can changes in mitochondrial DNA affect health and development?.Can changes in noncoding DNA affect health and development?.Can changes in the structure of chromosomes affect health and development?.Can changes in the number of chromosomes affect health and development?.Can a change in the number of genes affect health and development?.What kinds of gene variants are possible?.Do all gene variants affect health and development?.How can gene variants affect health and development?.What is a gene variant and how do variants occur?.Topics in the Variants and Health chapter This disease resistance helps explain why the variants that cause sickle cell disease are still found in many populations, especially in areas where malaria is prevalent. The best-studied example of this phenomenon is sickle cell disease: Having two altered copies of the HBBgene in each cell results in the disease, but having only one copy provides some resistance to malaria. For some conditions, having one altered copy of a gene in each cell is advantageous, while having two altered copies causes disease. For other harmful traits, a phenomenon called reduced penetrance, in which some individuals with a disease-associated variant do not show signs and symptoms of the condition, can also allow harmful genetic variations to be passed to future generations. For some conditions, such as the neurological condition Huntington disease, signs and symptoms occur later in life, typically after a person has children, so the gene variant can be passed on despite being harmful. So why do some harmful traits, like genetic diseases, persist in populations instead of being removed by natural selection? There are several possible explanations, but in many cases, the answer is not clear. Some differences introduced by variants may help an organism survive in one setting but not in another-for example, resistance to a certain bacteria is only advantageous if that bacteria is found in a particular location and harms those who live there. In addition, the environment in which a population of organisms lives is integral to the selection of traits. Also, many genetic changes have no impact on the function of a gene or protein and are not helpful or harmful. Some variants occur during a person’s lifetime in only some of the body’s cells and are not hereditary, so natural selection cannot play a role. Only hereditary variants, which occur in egg or sperm cells, can be passed to future generations and potentially contribute to evolution. Sometimes the population becomes so different that it is considered a new species. Over time, as generations of individuals with the trait continue to reproduce, the advantageous trait becomes increasingly common in a population, making the population different than an ancestral one. If a trait is advantageous and helps the individual survive and reproduce, the genetic variation is more likely to be passed to the next generation (a process known as natural selection). Genetic variations that alter gene activity or protein function can introduce different traits in an organism. ![]() Genetic variations can arise from gene variants (also called mutations) or from a normal process in which genetic material is rearranged as a cell is getting ready to divide (known as genetic recombination). Genetic variations underlie these changes. Evolution is the process by which populations of organisms change over generations.
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