Such changes are usually very harmful. One example of a chromosomal mutation is a condition called Down syndrome. In each cell, humans normally have forty-six chromosomes, consisting of two copies of the twenty-three kinds of chromosomes. Down syndrome usually results from the presence of one extra copy of a particular chromosome, or an extra portion of that chromosome. The presence of that extra chromosome leads to problems with certain organs of the body, such as the heart. It can also lead to leukemia—a cancer of the blood-forming cells—and produce mental disabilities.
Many people with Down syndrome also have distinct facial features. Mutations can be inherited or acquired during a person's lifetime.
Mutations that an individual inherits from their parents are called hereditary mutations. They are present in all body cells and can be passed down to new generations. Once an acquired mutation is passed down, it is a hereditary mutation. Acquired mutations are not passed down if they occur in the somatic cells, meaning body cells other than sperm cells and egg cells. Some acquired mutations occur spontaneously and randomly in genes.
Other mutations are caused by environmental factors, such as exposure to certain chemicals or radiation. Mutations occur throughout the natural world. Some mutations are beneficial and increase the possibility that an organism will thrive and pass on its genes to the next generation. When mutations improve survival or reproduction, the process of natural selection will cause the mutation to become more common over time. When mutations are harmful, they become less common over time.
Therefore, mutation is a force that helps drive evolution. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited. Tyson Brown, National Geographic Society.
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From Genetics Home Reference. When an egg and a sperm cell unite, the resulting fertilized egg cell contains DNA from both parents. Any variants that are present in that DNA will be present in the cells of the child that grows from the fertilized egg.
Because non-inherited variants typically occur in somatic cells cells other than sperm and egg cells , they are often referred to as somatic variants. These variants cannot be passed to the next generation. Non-inherited variants can be caused by environmental factors such as ultraviolet radiation from the sun or can occur if an error is made as DNA copies itself during cell division.
Topics in the Variants and Health chapter What is a gene variant and how do variants occur? How can gene variants affect health and development? Do all gene variants affect health and development? What kinds of gene variants are possible? Can a change in the number of genes affect health and development?
Can changes in the number of chromosomes affect health and development? Can changes in the structure of chromosomes affect health and development? Can changes in noncoding DNA affect health and development?
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