Can you roll your tongue? Not bend it so that the tip points toward the back of your mouth, but roll it, bringing the outside edges together to form a tube of your tongue? If you can, you are one of 7 in 10 people whose genetic constitution, called their genotype, contains the gene for tongue-rolling. As the numbers indicate, a lot of people are able to roll their tongues, or as a geneticist might say, the phenotype of many people includes the ability to tongue-roll. Phenotype comes from Greek phainein, “to show.” It describes a person’s physical and biochemical expression of his or her genotype as well as a person’s physical manifestation of various environmental influences. For example, a person’s natural color of hair and eyes, blood type, and fingerprints are phenotypic expressions of genetically determined traits. An individual’s hair style, altered hair or eye color, and style of eyeglasses or sunglasses are examples of phenotypic expressions of environmental influences.
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Analyzing a cell’s chromosomes, the cellular structures that convey genetic information, can give clues to a person’s genotype and, ultimately, his or her phenotype. One method for doing such an analysis involves making photographic enlargements of the chromosomes after they have been arranged in an orderly manner, such as from largest to smallest. Such photographs are called karyotypes. An example of such an analysis is a prenatal test, a procedure in which chromosomes taken from a fetus are analyzed to determine whether any genetic diseases or disorders are present. An analysis of the karyotype of these chromosomes can help determine whether the baby’s genotype would result in a phenotype marked by Down syndrome, Klinefelter’s syndrome, or a similar disorder that affects physical and mental development.
