Many human traits are controlled by a single gene with one dominant allele and one recessive allele. As with tall and short pea plants, these human traits have two distinctly different phenotypes, or physical appearances. For example, the allele for a widow's peak, which is a hairline that comes to a point in the middle of the forehead, is dominant over the allele for a straight hairline.
Some human traits are controlled by single genes with two alleles, and others by single genes with multiple alleles. Still other traits are controlled by many genes that act together. Height and skin color are both examples of human traits controlled by many genes. When more than one gene controls a trait, there are many possible combinations of genes and alleles. There is an enormous variety of phenotypes for height, for example, and human skin color ranges from almost white to nearly black, with many shades in between.
Some human traits are controlled by a single gene that has more than two alleles. Such a gene is said to have multiple alleles-three or more forms of a gene that code for a single trait. An example of a human trait that is controlled by a gene with multiple alleles is blood type. There are four main blood types-A, B, AB, and O-controlled by three alleles.
The sex chromosomes are one of 23 pairs of chromosomes in each body cell. The sex chromosomes carry genes that determine whether a person is male or female. They also carry genes that determine other traits. If you are female, you have two X chromosomes. If you are male, you have an X and a Y chromosome. Whether you inherited an X or Y chromosome from your father determines your sex.
Genes on the X and Y chromosomes are often called sex-linked genes. Traits controlled by sex-linked genes are called sex-linked traits. Because males have only one X chromosome, males are more likely than females to have a sex-linked trait that is controlled by a recessive allele. One example of a sex-linked trait that is controlled by a recessive allele is red-green colorblindness. A carrier is a person who has one recessive allele for a trait and one dominant allele. Although a carrier does not have the trait, the carrier can pass the recessive allele on to his or her offspring. In the case of sex-linked traits, only females can be carriers.
The effects of genes are often altered by the environment-the organism's surroundings. Many of a person's characteristics are determined by an interaction between genes and the environment. Several genes determine human height. However, environment also inﬂuences people's heights. People's diets can affect their height. A poor diet can prevent a person from growing as tall as might be possible.
A genetic disorder is an abnormal condition that a person inherits through genes or chromosomes. Some genetic disorders are caused by mutations in the DNA of genes. Other disorders are caused by changes in the overall structure or number of chromosomes.
Cystic ﬁbrosis is a genetic disorder in which the body produces abnormally thick mucus in the lungs and intestines, making it hard to breathe and digest food. The allele that causes cystic ﬁbrosis is recessive. Currently there is no cure for cystic ﬁbrosis, although there are treatments to help control the symptoms.
Sickle-cell disease is a genetic disorder that affects hemoglobin, the protein in blood that carries oxygen. People with sickle-cell disease suffer from lack of oxygen in the blood and experience pain and weakness. The allele that causes sickle-cell disease is codominant with the normal allele. People with two sickle-cell alleles have the disease. People with one sickle-cell allele produce both normal and abnormal hemoglobin but usually do not have symptoms of the disease. Currently there is no cure for sickle-cell disease. However, treatments can lessen the pain and other symptoms.
Hemophilia is a genetic disorder in which the blood clots very slowly or not at all. People with the disorder do not produce one of the proteins needed for normal blood clotting. Hemophilia is caused by a recessive allele on the X chromosome. Because it is a sex-linked disorder, it occurs more often in males than in females. With treatment, people with hemophilia can lead normal lives.
Down syndrome is a genetic disorder that is due to an extra copy of chromosome 21. Most often Down syndrome occurs when the chromosomes fail to separate properly during meiosis. People with Down syndrome have a distinctive physical appearance and some degree of mental retardation. Many people with Down syndrome lead full, active lives.
Geneticists trace the inheritance of traits through several generations of a family. One important tool that geneticists use to trace the inheritance of traits in humans is a pedigree. A pedigree is a chart or "family tree” that tracks which members have a particular trait.
Today, doctors use tools such as karyotypes to help diagnose genetic disorders. People with genetic disorders are helped through medical care, education, job training, and other methods. To detect chromosomal disorders such as Down syndrome, a doctor examines the chromosomes from a person's cells. The doctor uses a karyotype, or picture of all the chromosomes in a cell, to examine the chromosomes. The chromosomes are arranged in pairs. A karyotype can reveal whether a developing baby has the correct number of chromosomes in its cells.
A couple that has a family history or concern about a genetic disorder may turn to a genetic counselor for advice. Genetic counselors help couples understand their chances of having a child with a particular genetic disorder. Genetic counselors use tools such as karyotypes, pedigree charts, and Punnett squares.
For thousands of years, people have tried to produce plants and animals with desirable traits. Selective breeding, cloning and genetic engineering are three methods for developing organisms with desirable traits.
The process of selecting organisms with desired traits to be parents of the next generation is called selective breeding. People have used selective breeding with many different plants and animals. One selective breeding technique is called inbreeding. Inbreeding involves crossing two individuals that have similar characteristics. One goal of inbreeding is to produce breeds of organisms with speciﬁc traits. For example, by only crossing horses with exceptional speed, breeders can produce purebred horses that can run very fast. Unfortunately, inbreeding also increases the probability that organisms may inherit alleles that lead to genetic disorders. Another selective breeding technique is called hybridization. In hybridization, breeders cross two genetically different individuals. The hybrid organism that results is bred to have the best traits from both parents. For example, a farmer might cross corn that produces many kernels with corn that is resistant to disease.
For some organisms, another technique, called cloning, can be used to produce offspring with desired traits. A clone is an organism that is genetically identical to the organism from which it was produced. One way to produce a clone of a plant is to cut and grow a small part of a plant, such as a leaf or stem. Several types of animals have been cloned in recent years.
Another technique for producing organisms with desired traits is called genetic engineering. In genetic engineering, genes from one organism are transferred into the DNA of another organism. Genetic engineering can produce medicines and improve food crops, and may some day correct human genetic disorders. In a type of genetic engineering called gene therapy, working copies of a gene may be inserted directly into the cells of a person with a genetic disorder. Some people are concerned about the long-term effects of genetic engineering.
A genome is all the DNA in one cell of an organism. The main goal of the Human Genome Project has been to identify the DNA sequence of every gene in the human genome. From the Human Genome Project, scientists hope to learn more about what makes the body work and what causes things to go wrong. A genetic technique called DNA ﬁngerprinting is used to identify people. No two people, except for identical twins, have the same DNA.
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