单选题
Genes
Until only just a few centuries ago, people believed that the genes of the parents blended together and resulted in the exact same genetic makeup. Then, in the early 1800s, Gregory Mendel, a renowned Austrian scientist, discovered that only certain traits such as the shape of the nose, or a particular eye color are passed directly from parent to child. Because of his work, and the work of many scientists since, we now know that many of our physical and behavioral characteristics are determined by genetic factors which have been handed down to us from our parents and grandparents. We also know that our genes, as well as our lifestyle, are behind many common diseases that afflict us as individuals.
The genes we inherit from our parents program our development from conception to adulthood. In each cell, we have about 90,000 pairs of genes arranged on 23 pairs of chromosomes, one of the pair being maternal and the other being paternal. Although our brothers and sisters inherit genes from the same parents, the mixture of genes is slightly different in each sibling. It is the reason every organism is different.
Each gene provides instructions to a cell to carry out a single chemical process; it also controls the growth and reproduction of multiple cells. Genes are thus responsible for the development of the embryo into a baby and a child, then eventually into an adult. Throughout our lives, genes control cell function and the repair and replacement of damaged or dead cells. Blood relatives have many genes in common, and these genes help to determine family physical characteristics and other traits. Most of these traits, such as the shape of the nose, are trivial and have no significant effect on health. Other traits, such as being abnormally short or tall, or having a tendency to be overweight, can be associated with an increased risk of certain diseases.
Some diseases, such as hemophilia and cystic fibrosis, are directly caused by a mutation in a single gene or a pair of genes. These rare diseases follow a predictable pattern of inheritance, and this means that families in which the gene is present can usually be given clear, reliable information regarding the risk of the disease affecting their future offspring. Therefore, gene therapy holds great promise for the cure of diseases, and researchers will eventually come up with a way to regulate abnormal gene behavior without disrupting normal ones. Some, gene therapists have looked at viruses as a way to cure certain diseases. However, viruses replicate easily and move around the body to interact with other genes, so a virus that is used to cure hemophilia can replicate and enter an area where a tumor gene is located and stimulate it to cause a cancer. Gene therapy using bacterial DNA yields opposite results.
More common than these genetic disorders are those in which genes, along with other factors, contribute to a family"s susceptibility to certain diseases. For example, some disorders, such as coronary artery disease, tend to run in families, but lifestyle factors such as a high-fat diet, smoking, and lack of exercise also play a part in determining whether these diseases develop. In some diseases that have a genetic component, including asthma, environmental factors, such as living in a polluted area, also play a crucial role. Cancer is not caused simply by one gene, but rather it is a result of a whole variety of circumstances. In a lab, mice were genetically altered to include a gene that causes tumors in the retina of the eye in humans, but surprisingly, none of the animals developed any symptoms of this disorder. Such complex interplay between genetic susceptibility and environment makes it difficult to predict the risks in adult life for children who are born into families affected by disorders of this kind.
We are now aware of the fact that we are susceptible to diseases passed through families, and that our lifestyles can serve to exacerbate symptoms and or bring about the early onset of say heart disease. The good news is that we can use this information to arm ourselves against diseases brought on by an unhealthy lifestyle, if we know that our genes carry a likelihood of us developing heart disease then it would be prudent to try to live a healthy lifestyle.
Glossary
conception:
the act of becoming pregnant; fertilization of an ovum by a spermatozoid
chromosome:
a thread-like body in the cell nucleus that carries the genes in a linear order
embryo:
an organism in its earl) stages of development, especially before it has reached a distinctively recognizable form
hemophilia:
congenital tendency to uncontrolled bleeding; usually affects males and is transmitted from mother to son
cystic fibrosis:
the most common congenital disease, the child"s lungs and intestines and pancreas become clogged with thick mucus; caused by defect in a single gene
coronary artery:
the artery that branches from the aorta to supply blood to the heart