Given the choice, would you rather have been born with a different eye color, hair color or skin tone? Maybe you would have chosen to be taller, thinner or more muscular. Of course, you didn't have these options. The physical and personal traits a person winds up with are just one big roll of the dice, with only the biological parents' genes to draw from. However, within a few decades, there's a good chance that biotechnology could give us the ability to pre-choose our children's physical and personality traits like we pick out options on a new car.


Photo courtesy DOE Joint Genome Institute
Scientists are working to unlock the secrets hidden in our genome. This research could give us the ability to genetically engineer our children.

Scientists have only begun to unravel the secrets hidden within the human genome -- the genetic blueprint for a human being. A working draft of the human genome has already been published. Once the mapping of the human genome is finished, scientists will begin to discover what each gene does and how it functions. At that point, it might be possible to manipulate the genes of embryos. Imagine your doctor taking your order: "Okay, that's blue eyes, blonde hair, button nose. And will that be 6 feet 2 inches or 6 feet 4 inches?"

The idea of designing our babies is not as far-fetched as it may have seemed just a decade ago. Scientists are already tinkering with the genetic makeup of animals. In this edition of How Stuff WILL Work, you'll learn about the progress and goals of human-genome research, how we already can weed out genetic diseases and how the engineering of human genes will work.

Mapping the Human Genome
If you think of the human body as big, complicated, encrypted code, then the scientists mapping the human genome are attempting to break that code. Once the code is broken, it will reveal many secrets of how the human body works, and it could lead to greater disease prevention. In June 2000, scientists from the Human Genome Project and from Celera Genomics both announced that they had assembled a working draft sequence of the human genome, a major step in cracking the code.

What researchers are trying to do is construct a detailed genetic map of the human genome and determine the entire nucleotide sequence of human deoxyribonucleic acid (DNA). A nucleotide is the basic unit of nucleic acid, which is found in the 23 pairs of chromosomes in the human body. According to the Human Genome Project, there are between 26,000 and 40,000 genes in the human body. Each of these genes is composed of a unique sequence of pairs, each with four bases, called base pairs.


Photo courtesy DOE Joint Genome Institute
Human genes are found in the rungs of a DNA double helix. DNA makes up the 23 pairs of chromosomes in the human body.

In a DNA molecule, which is shaped like a twisted ladder, the bases are the chemicals that interlock to form the rungs of the ladder. The sides of the ladder are made of sugar and phosphate molecules. The human body has about 3-billion base pairs, but only about 4 percent of those pairs constitute DNA that affects gene function. We don't have any idea about the purpose of the other 96 percent of base pairs, consequently termed junk-DNA.

That researchers have a "working draft" means that they have spotted most of the human genes but must go further to assemble a finished sequence. A finished sequence will be considered the "gold standard" of the human-gene map. This finished sequence is expected to be complete, with 99.99-percent accuracy, in 2003. About 20 percent of the genome, including two entire chromosomes, has been completed to this level.

Solving the human genome will tell us a lot about how life works. It could lead to preventing or curing diseases, because genetics is what getting sick is all about -- our genes are trying to fight off the genes of a virus or bacteria. The next step will be to determine how this battle is played out. Today, researchers know the positions of some genes that control our medical traits, but don't know the exact gene sequence. Other genes have been located but their functions are unknown, and still others remain entirely elusive. The point of genome research is to locate the genes and determine just how the four bases are sequenced, and then, to learn what the genes actually do.

Genetic Prescreening
When doctors first performed in vitro fertilization (IVF) in 1978, it gave many otherwise-infertile couples a way to have a child of their own. IVF works by removing the eggs from the woman's uterus, fertilizing them in a laboratory and then, a few days later, transferring the fertilized egg, called a zygote, back into the uterus. IVF has also led to a procedure that allows parents to weed out genetically defective embryos. This procedure is called preimplantation genetic diagnosis (PGD).

PGD is often used during IVF to test an embryo for genetic disorders before inserting it into the woman's uterus. Once the egg is fertilized, a cell from each embryo is taken and examined under a microscope for signs of genetic disorders. Many couples use this procedure if there are any inherited disorders in their genes to decrease the possibility that the disorder will passed to their child. Currently, PGD can be used to detect many disorders, including cystic fibrosis, Down syndrome, Tay-Sachs disease and Hemophilia A.


Preimplantation genetic diagnosis involves screening embryos for genetic defects

Some genetic disorders are specific to one gender or another, such as hemophilia, which only affects boys. Doctors may examine the cells to determine the gender of the embryo. In a case where a family has a history of hemophilia, only female embryos are selected for placement in the uterus. This practice is at the center of a larger debate about whether parents should be able to choose embryos purely on the basis of gender. Some people worry that it could lead to an imbalance between genders in the general population, especially in societies that favor boys over girls, such as China.

While PGD enables us to pick out embryos that don't have genetic disorders, and even chose the gender we want, it is only the beginning of what genetic engineering will be able to do. In the next section, we will look at how parents may be able to custom-order babies within a few decades.

Choosing from the Genetic Menu
The idea that we could one day manipulate the genes of humans should not surprise us. Scientists have been altering animal genes for years. Goats and cows have been manipulated so that they produce more milk or more proteins in their milk. Mice have been injected with genes that may cause Alzheimer's Disease in an effort to find a cure. Jellyfish genes have been injected into monkeys, which are in the same class of animals as humans, to show that genes could be inserted into a monkey genome.

One of the more interesting transgenic animals was created by injecting a spider gene into a goat's genome. Spider silk is very strong and, if produced in enough quantity, could create a very powerful type of body armor. And while spiders don't make enough silk to produce this super body armor, scientists discovered that spider silk is a protein similar to goat milk. When the spider gene is inserted into a goat, the goat produces a protein that is identical to that found in spider silk. This protein is extracted from the goat's milk to produce silk fibers, called BioSteel, which is used to make bulletproof vests.


Photo courtesy Nexia Biotechnologies
Spider genes being injected into a goat's egg to produce a transgenic goat

So it is already possible to alter the genetic properties of living animals, some of whom have genetic makeups similar to that of humans. It's only a small leap from here to tampering with human genetics. Humans could be genetically altered to jump higher, see farther, hear better or run faster. Before these super humans can be created, though, we have to learn more about the human genetic code.

One method that could soon be used to manipulate human genetics is called germline gene therapy, and would involve adding an additional step to PGD. Beyond just screening embryos, germline therapy would actually add new genes to the cells. It's possible that almost any trait could be added to an embryo to create a tailor-made child.

Germline therapy, which is already being performed on animals, means manipulating genes of the sperm, egg or early embryo. The name germline comes from germinal cells, which are our reproductive cells. Genetic changes to these cells may not show up in the animal that results from the embryo, but instead show up in later generations.

As more is found out about the human genome and how are genes actually function, we will have to begin to face the possibility of made-to-order children. However, that time is still decades away.

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