Frequently Asked Questions
There are a lot of misconceptions about genetic disorders and inheritance. Here are some answers to commonly asked questions that may help to clear up confusion.
What's the difference between DNA and a gene?
Your DNA is like a book of instructions that directs what traits you will get. In humans, DNA is very organized and packaged into chromosomes. We get 23 chromosomes from our mother and 23 from our father for a total of 46 chromosomes. Each chromosome has very specific chapters that tell the cell what kinds of characteristics you will have (like eye color and hair color). Each of these chapters that codes for a specific trait is called a gene. So, a gene is a small piece of DNA that has information about one of your traits.
Genes come in pairs. Each person receives one copy of a gene pair from his mother and one from his father. Even though you get half of your genes from your mother and half from your father, each of your parents contributes to all your traits. For the each trait, there are different versions of a gene. Different versions of a gene are called alleles. Although you can have two different alleles, you usually only express one of them. Alleles can be dominant or recessive. Dominant alleles are always expressed, regardless of what the other allele is that a person has. Recessive alleles are only expressed if both of a person's alleles are recessive. If a person has one dominant allele and one recessive, he is called a carrier. This is because he expresses the dominant trait, but "carries" the recessive trait.
An example of this is albinism. Albinism is a condition where a person does not produce any melanin, the protein that gives color to our skin, hair and eyes. Albinos have very pale skin, reddish eye color (because you see straight through to the blood vessels in their eyes) and white hair. Classic albinism is a recessive allele. This means that a person will have albinism only if he gets the "albino" trait from both his mother and his father. If a person gets the albino trait from only one of his parents, he does not express the albino condition and is considered a carrier. This is how someone can pass on a trait that, he does not appear to have.
Albinism is an example of a common type of inheritance called autosomal recessive inheritance. Many genetic conditions are also inherited this way, such as: Cystic Fibrosis, Sickle Cell Anemia and Tay-Sachs Disorder. These conditions have an equal chance of appearing in both males and females.
Before parents can make a baby, certain cells must divide so that the child does not get too much DNA. Each cell that will make the baby must have exactly 23 chromosomes. When this cell from the mother merges with this cell from the father, the baby will have its full 46 chromosomes. These parental cells must go through a very specific kind of division called meiosis where new cells have exactly half the genetic material. During this division, chromosomes that encode the same kinds of genes must line up next to each other. They then split away to the 2 new cells.
Sometimes they "stick" together, rendering one cell with one extra and one cell with one fewer chromosome. This is called nondisjunction. Then the baby will have too many or not enough chromosomes when the father's cell merges with the mother's. This is what happens in Down Syndrome. Chromosome 21 doesn't separate properly. The child then gets one extra chromosome from either the mother or the father. Instead of having 2 copies, it gets 3.
Genetics is related to cancer in that cancer results from mutated genes. Some times these genes are passed down through families putting them at greater risk for getting cancers. Three major events happen to turn a normal cell into a cancerous (or malignant) one.
- Dividing out of control
When a cell divides, it must get the OK from certain molecules (proteins) on its surface. These substances help regulate the cell so it does not divide too much. Specific genes are responsible for making these proteins. If these genes have defects, or mutations, in them, they produce defective proteins. Defective proteins lose their ability to perform their job correctly. The cell then begins to divide more than it should.
- Lose awareness of surrounding cells
Like the proteins that help control how a cell divides, other proteins are involved in letting a cell know that other cells are around. This helps cells to know how much space it has to grow and when it should slow down or stop. Mutations in these proteins cause cell growth to spill over into surrounding tissue. This creates a tumor where cells grow on top of each other instead of in a single layer.
- Lose ability to stay put
Though the first two events may happen, cancer might not result, like a mole. This is an example of a benign tumor. When a tumor is malignant, it has gained the ability to spread throughout the body. Cancerous tumors can cause blood vessels to grow toward them. Then some of the cells can move to other areas of the body by traveling in the blood. This is called metastasis. Since other proteins help a tumor "metastasize", it results from mutations in genetic material.
Every one of your cells has the exact same DNA. As you grow, different factors cause the DNA to be read and make new proteins. However, the DNA never changes. In other words, any gene that you are born with will never go away. Scientists are working on research that may change this in the future (see the Human Genome Project). Even though you keep your genes for life, many genetic disorders can now be well managed.