One hundred years ago, the term genetics didn’t even exist in the English language. At that time, scientists were just beginning to see and understand what would eventually blossom into the young science of human genetics.
Today, a few short decades later, we know that there are thousands of genetic diseases, and we seem to hear or read about new genetic medical discoveries and breakthroughs every month. We’ve come a long way in a short time.
But just what does the term genetics actually mean?
Let’s take a look at hemophilia. Since Biblical times, people have recognized that hemophilia runs in families. Men with hemophilia tended to have male relatives with the same disease, usually on the mother’s side of the family. Despite this recognition, the way in which a disease like hemophilia was transmitted within families remained unclear until well into the 20th century.
Now we know that hemophilia is a genetic disease, which means the disease occurs because specific genes are not properly working. Genes and other pieces of the genetic puzzle may sound intimidating, but they’re really not, as you’ll see below.
Five easy pieces
For many people, understanding the term genetics represents a giant, unsolvable puzzle. But genetics doesn’t have to be complex. To begin building a solid understanding of the genetic nature of hemophilia, you really need to be familiar with only five straightforward pieces of the puzzle.
1. Genes: Genes are the master planners of the body. They are tiny units of heredity within each of the millions of cells in your body. Genes are the blueprints that tell cells what to do. The 50,000 or so genes in our bodies contain instructions that determine everything physical about us. For example, some genes tell eyes to be blue or brown. Other genes result in brown, blonde, or red hair. Some genes tell cells to make clotting factors to help blood clot. We get our genes from our parents.
2. Chromosomes: Genes live on structures within each cell called chromosomes. Think of chromosomes as tiny, threadlike packages of genes. Each cell contains 46 of these packages arranged in 23 pairs. You get half of each pair from your mother and the other half from your father. One pair of these chromosomes determines your gender. Females have two X chromosomes, while males have an X and a Y chromosome.
3. X-linked: The genes responsible for the production of Factors VIII and IX reside on the X chromosome. That’s why hemophilia is sometimes called an X-linked disorder. It’s also why males typically get hemophilia, as you’ll see below.
4. Mutation: We’ve already discussed how genes determine your physical traits and tell cells what to do. Sometimes there are problems with the genes that result in disease. Genetic disorders are caused by mutations in a gene or a set of genes. Mutations are changes in the gene that can happen at any time, from when we start out as a single cell to when we are 100 years old. In the case of hemophilia, a mutation causes a single gene to be altered or missing.
5. Carrier: Hemophilia is an X-linked disorder. Hemophilia is more common in males because males have only one X chromosome. Hemophilia is usually not observed in females because females have two X chromosomes and one X chromosome can mask the problem of the other. Instead, the female is a carrier of the disease. She has the potential of passing on hemophilia to her children. Recent advances in genetic testing allow women from families with hemophilia to learn whether they are carriers and therefore at risk for having children with hemophilia.
Putting it all together
Now that we’ve described key terms and ideas in genetics, let’s see how genetics works in the inheritance of hemophilia using the scenarios below. Remember that hemophilia results from a mutated gene on an X chromosome. Mothers always contribute an X chromosome to their offspring, while fathers contribute either an X or a Y chromosome. If hemophilia is found on any of these X chromosomes, it can be passed on to the child.
Normal Mother + Father with Hemophilia
Each pregnancy has a 50% chance of resulting in a female carrier and a 50% chance of resulting in a normal male. Sons of hemophiliac fathers and normal mothers will not have hemophilia.
Carrier Mother + Normal Father
Each pregnancy has a 25% chance of resulting in a normal female, a 25% chance of resulting in a female carrier, a 25% chance of resulting in a normal male, and a 25% chance of resulting in a male with hemophilia.
Carrier Mother + Father with Hemophilia
Each pregnancy has a 25% chance of resulting in a female carrier, a 25% chance of resulting in a female with hemophilia, a 25% chance of resulting in a normal male, and a 25% chance of resulting in a male with hemophilia.
Mother with Hemophilia + Father with Hemophilia
Each pregnancy has a 50% chance of resulting in a female with hemophilia and a 50% chance of resulting in a male with hemophilia (actual occurrence is extremely rare).
Mother with Hemophilia + Normal Father
Each pregnancy has a 50% chance of resulting in a female carrier and a 50% chance of resulting in a male with hemophilia (actual occurrence is extremely rare).
In summary, knowledge of genetics lets us make the following statements about hemophilia:
- Nearly all affected people are male.
- Hemophilia may represent a new mutation in the affected male.
- An affected male never transmits the trait to his sons.
- All daughters of an affected male will be carriers (if the mom is not a carrier).
- A carrier female transmits the trait to her sons 50 percent of the time.
- No daughters of a carrier female will show the trait, but a daughter in this case (if the dad is not affected) will be a carrier 50 percent of the time.