How is Sickle Cell Anemia Inherited?

What is sickle cell anemia?

Sickle cell anemia is a genetic condition that’s present from birth. Many genetic conditions are caused by altered or mutated genes from your mother, father, or both parents.

People with sickle cell anemia have red blood cells that are shaped like a crescent or sickle. This unusual shape is due to a mutation in the hemoglobin gene. Hemoglobin is the molecule on red blood cells that allows them to deliver oxygen to tissues throughout your body.

The sickle-shaped red blood cells can lead to a variety of complications. Due to their irregular shape, they can become stuck within blood vessels, leading to painful symptoms. Additionally, sickle cells die off faster than typical red blood cells, which can lead to anemia.

Some, but not all, genetic conditions can be inherited from one or both parents. Sickle cell anemia is one of these conditions. Its inheritance pattern is autosomal recessive. What do these terms mean? How exactly is sickle cell anemia passed on from parent to child? Read on to learn more.

What’s the difference between a dominant and recessive gene?

Geneticists use the terms dominant and recessive to describe the likelihood of a particular trait being passed on to the next generation.

You have two copies of each of your genes — one from your mother and another from your father. Each copy of a gene is called an allele. You may receive a dominant allele from each parent, a recessive allele from each parent, or one of each.

Dominant alleles usually override recessive alleles, hence their name. For example, if you inherit a recessive allele from your father and a dominant one from your mother, you’ll usually display the trait associated with the dominant allele.

The sickle cell anemia trait is found on a recessive allele of the hemoglobin gene. This means that you must have two copies of the recessive allele — one from your mother and one from your father — to have the condition.

People who have one dominant and one recessive copy of the allele won’t have sickle cell anemia.

Is sickle cell anemia autosomal or sex-linked?

Autosomal and sex-linked refer to the chromosome that the allele is present on.

Each cell of your body typically contains 23 pairs of chromosomes. Out of each pair, one chromosome is inherited from your mother and the other from your father.

The first 22 pairs of chromosomes are referred to as autosomes and are the same between males and females.

The last pair of chromosomes are called sex chromosomes. These chromosomes differ between the sexes. If you’re female, you’ve received an X chromosome from your mother and an X chromosome from your father. If you’re male, you’ve received an X chromosome from your mother and a Y chromosome from your father.

Some genetic conditions are sex-linked, meaning that the allele is present on the X or Y sex chromosome. Others are autosomal, meaning that the allele is present on one of the autosomes.

The sickle cell anemia allele is autosomal, meaning it can be found on one of the other 22 pairs of chromosomes, but not on the X or Y chromosome.

How can I tell if I’ll pass on the gene to my child?

In order to have sickle cell anemia, you must have two copies of the recessive sickle cell allele. But what about those with only one copy? These people are known as carriers. They’re said to have sickle cell trait, but not sickle cell anemia.

Carriers have one dominant allele and once recessive allele. Remember, the dominant allele usually overrides the recessive one, so carriers generally don’t have any symptoms of the condition. But they can still pass the recessive allele on to their children.

Here are a few example scenarios to illustrate how this might happen:

• Scenario 1. Neither parent has the recessive sickle cell allele. None of their children will have sickle cell anemia or be carriers of the recessive allele.
• Scenario 2. One parent is a carrier while the other isn’t. None of their children will have sickle cell anemia. But there’s a 50 percent chance that children will be carriers.
• Scenario 3. Both parents are carriers. There’s a 25 percent chance that their children will receive two recessive alleles, causing sickle cell anemia. There’s also a 50 percent chance that they will be a carrier. Lastly, there’s also a 25 percent chance that their children won’t carry the allele at all.
• Scenario 4. One parent isn’t a carrier, but the other has sickle cell anemia. None of their children will have sickle cell anemia, but they’ll all be carriers.
• Scenario 5. One parent is a carrier and the other has sickle cell anemia. There’s a 50 percent chance that children will have sickle cell anemia and a 50 percent chance they’ll be carriers.
• Scenario 6. Both parents have sickle cell anemia. All of their children will have sickle cell anemia.

How do I know if I’m a carrier?

If you have a family history of sickle cell anemia, but you don’t have it yourself, you may be a carrier. If you know others in your family have it, or you’re not sure about your family history, a simple test can help to determine whether you carry the sickle cell allele.

A doctor will take a small blood sample, usually from a fingertip, and send it off to a laboratory for analysis. Once the results are ready, a genetic counselor will go over them with you to help you understand your risk of passing the allele on to your children.

If you do carry the recessive allele, it’s a good idea to have your partner take the test as well. Using the results of both of your tests, a genetic counselor can help you both understand how sickle cell anemia may or may not affect any future children you have together.

The bottom line

Sickle cell anemia is a genetic condition that has an autosomal recessive inheritance pattern. This means that the condition isn’t linked to the sex chromosomes. Someone must receive two copies of a recessive allele in order to have the condition. People that have one dominant and one recessive allele are referred to as carriers.

There are many different inheritance scenarios for sickle cell anemia, depending on the genetics of both of the parents. If you’re concerned that you or your partner could pass the allele or condition on to your children, a simple genetic test can help you navigate all the potential scenarios.

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