Understanding the Odds in Carrier Genetics
You may find that you have basic knowledge about genetics from your high school days, like how particular genetic traits (for instance, hair colour) are inherited. While this rudimentary understanding is important and useful, it can seem inadequate when you need to understand a genetic condition, the results of a carrier screening, or your child’s likelihood of developing a genetic condition.
The reality is that your child’s likelihood of being born with a genetic disorder is dependent on several factors, including maternal and environmental influences. Below is a brief summary of the most common genetic kinds of inheritance.
Autosomal Dominant Inheritance
Children inherit two copies of a gene — one from each parent. In autosomal dominant inherited genetic conditions, if only one parent has a particular mutation that causes the condition, then the child has a 50% probability of inheriting that condition and being affected by it.
Autosomal Recessive Inheritance
In an autosomal recessive inherited genetic condition, if both parents have a particular mutation that causes the disorder, then each child born to the couple has a:
- 25% chance of inheriting (and being affected by) the disease
- 50% chance of inheriting the disease as a carrier, but not being affected by its symptoms
- 25% chance of not inheriting (or being affected by) the disease and not being a carrier
- If only one parent has the mutation, the child has no chance of developing the disease
but has a 50% chance of becoming a carrier.
X-linked Dominant Inheritance
With x-linked inherited disease, it matters who (mother or father) has the genetic mutation and if the child in question is a male or a female. This is because women have two x chromosomes and men have one x chromosome and one y chromosome. When a woman and a man have a male child, the mother passes on one of her x chromosomes and the father passes on his only y chromosome. When a woman and a man have a female child, the mother passes on one of her x chromosomes and the father passes on his only x chromosome.
If a man has an x-linked dominant inherited condition, any male child he has will not be a carrier and will not be affected. This is because a boy will not inherit any genes from his father’s x chromosome. If that same man has a female child, the father will pass on the genes from his x chromosome and his daughter will have a 100% chance of inheriting and being affected by the genetic disorder. Because a woman has two x chromosomes to potentially pass on to either a son or daughter, there is a 50% chance that a woman with an x-linked dominant inherited condition will pass on the mutation for each of her children and they will be affected, regardless of whether they are male or female.
X-linked Recessive Inheritance
If a man has an x-linked recessive inherited condition, their sons will not be affected at all and their daughters will have a 100% chance of getting the mutation and being carriers for the genetic condition (but they will not necessarily be affected — this will depend on the mother’s genes). If a woman has an x-linked recessive inherited condition, this means that she inherited the mutation on both x chromosomes, so she will pass the disorder and mutation onto her son — who only gets an x chromosome from her — but her daughter may only become a carrier (depending whether the father also passed on the disorder with his x chromosome). If a woman is only the carrier (not affected) for an x-linked recessive inherited condition, she has a 50% chance of passing on the disorder to her son (who will be affected), and a 50% chance of passing the mutated gene onto her daughter (who may be a carrier or may be affected depending on whether the father also passed on the disorder with his x chromosome).
Y-linked Inheritance
In y-linked inherited conditions, only men have and inherit the Y-chromosome — and any genetic mutations located therein. This type of inheritance can only be passed from male to male. If a father is affected by the disorder, any sons he has will also be affected.
Talk to Your Doctor
The thought of passing on an inheritable genetic condition to your child is enough to cause considerable stress and uncertainty to a couple who definitely wants to have children. These doubts can affect all aspects of the pregnancy and the relationship, so it’s important to arm yourself with knowledge and always ask questions. If you find yourself with doubts or concerns, speak with your doctor or genetic counselor. They will be able to better explain any particular genetic risk factors that you and your partner are facing, in addition to providing a strong working knowledge of genetics.
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