Long before the advent of genetic testing, or even knowledge of DNA and RNA, astute observers noticed that many traits were passed from one generation to another. But it still can be difficult to understand why some people inherit a genetic disease and others do not. Also, it’s often not clear which family members are at risk of inheriting a condition.
To understand heredity, you have to know a little about genetics. Below is a review of the three major genetic-disease inheritance patterns. But, first, a little background:
Genes and Mutations
Genetic conditions, including many retinal diseases, are caused by genetic variations, or mutations, passed down from one or both parents. Everyone has about 25,000 pairs of genes, which play a major role in determining who we are—our height, hair color, etc. We all have many variations and mutations in our genes, and most do not affect health or well-being. However, some cause problems and lead to diseases, including those of the retina.
Genes are wrapped in structures called chromosomes, of which humans normally have 23 pairs. We inherit one of each chromosome from our mother and father. Chromosomal pairs 1 through 22 are known as autosomal (non-sex) chromosomes. The X and Y chromosomes on pair 23 are known as the sex chromosomes, because one of their functions is to determine our sex.
To get an autosomal dominant disease, a child receives one copy of a mutated gene from an affected parent. It’s likely the affected parent knows he or she has the disease, or at least has related symptoms. The other parent is usually neither affected nor a carrier.
With dominant diseases, there’s a 50 percent chance that a child will receive the mutated gene and be affected by the disease. Dominant diseases frequently appear in several generations of a family, with many family members being affected.
To get a recessive disease, a child must inherit a mutated copy of the gene from both parents. With recessive conditions, each parent has one mutated copy and one normal copy of the relevant gene. Because each parent has only one mutated copy of the gene, they’re unaffected carriers of the condition. They usually have no idea that they’re carriers because they have no related vision problems.
If their child inherits only one mutated gene from one parent, then the child will only be a carrier of the disease and usually won’t experience any vision problems. If their child inherits a mutated copy from each parent, then the child will develop the disease and associated vision loss.
With recessive disease, there’s a 25 percent chance that the child will be affected and a 50 percent chance he or she will be an unaffected carrier. Recessive diseases are more likely to surprise a family because parents of an affected child usually don’t know they’re carriers.
Inheritance patterns for X-linked diseases are more complex than the others, because the gender of both parents and their children often determine if the disease will be passed down.
As mentioned previously, men have an X and Y chromosome and women have two Xs. In most cases, mothers pass X-linked diseases down to their sons. Daughters usually don’t get the disease, because they also have a healthy X chromosome from their fathers. However, researchers have found that women can sometimes have mild vision loss or, in rarer cases, severe vision loss from X-linked retinal diseases.
If an X-linked carrier mother has a son, there’s a 50 percent chance he will be affected. If an X-linked carrier mother has a daughter, there’s a 50 percent chance the daughter will be a carrier.
A Word on Age-Related Macular Degeneration (AMD)
AMD is the leading cause of blindness in people over 55 living in developed countries. It’s different from inherited retinal diseases, because risk of the condition is caused by a complex combination of genetic and lifestyle factors, including advancing age and smoking. The best way to determine risk of vision loss from AMD is through an examination of the retina by an eye doctor.
Image, above, courtesy of the National Institutes of Health.