How genetic diseases are inherited

Genetic disorders are exactly what they sound like: diseases caused by a genetic mutation. When these diseases are inherited (and not as a result of a random mutation), this means that they are passed to the child from one or both parents according to certain inheritance patterns.

These patterns are determined by the gene involved, the presence of a gene in one or both parents, the chromosome on which it is found and other factors, the presence of a mutation does not always mean the disease it is associated with. For example, Huntington's disease , breast cancer, and autoimmune diseases are associated with certain genes, but the person who inherits them does not necessarily develop these conditions.

On the other hand, some genetic mutations, such as those associated with hemophilia , will always present with disease. Additionally, environment can influence the severity of a genetic mutation, which explains why, in some cases, family members with the same genetic mutation may carry the inherited disease differently .

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Inheritance patterns

Various inheritance patterns are attributed to the Austrian scientist Gregor Mendel, who discovered them while working with garden pea hybrids in the 19th century. Mendel is sometimes called the father of modern genetics; similarly, the inheritance patterns of single gene diseases are often described as Mendelian .

According to Mendel's work, there are five different patterns of inheritance: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, and mitochondrial.

The likelihood that a person inherits a genetic disorder depends on two main factors:

  • Whether one copy of the mutated gene (from one parent) or two copies (one from both parents) is passed on
  • Whether the mutation is on one of the sex chromosomes (X or Y) or on one of the other 22 pairs of non-sex chromosomes (called autosomes)

Autosomal dominant

In autosomal dominant disorders, only one copy of the mutated gene is needed, and men and women are equally likely to be affected. Children whose parents have an autosomal dominant disorder have a 50% risk of inheriting the disorder. However, sometimes these disorders are the result of a new mutation and occur in people with no family history. Examples of autosomal dominant disorders include Huntington's disease and Marfan syndrome.

Autosomal recessive

In autosomal recessive diseases, both copies of the mutated gene are present, one from each parent. The bearer will be a person who has only one copy. Carriers will not have any signs or symptoms of the disease. However, they can pass the mutation on to their children.

If in families in which both parents are carriers of the mutation of an autosomal recessive disease, the probability that the children have this disease is as follows :

  • 25% risk of inheriting both mutations and contracting the disease
  • 50% risk of inheriting just one copy and becoming a carrier
  • 25% risk of not inheriting the mutation at all

Examples of autosomal recessive diseases include cystic fibrosis , sickle cell anemia, Tay-Sachs disease, and phenylketonuria (PKU).

X-linked dominant

Dominant X-linked disorders are caused by mutations in genes on the X chromosome (female). In women with two X chromosomes, a mutation in only one of the two copies of the gene is required for the disease to manifest. In men (who have one X chromosome and one Y chromosome), mutations in a single copy of a gene in each cell are enough to cause disease.

In most cases, men have more severe symptoms of X-link disorder than women. However, a characteristic of X-linked inheritance is that parents cannot pass these traits on to their children. Fragile X syndrome is an example of a dominant X-linked disorder.

X-linked recessive

In X-linked recessive diseases, the mutated gene is located on the X chromosome. Because males have both an X chromosome and a Y chromosome, one mutated gene on the X chromosome is enough to cause a recessive chromosome-linked disorder. X.

In contrast, females have two X chromosomes, so a mutated gene on one X chromosome generally affects the female less, because a non-mutant copy on the other largely negates the effect.

However, a woman with a genetic mutation on an X chromosome is a carrier of this disease. Statistically speaking, this means that 50% of his sons will inherit the mutation and develop the disease, while 50% of his daughters will inherit the mutation and become carriers. Examples of X-linked recessive diseases are hemophilia and red-green color blindness .

Mitochondrial

Mitochondria are structures called organelles that exist in every cell in the body where they convert molecules into energy. Each mitochondrion contains a small amount of DNA: a mutation in this DNA is responsible for mitochondrial damage .

Mitochondrial abnormalities are passed down from the mother: only females can share mitochondrial mutations with their offspring because the eggs carry mitochondria to the developing embryo; no sperm .

Conditions resulting from mutations in mitochondrial DNA can occur in every generation of a family and can affect both men and women. An example of an inherited mitochondrial disorder is Leber's inherited optic neuropathy, a form of sudden vision loss .

Other inheritance patterns

In addition to the five main inheritance patterns, there are several more, sometimes recognized by geneticists.

Y-linked disorders

Since only males have a Y chromosome, only males can be affected and pass on Y-linked disorders. All children of a man with a Y-linked disorder will inherit the disorder from their father. Some examples of Y-linked disorders are Y chromosome infertility and cases of Swier syndrome, in which a man's testicles do not develop normally .

Codominance

Codominant inheritance involves the relationship between two versions of a gene. Each version of a gene is called an allele. If the alleles inherited from the parents do not match, the dominant allele will generally be expressed, while the effect of the other allele, called recessive, will be inactive. However, with codominance, both alleles are dominant and therefore the phenotypes of both alleles are expressed. An example of a codominance state is alpha-1 antitrypsin deficiency .

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