Question

Why are extremely rare autosomal recessive disorders more likely to appear in families in which blood relatives have children together?

Short answer

Extremely rare autosomal recessive disorders are more likely to appear in families where blood relatives have children together because these relatives have a higher probability to be carriers of the same mutated gene. Therefore, their offsprings have a significantly higher chance of inheriting two copies of the same mutated gene and developing the disorder.

Step-by-step solution

Understanding Autosomal Recessive Disorders

Autosomal recessive disorders are types of genetic diseases that occur when an individual inherits two gene copies, one from each parent, that are both mutated. This can occur when two carries (individuals who both have a single mutated gene copy) have a child together, the child will have a 25% chance of inheriting both mutated gene copies, thereby developing the disorder.

Progeny from Blood Relatives

Blood relatives share a larger proportion of their genes compared to non-related individuals. Therefore, it's more likely that both parents (who are blood relatives) could be carriers of the same mutated gene. This increases the chance that their child will inherit two copies of the same mutated gene.

The Increased Prevalence

In non-related individuals, the chance of both parents being carriers for the same extremely rare autosomal recessive disorder is very low, making the occurrence of these disorders also very low. However, this chance is significantly increased when blood relatives have children together, due to the higher genetic similarity. This is why extremely rare autosomal recessive disorders are more likely to appear in these families.

Key concepts

Genetic Inheritance

Genetic inheritance refers to the process by which genetic information is passed on from parents to offspring. This biological transmission of traits happens via the cellular units called genes. Most people have two copies of each gene, one inherited from each parent.

In the context of autosomal recessive disorders, two copies of a mutated gene must be present for an individual to exhibit the disorder. The autosomal part of the term implies that the genes responsible are not located on the sex chromosomes, but on any of the other 22 pairs of chromosomes that humans typically have. A recessive gene means that two copies are needed for it to impact the individual's health.

For example, let's consider the genetic inheritance of cystic fibrosis, an autosomal recessive disorder. A child must receive one copy of the mutated CF gene from each parent to have this condition. If the child has just one mutated gene, they are a carrier but do not show symptoms of the disease.

Carrier Status

Carrier status indicates that a person has a single copy of a mutated gene for an autosomal recessive disorder but does not show symptoms because the second copy is normal. Carriers can still pass the mutated gene to their children. If two carriers have a child, there is a 25% chance the child will inherit both mutated genes, leading to the disorder, as explained in step 1 of our solution.

Extending upon our cystic fibrosis example, if both parents are carriers, there's a 1 in 4 chance their child will have cystic fibrosis, a 1 in 4 chance the child will be unaffected and non-carrier, and a 50% chance the child will also be a carrier. Understanding carrier status is crucial for genetic counseling and reproductive decisions, as the knowledge helps to assess potential risks for offspring.

Consanguinity

Consanguinity is the term used to describe the relationship between individuals who are blood relatives. When blood relatives reproduce, their offspring are more likely to inherit the same autosomal recessive mutations. This is because relatives have a higher probability of carrying the same ancestral mutated gene that's been passed through generations.

Consanguineous relationships increase the risk of genetic disorders due to this shared genetic heritage. In step 2, we discussed that blood relatives would likely share more genetic similarities, meaning if a mutation exists in the family gene pool, it has a higher chance of being inherited from both parents. This is why genetic counseling and understanding your family health history is important, particularly in communities where consanguinity is common, as preventative measures or early interventions can be applied.