19 Jan Autosomal Recessive Traits
Autosomal Recessive Traits
Autosomal Recessive Traits Autosomal recessive diseases are genetic diseases that are passed to a child through both parent’s chromosomes. Each person inherits 23 chromosomes from each parent and so has 23 pairs of chromosomes. Autosomal recessive traits appear only when an individual carries both copies of a recessive allele. They share some characteristics with autosomal dominant traits since both are encoded by autosomal genes; they occur with equal frequency in both sexes and are transmitted by both males and females. Trait is rare in pedigree Trait often skips generations (hidden in heterozygous carriers). Recessive inheritance means both genes in a pair must be abnormal to cause disease. People with only one defective gene in the pair are called carriers. These people are most often not affected with the condition. However, they can pass the abnormal gene to their children. These include disorders the genes for which are present on autosomes and are recessive to their alleles. These disorders are expressed only in a homozygous condition like the typical Mendelian recessive disorders.
Example; 1. Albinism 2. Tay – Sachs disease 3. Cystic fibrosis 4. Sickle cell anemia 5. Phenylketonuria 6. Autosomal recessive polycystic kidney disease (ARPKD)
Characterstics-
ü They tend to skip generations.
ü The affected individuals are generally born to unaffected parents (heterozygous carriers that are usually asymptomatic).
ü The incidence of the trait increases in consanguineous marriages (matings between close relatives), due to shared alleles.
ü In a mating between heterozygous carriers, there is 25% chance of having affected offsprings which may not always be evident in small families.
SICKLE CELL ANAEMIA Sickle Cell Anaemia (SCA) is a severe, inherited blood disorder. It is part of a group of conditions known as Sickle Cell Disease (SCD). Normal hemoglobin cells are smooth, round and flexible, like the letter “O.” They can easily move through the vessels in our bodies. Sickle cells are stiff and sticky. When they lose their oxygen, they form into the shape of a sickle, or the letter “C”. These sickle cells tend to cluster together and can’t easily move through the blood vessels. The cluster causes a blockage and stops the movement of healthy, normal, oxygen-carrying blood. This blockage is what causes the painful and damaging complications of sickle cell disease. In people with SCA, the red blood cells—which are typically flexible and disc-shaped—become rigid, sticky, and shaped like a crescent or “sickle.” Sickle cell anemia is another common, inherited, single-gene disorder in African-Americans. About 1 in 500 African-American babies is born with sickle cell anemia. About 1 in 12 AfricanAmerican people carries the gene for this disease. Sickle cell disease involves the red blood cells or hemoglobin and their ability to carry oxygen.
1. The Cause: Molecular Level
The root cause of sickle cell anaemia is a point mutation in the DNA.
· The Gene: It affects the HBB gene on chromosome 11, which provides instructions for making beta-globin (a part of hemoglobin).
· The Switch: A single nucleotide base is swapped: the codon GAG (which codes for Glutamic Acid) becomes GTG (which codes for Valine).
· The Result: This tiny change creates an abnormal version of hemoglobin called Hemoglobin S (HbS). Unlike normal Hemoglobin A, HbS molecules “clump” together into long, stiff rods when oxygen levels are low. These rods physically stretch and distort the red blood cell into a sickle shape.
2. Genetics and Heredity
Sickle cell anaemia is an autosomal recessive disorder. This means a person must inherit two copies of the “sickle” gene (one from each parent) to have the disease
· Sickle Cell Trait (AS): If you inherit only one sickle gene, you are a “carrier.” You generally lead a normal life without symptoms, but you can pass the gene to your children.
· Sickle Cell Anaemia (SS): If you inherit two sickle genes, you have the full disease.
· The Evolutionary Link: The sickle cell trait is more common in people whose ancestors come from regions where malaria is prevalent (Africa, India, Mediterranean). Interestingly, being a carrier provides a survival advantage against malaria, which is why the gene has persisted in the population.
3. The Disorder (Pathophysiology)
The “sickling” of cells causes two primary problems in the body:
1. Vaso-occlusion (Blockage): Sickled cells are rigid and sticky. They get stuck in small blood vessels (capillaries), blocking the flow of oxygen-rich blood to tissues and organs. This leads to intense pain and organ damage.
2. Hemolysis (Early Cell Death): Normal red blood cells live for about 120 days. Sickle cells are fragile and break apart (hemolyze) after only 10 to 20 days. The bone marrow cannot keep up with this pace, leading to a chronic shortage of red blood cells (anaemia).
3. Spleen- Normal hemoglobin cells can live up to 120 days. Sickle cells risk being destroyed by the spleen because of their shape and stiffness. The spleen helps filter the blood of infections. Sickle cells get “stuck” in this filter and die. Due to the decreased number of hemoglobin cells circulating in the body, a person with sickle cell is chronically anemic. The spleen also suffers damage from the sickle cells, which block the healthy oxygen-carrying cells. After repeated blockages, the spleen becomes very small and does not work properly. Without a functioning spleen, these people are more at risk for infections. Infants and young children are at risk for life-threatening infections.
4. Symptoms and Complications
Symptoms usually begin around 5 to 6 months of age as “fetal hemoglobin” (which doesn’t sickle) is replaced by the adult version.
· Pain Crises: Sudden, severe episodes of pain in the chest, joints, or abdomen.
· Dactylitis: Painful swelling of the hands and feet (often the first sign in infants).
· Fatigue and Paleness: Common signs of anaemia.
· Jaundice: Yellowing of the eyes and skin due to the rapid breakdown of red blood cells.
· Acute Chest Syndrome: A life-threatening condition similar to pneumonia, caused by infection or trapped sickle cells in the lungs.
· Splenic Sequestration: The spleen traps a large amount of sickled cells, leading to a sudden drop in hemoglobin that can be fatal.
Treatment includes prompt emergency care for fevers and infections, appropriate vaccinations, penicillin and management of anemia. The Future: Gene Therapy Recent breakthroughs (including CRISPR technology) allow doctors to modify a patient’s own stem cells to produce healthy hemoglobin or “turn back on” fetal hemoglobin. These cells are then infused back into the patient, potentially curing the disease without the need for a donor.
From this topic the there are already PYQs in UPSC Zoology Optional Paper 2, also as segments wise.
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