Introduction
G.J Mendel is known as the father of genetics. He was the person who introduced the world to the world of genetics. Genes are made up of two alleles. When a gene is not working,it results in deficiency in the formation of the enzymes or proteins formed by the gene.This is known as disorder.
Mendelian disorders
There are two types of human genetic disorders. The first is chromosomal disorders and the second is Mendelian disorders. Chromosomal disorders are related to autosomes and sex chromosomes abnormalities. Mendelian disorders are related to abnormalities in genes. Gene related disorders are of two types:
- Gene mutations in Autosomes
- Gene mutations in sex chromosomes
Types of Mendelian disorders
Gene mutations in autosomes are further divided into two parts. The first is recessive, and the second is dominant. The most common recessive Mendelian disorders are Gaucher’s Disease, Tay-Sachs Disease, Cystic Fibrosis, Thalassaemia, sickle cell Anaemia, Albinism, Alkaptonuria and Phenylketonuria.
Phenylketonuria
It is a recessive, autosomal, inborn metabolic disorder in which the phenylalanine hydroxylase enzyme is lacking in homozygous recessive individuals. The enzyme changes phenylalanine to tyrosine in the liver. Abnormal autosomal recessive gene on chromosome 12 is the cause of the lack of this phenylalanine and related compounds in hyperphenylalaninemia. Babies affected by this disease are normal at birth, but sometimes, in a few weeks, there starts a rise in plasma almost 30-50 times, resulting in an impairment in brain development.
Symptoms
- A baby of six months has developed severe mental retardation in this disease.
- If not treated on time, the child will be unable to walk or talk.
- Brain damage
- Poor development
- Learning disabilities
- Behavioural problems
Alkaptonuria
Alkaptonuria was described by Garrod in 1908. The cause of the metabolic, recessive, autosomal disorder is a result of a lack of an oxidase enzyme needed for the breakdown of tyrosine. The deficiency of the enzyme is a result of the absence of a normal gene on chromosome 3, which controls the enzyme’s synthesis.
Symptoms
- Affected parts are heart cartilages, capsules of joints, ligaments, valves and tendons.
- If their urine is left in the air for some hours, it becomes black due to homogentisic acid.
Albinism
The cause of this autosomal, recessive genetic disorder is the lack of tyrosinase enzyme, which is required for the synthesis of melanin pigment from dihydroxyphenylalanine. This is why only homozygous recessive individuals are affected by the disease. The lack of dark pigment melanin in the iris and hair is visible. Individuals with Albinism have poor vision but can live everyday life.
Symptoms
- Sensitivity to light
- Poor vision
- Strabismus
- Nystagmus
- Increased risk of skin cancer
Sickle cell anaemia
In this autosomal recessive disorder disease, the affected individual produces an altered form of haemoglobin. Individuals affected by this disease produce only haemoglobins. The result is their RBC. Red blood cells change into shape as sickle due to less oxygen concentration. Sickle cell allele is found more frequently in people who have suffered from malaria.
Thalassaemia
Thalassaemia was found by Thomas Cooley. Bradford and Whipple coined the term. It is an autosomal recessive blood disease found in children of a pair of heterozygous parents. This disease occurs due to the deletion and mutation of the genes that control haemoglobin’s globin chain formation. The imbalanced synthesis of the globin chain of haemoglobin causes anaemia.
There are three types of thalassaemia:
- Alpha thalassaemia
- Beta-thalassaemia
- Delta thalassaemia
Cystic Fibrosis
This disorder is seen in infants, children and young adults. It is an abnormal recessive disorder. The cause of the disease is the recessive autosomal allele present on chromosomes. Because of this disease, defective glycoprotein is produced. The result is thick mucus in the liver, skin, pancreas, lungs, and other organs.
Symptoms
- Failure of the chloride ion transport mechanism.
- The sweat of the patient contains a significant level of Na+ and Cl-
- Because of mucus, obstruction of airways.
- Recurrent pulmonary infection and lung damage.
- Maldigestion of food.
Dominant Mendelian disorders
Some of the common dominant Mendelian disorders are Huntington’s Chorea, Alzheimer’s disease, Hypercholesterolemia, Neurofibromatosis, Achondroplasia, Marfan Syndrome, Aniridia, Myotonic Dystrophy.
Huntington’s Chorea
It is located on chromosome 4. It results in uncontrollable shaking, which is a loss of motor control. It results in slurring of speech, loss of memory and hallucinations.
Alzheimer’s disease
There are two causes of it. The first is ageing, and the second is two defective autosomal – chromosome 21 and chromosome 19. The cause of this disease is amyloid protein plaque in the brain, resulting in neuron degeneration.
Hypercholesterolemia
It is due to the lack of receptors on liver cells that take up LDL, low-density lipoprotein from the bloodstream. The children having two mutant alleles die due to heart attack. With one mutant allele, he dies of a heart attack in young adulthood.
Neurofibromatosis
It causes tumours on nerve tissue. These tumours are non-cancerous. Symptoms are hearing loss, learning impairment, cardiovascular problems, vision loss, and severe pain.
Aniridia
It is an eye disorder in the partial or complete absence of the coloured part of the eye.
Conclusion
Mendelian disorders are related to genes disorders. Gene related disorders of two types. The first is Gene mutations in Autosomes, and the second is gene mutations in sex chromosomes.
The most common recessive Mendelian disorders are Gaucher’s Disease, Tay-Sachs Disease, Cystic Fibrosis, Thalassaemia, sickle cell Anaemia, Albinism, Alkaptonuria, and Phenylketonuria.
Phenylketonuria is an autosomal disorder in which the phenylalanine hydroxylase enzyme is lacking in homozygous recessive individuals.
The cause of Albinism is an autosomal, recessive genetic disorder due to the lack of tyrosinase enzyme, which is required for the synthesis of melanin pigment from dihydroxyphenylalanine.