Mendelian Disorders: Principles, Types and Examples

Mendelian Disorders or Mendelian Diseases are the kinds of disorders or diseases that are caused by the mutation in one gene. These disorders sometimes run in the families. Mendel’s studies of inheritance in pea plants are the foundation of the study of Single-gene diseases or Mendelian Disorders. 

Keyterms: Mendelian Diseases, Mutation, Gene, Pea plants, Single-gene disease, Genome, Pedigree Analysis, Genetic disorder, Chromosome, Sex chromosome

Read More: Difference between Dominant and Recessive Traits

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What are Mendelian Disorders? 

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Mendelian Disorder is a type of genetic disorder. It is caused due to alterations in one gene or abnormalities in the genome. These disorders can be seen since birth and are deduced using the Pedigree Analysis. 

In Pedigree Analysis, the family history is determined through the family tree. Mendelian Disorders affect one in thousands or even millions, making them quite rare. 

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Principles of Mendelian Disorders

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Mendel has performed various studies on pea plants that has helped us to understand inheritance patterns and single-gene diseases in humans. Mendelian Disorders are the result of mutations in one gene and the effect of these diseases run in families. These mutations could be on an autosome or a sex chromosome. With the help of Pedigree Analysis on large families, it can be located whether the disease-associated gene is present on an autosome or a sex chromosome. 

Mendel also gave us Mendel’s Laws of Inheritance which are listed as follows: 

• Law of Dominance and Uniformity
• Law of Segregation
• Law of Independent Assortment 

Read More: Difference Between Nucleotide and Nucleoside

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Types of Mendelian Disorders 

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The various types of Mendelian Disorders are as follows: 

• Autosomal Dominant
• Autosomal Recessive 
• Sex-Linked Dominant
• Sex-Linked Recessive 
• Mitochondrial 

These different types of Mendelian Disorders can be identified with the help of the Pedigree Analysis. 

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Examples of Mendelian Disorders 

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The examples of Mendelian Disorders in humans are as follows: 

• Sickle Cell Anaemia
• Muscular Dystrophy 
• Cystic Fibrosis
• Thalassemia
• Phenylketonuria
• Colour Blindness
• Skeletal Dysplasia 
• Haemophilia 

Sickle Cell Anaemia 

• Sickle Cell Anaemia is a type of autosomal recessive genetic disorder and its inheritance pattern follows the inheritance from two carrying parents. 
• Sickle Cell Anaemia is caused when valine replaces the glutamic acid in the sixth position of the beta-globin chain of the haemoglobin molecule. 
• Due to this mutation, the biconcave shape of the haemoglobin molecule undergoes physical change and converts into the sickle shape. 
• This mutation results in the reduction of the oxygen-binding capacity of the haemoglobin molecule. 

Cystic Fibrosis 

• Cystic Fibrosis is a type of autosomal recessive genetic disorder.
• Cystic Fibrosis attacks the lungs and the digestive system. The body of a person suffering from Cystic Fibrosis produces thick and sticky mucus that jams the lungs and pancreas. 
• People that are diagnosed with Cystic Fibrosis have a short lifespan. 

Thalassemia 

• Thalassemia is a type of autosomal recessive genetic disorder.
• Thalassemia results in the production of an abnormal amount of haemoglobin by the body. This destroys a large number of red blood cells and causes anaemia. 
• The symptoms of Thalassemia include facial deformities, abdominal swelling, dark urine, etc. 

Phenylketonuria 

• Phenylketonuria is a type of autosomal recessive genetic disorder.
• Phenylketonuria is caused by the decreased metabolism level of the amino acid phenylalanine. 
• The person suffering from Phenylketonuria does not have the enzyme that converts phenylalanine into tyrosine. This causes an accumulation of phenylalanine in the body which results in mental retardation. 

Haemophilia

• Haemophilia is a type of sex-linked recessive genetic disorder and its inheritance pattern follows the inheritance from the unaffected carrier mother to the son. The hemophilic genes are passed down to the sun through the mother.
• Haemophilia results in the lack of blood-clotting protein. Because of that, the person suffering from this disorder has symptoms of excessive bleeding from injuries. 
• Haemophilia is caused when the affected genes are located on the X chromosomes and hence, making males more prone to it. 
• Females are rarely affected by it. For a female to get this disorder, the mother should either be hemophilic or a carrier and the father should be haemophilic. 

Also Read:

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Things to Remember

• Mendelian Disorders are a type of genetic disorders that are caused due to alterations in one gene or abnormalities in the genome. 
• Mendelian disorders can be seen since birth and are deduced using the Pedigree Analysis. 
• Mendelian Disorders are the result of mutations in one gene and the effect of these diseases run in families.
• The various types of Mendelian Disorders. These are- Autosomal Dominant, Autosomal Recessive, Sex-Linked Dominant, Sex-Linked Recessive and Mitochondrial 
• Sickle Cell Anaemia is caused when valine replaces the glutamic acid in the sixth position of the beta-globin chain of the haemoglobin molecule. 
• Cystic Fibrosis is a type of autosomal recessive genetic disorder.
• Thalassemia results in the production of an abnormal amount of haemoglobin by the body. This destroys a large number of red blood cells and causes anaemia. 
• Phenylketonuria is caused by the decreased metabolism level of the amino acid phenylalanine.
• Haemophilia results in the lack of blood-clotting protein.

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Previous Year Questions

1. A woman with normal vision, but whose father was colour blind, marries a colour blind man. Suppose that the fourth child of this couple was a boy. This boy...[NEET 2005]
2. A woman with two genes for haemophilia and one gene for colour blindness on one of the 'X' chromosomes marries a normal man. How will the progeny be?...[NEET 1998]
3. ABO blood group system is due to...[NEET 1990]
4. Since there are three different alleles, six different genotypes are possible. How many phenotypes can occur ?...[NEET 2010]
5. According to Mendelism, which character shows dominance?...[NEET 2000]
6. Albinism is a congenital disorder resulting from the lack of which enzyme?...[NEET 1994]
7. Alleles are..[NEET 2015]
8. An allele is dominant if it is expressed in….[NEET 1992]
9. An organism with two identical alleles is...[NEET 1992]
10. XO-chromosomal abnormality in human beings causes….[NEET 1992]
11. In Drosophila, the sex is determined by...[NEET 2003]
12. Name the correct parental of coupling type alongwith its cross over or repulsion...[NEET 1990]
13. Blue eye colour is recessive to brown eye colour. | A brown eyed man whose mother was blue eyed marries a blue-eyed women. The children will be...[NEET 1991]
14. Both sickle cell anaemia and Huntington's chorea are-...[NEET 2006]
15. Cri-du-chat syndrome in humans is caused by the-....[NEET 2006]
16. Cross between AaBB and aaBB will form...[NEET 1990]
17. Crossing over in diploid organism is responsible for..[NEET 1998]
18. Down's syndrome in humans is due to….[NEET 2013]
19. Due to the cross between TTRr x ttrr the resultant progenies show what percent of tall, red flowered plants….[NEET 2000]
20. Experimental verification of the chromosomal theory of inheritance was done by:….[NEET 2020]