Key Difference – Mendelian vs Chromosomal Disorders
DNA is the nucleic acid that is used to store genetic information of most living organisms. Genetic information flows from parent to offspring during the inheritance. DNA molecule is tightly arranged into chromosomes which bear thousands of genes. Each gene consists of two alleles received from each of the two parents. Genetic disorders are caused due to the changes in the genetic information of the organism. It can be due to chromosomal abnormalities or gene (allele) abnormalities. Mendelian disorders are allelic abnormalities which cause genetic diseases in organisms due to inheritance from parent to offspring. Chromosomal disorders are numerical and structural abnormalities of chromosomes which cause genetic diseases due to defective synapsis and disjunction. This is the key difference between mendelian and chromosomal disorders.
CONTENTS
1. Overview and Key Difference
2. What are Chromosomal Disorders
3. What are Mendelian Disorders
4. Side by Side Comparison – Mendelian vs Chromosomal Disorders
5. Summary
What are Chromosomal Disorders?
A chromosome is a thread-like structure of DNA. It is located in the nucleus of living organisms. Some organisms consist of a small number of chromosomes while others have a comparatively large number of chromosomes. Humans have a total of 46 chromosomes arranged in 23 pairs. Chromosomes are composed of DNA sequences and DNA sequences are characterized into genes. Gene is a part of a chromosome which has a distinct nucleotide sequence to determine a specific protein. Chromosomes are really important because they contain the entire genetic information of the organism. Hence, chromosomal structure, composition, and the number affect the content of the organisms’ genetic information.
Chromosomal disorders refer to any abnormal conditions in chromosomes of an organism. They create changes in the amount or the arrangement of genetic information of the organisms which cause problems in growth, development, and functions of the body systems. They occur when there is a change in the number (numerical aberration) or structure (structural aberration) of the chromosomes. Chromosomal physical structure changes when there is a structural aberration. It is caused by several reasons such as deletions, duplications, insertions, inversions, translocations, etc. These changes result in different disease conditions such as birth defects, pregnancy loss, mental retardation, risk of fertility, etc. Down syndrome, Klinefelter syndrome and Turner syndrome are common syndromes caused due to numerical abnormalities of chromosomes. Cri-du-chat syndrome and several other syndromes are caused due to structural abnormalities of chromosomes.
Karyotyping is a key technique employed to identify chromosomal disorders. It reveals information regarding the size, position of the centromere, and banding patterns of the chromosomes. Blood test, prenatal tests, and FISH cytogenetic studies also can be performed to identify chromosomal disorders in humans.
Figure 01: Single chromosomal disorders
What are Mendelian Disorders?
There are thousands of genes in a chromosome throughout its length. A gene has two alleles which indicate the sites of the gene in the chromosome pairs. These loci are known as alleles and they are located in the homologous chromosome pair. Each parent contributes one allele for this allele pair as described in Mendelian inheritance. Diseases can be caused due to mutations in one or more genes on the chromosomes though there are normal chromosome structure and number. These mutations occur due to the alteration of the base sequence of the alleles. They are inherited by the new generation and can lead to defective offspring. These allelic abnormalities are known as Mendelian disorders and can create evolutionary changes in new species.
These allelic abnormalities can be detected in a family by pedigree analysis because these disorders are transmitted from parent to the progeny. Colour blindness, hemophilia, cystic fibrosis, thalassemia, sickle cell anemia are examples of Mendelian disorders.
Figure 02: Mendelian disorder
What is the difference between Mendelian and Chromosomal Disorders?
Mendelian vs Chromosomal Disorders |
|
| Mendelian disorders are the genetic diseases caused by allelic abnormalities. | Chromosomal disorders are the genetic diseases caused by abnormal number and structure of chromosomes. |
| Causes | |
| They are due to mutations in the genes. | They are due to defective synapsis and disjunction during cell division. |
| Transmission to Next Generation | |
| These disorders are transmitted to progeny. | These disorders may or may not be transmitted to next generation. |
Summary – Mendelian vs Chromosomal Disorders
Genetic disorders occur due to abnormalities in genes and chromosomes. Genetic diseases which are caused due to allelic abnormalities are known as Mendelian disorders and are inherited from parent to progeny. Genetic disorders that arise due to chromosomal abnormalities are known as chromosomal disorders and they may or may not be inherited to progeny. Chromosomal disorders mostly occur during cell division and tend to be restricted to a particular individual. This is the difference between Mendelian and chromosomal disorders.
Reference:
1.Schoumans, J., and C. Ruivenkamp. “Laboratory methods for the detection of chromosomal abnormalities.” Methods in molecular biology (Clifton, N.J.). U.S. National Library of Medicine, n.d. Web. 30 Apr. 2017
2.”Cytogenetic Tests for Chromosomal Abnormalities.”Cytogenetic Tests for Chromosomal Abnormalities Eastern Biotech Dubai. N.p., n.d. Web. 30 Apr. 2017.
Image Courtesy:
1. “Single Chromosome Mutations” By Richard Wheeler (Zephyris)Vector version: NikNaks – File:Single Chromosome Mutations.png. (CC BY-SA 3.0) via Commons Wikimedia
2, “Autorecessive” (CC BY-SA 2.0) via Commons Wikimedia