Genetic disorders may be grouped into two categories: Mendelian disorders and Chromosomal disorders.

Mendelian disorders are mainly determined by alteration or mutation in the single gene. These disorders are transmitted to the offspring on the same lines. The pattern of inheritance of such Mendelian disorders can be traced in a family by the pedigree analysis.
Examples of Mendelian disorders: Haemophilia, Cystic fibrosis, Sicklecell anaemia, Colour blindness, Phenylketonuria, Thalassemia, etc.
Mendelian disorders may be dominant or recessive. By pedigree analysis one can easily understand whether the trait in question is dominant or recessive. Similarly, the trait may also be linked to the sex chromosome, as in the case of haemophilia.
The chromosomal disorders are caused due to absence or excess or abnormal arrangement of one or more chromosomes. Failure of segregation of chromatids during the cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy.
Example of Chromosomal disorders:
- Down’s syndrome results in the gain of extra copy of chromosome 21.
- Turner’s syndrome results due to loss of an X chromosome in human females.
- Klinefelter's syndrome
Mendelian
Disorders |
Chromosomal
Disorders |
This disorder is mainly due to alternation
or mutation in a single gene. |
This disorder is caused due to absence or
excess or abnormal arrangement of one or more chromosome |
This follows Mendel’s principles of
inheritance. |
This does not follow Mendel’s principles of inheritance
|
This may be recessive or dominant in nature. |
This is always dominant in nature |
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