Mendelian Disorders – Class 12 | Chapter – 5 | Biology Short Notes Series PDF
Mendelian 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 as we have studied in the principle of inheritance. Most common and prevalent Mendelian disorders are Hemophilia, Cystic fibrosis, Sickle-cell anemia, Colour blindness, Phenylketonuria, Thalassemia, etc. It is evident that this X-linked recessive trait shows transmission from carrier female to male progeny.
Examples of Mendelian Disorders
| Name | Genetic Trait | Cause | Effects | Inheritance pattern |
| Sickle cell anaemia | Autosome-linked recessive | A single point mutation in the beta-globin chain of haemoglobin | Anaemia | Offsprings may get the disease when both the parents are a carrier (heterozygote) |
| Thalassemia | Autosome-linked recessive | Mutation in the genes HBA1 and HBA2 present on the chromosome 16 | Formation of abnormal haemoglobin molecule resulting in anaemia | Offsprings may get the disease when both the parents are a carrier (heterozygote) |
| Phenylketonuria | Autosome-linked recessive | Lack of an enzyme that converts phenylalanine to tyrosine | Mental retardation. Accumulation and excretion of phenylalanine and its derivatives in urine | Offsprings may get the disease when both the parents are a carrier (heterozygote) |
| Colour blindness | X-linked recessive | Defect in the green or red cone of the eye | Unable to discriminate between red and green colour | A daughter will be colour blind only if the father is colour blind
There is a 50 percent probability of a carrier female to transfer the disease to sons |
| Haemophilia | X-linked recessive | Defect in one protein involved in the clotting of blood | Continuous bleeding from wounds | A daughter will be colour blind only if the father is colour blind
There is a 50 percent probability of a carrier female to transfer the disease to sons |
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