Genetic Disorders – Class 12 | Chapter – 5 | Biology Short Notes Series PDF
Genetic Disorders: There are many disorders in the human being that are inherited and caused due to mutation in the gene or alteration in chromosomes.
Types of Genetic Disorders
Pedigree Analysis
It helps in determining the risk of getting a genetic disorder in the offspring by studying the inheritance pattern of a particular trait present in various generations of an individual.
Types of Genetic disorders
Genetic disorders can be grouped into two types:
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:
| 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 |
Chromosomal Disorders
- The chromosomal disorders on the other hand are caused due to absence or excess or abnormal arrangement of one or more sex chromosomes.
- Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy.
- Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism and, this phenomenon is known as polyploidy.
Examples:
- Down’s syndrome- Trisomy of chromosome 21. Symptoms include mental retardation, short stature, furrowed tongue, partially opened mouth.
- Klinefelter’s syndrome- Total 47 chromosomes with one extra X chromosome, i.e. XXY, They are sterile, tall, overall masculine with feminine characteristics such as breast development (gynecomastia).
- Turner’s syndrome- Total 45 chromosomes. One X chromosome is missing, i.e. XO. females are sterile, short stature and under-developed sexual characters.
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