Types of Genetic disorders

 

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.