Mendelian Disorder:
- Genetic disorders grouped into two categories –
- Mendelian disorder
- Chromosomal disorder
- Mendelian disorders are mainly determined by alteration or mutation in the single gene.
- Obey the principle of Mendelian inheritance during transmission from one generation to other.
- Can be expressed in pedigree analysis.
E.g. Haemophilia, colorblindness, Cystic fibrosis, Sickle cell anemia, Phenylketonuria, Thalasemia etc.
Hemophilia:
In this disease a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected. Due to this in an affected individual a simple cut will result in non-stop bleeding.
- Sex linked recessive disease.
- The diseases transmitted from unaffected carrier female to some of the male progeny.
- Female becoming hemophilic is extremely rare because mother of such a female at least carrier and the father should be hemophilic.
- Affected transmits the disease only to the son not to the daughter.
- Daughter can receive the disease from both mother and father.
Sickle cell anaemia:
- The defect is caused due to substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule.
- Substitution of amino acid takes place due to the single base substitution at the sixth codon of the beta globin gene from GAG to GUG.
- The mutant haemoglobin molecule undergoes polymerization under low oxygen tension causing the change in the shape of the RBC from biconcave disc to elongated sickle like structure.
- This is an autosomes linked recessive trait.
- Transmitted from parents to the offspring when both the parents are carrier for the gene (heterozygous).
- This disease is controlled by single pair of allele, HbA, and HbS.
- There are three possible genotypes (HbA HbA, HbA HbS, and HbSHbS.
- Only homozygous individuals for HbS (HbS HbS) show the diseased phenotype.
- Heterozygous (HbA HbS) individuals appear apparently unaffected but they are carrier of the disease as there is 50 percent probability of transmission of the mutant gene to the progeny.
Phenylketonuria:
- Autosomal recessive trait.
- Inborn error of metabolism.
- The affected individual lack one enzyme called phenyl alanine hydroxylase that converts the amino acid phenyl alanine to tyrosine.
- In the absence of the enzyme phenyl alanine accumulated and converted into phenylpyruvic acid and other derivatives.
- Accumulation of these results in mental retardation.
- These derivatives excreted through kidney.
Chromosomal disorders:
- Caused due to absence or excess or abnormal arrangement of one or more chromosome.
- Failure of segregation of chromatids during cell division cycle results in the gain or loss of chromosome(s), called Aneuploidy.
- Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosome in an organism and this phenomenon is called polyploidy.
Trisomy: additional copy of a chromosome may be included in an individual (2n+1).
Monosomy: an individual may lack one of any one pair of chromosomes (2n-1)Down syndrome:
- Caused due to presence of an additional copy of the chromosome number 21 (trisomy of 21).
- This disorder was first described by Langdon Down (1866).
- Short stature with small round head.
- Furrowed tongue
- Partially opened mouth
- Palm is broad with characteristic palm crease.
- Physical, psychomotor and mental development is retarded.
Klinefelter’s syndrome:
- Caused due to the presence of an additional copy of X-chromosome resulting into a karyotype of 47, (44+XXY).
- Overall masculine development.
- Also develop feminine character (development of breast i.e. Gynaecomastia)
- Individuals are sterile.
Turner’s syndrome:
- Caused due to the absence of one of the X- chromosomes i.e. 45 (44 + X0).
- Such females are sterile as ovaries are rudimentary.
- Lack of other secondary sexual characters.
CBSE Biology (Chapter Wise) Class XII ( By Mr. Hare Krushna Giri )
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