Pedigree Analysis:

  • Analysis of traits in several of generations of a family is called the pedigree analysis.
  • In the pedigree analysis the inheritance of a particular trait is represented in the family tree over generations.

Autosomal Dominant:

  • Affected individuals have at least one affected parent
  • The phenotype generally appears every generation
  • Two unaffected parents only have unaffected offspring
  • Traits are controlled by dominant genes
  • Both males and females are equally affected
  • Traits do not skip generations
  • e.g. polydactyly, tongue rolling ability etc

Autosomal recessive:

  • Unaffected parents can have affected offspring
  • Traits controlled by recessive  genes and
  • Appear only when homozygous 
  • Both male and female equally affected
  • Traits may skip generations
  • 3:1 ratio between normal and affected. 
  • Appearance of affected children from normal parents (heterozygous)
  • All children of affected parents are also affected.
  • e.g.- Albinism, sickle cell anaemia etc



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.


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.


  • 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.

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Gregor Jhon Mendel conducted hybridization experiments on garden peas(Pisum sativum) and proposed laws of inheritance.He proposed three laws of inheritance;law of Dominance, law of segregation, law of independent assortment.


Chromosomal theory of inheritance states that genes are located on specific site on chromosomes which are responsible for the inheritance of characters. Linkage and crossing over, polygenic inheritance, pleiotropy are based on this theory. Sex determination and mutation are the phenomenon which which influence the inheritance pattern of organisms.


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