What are Principles Of Inheritance And Variation?

Laws Of Heredity

Law of Dominance

Out of two contrasting genes for a particular trait, only one is expressed. This gene is called dominant and the other gene of the pair which does not express and remains unexpressed is called recessive.

Law of Segregation or Purity of Gametes

  • Both the alleles (recessive and dominant) separate and then expressed phenotypically in F2 generation. When F1 hybrids were allowed to self-pollinate and F2 progenies were obtained, Conclusion: Genotypic ratio: 1:2:1, Phenotypic ratio: 3:1

Law of Independent Assortment

  • The distribution of two or more genes in the gametes and in the progeny of subsequent generations is independent of each other. Conclusion: Genotypic ratio: 1:2:1:2:4:2:1:2:1., Phenotypic ratio: 9:3:3:1

Test Cross

A cross between F1 hybrid (Aa) and its homozygous recessive parent (aa) is called Test Cross. Used to find out whether the given dominant phenotype is homozygous or heterozygous.

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Incomplete Dominance

When neither of the alleles of a character is completely dominant over the other and the F1 hybrid is intermediate between the two parents, the phenomenon is called incomplete dominance. E.g. flower colour in 4’O clock plant, snapdragon.
phenotypic ratio and genotypic ratio in F2 generation in 4’O clock plant is 1:2:1.

Chromosome Theory Of Inheritance

Proposed by Sutton and Boveri independently in 1902.

Salient Features

  • Both chromosomes as well as genes occur in pairs in the somatic or diploid cells.
  • A gamete contains only one chromosome of a type thus it maintains half set of chromosome.
  • Pairing of chromosomes is restored during fertilization which makes individual diploid.

Multiple Allelism/codominance

When a gene exists in more than two allelic forms, it shows the phenomenon of multiple allelism. E.g. A, B and O blood groups in human being.

Linkage And Recombination

Linkage is the phenomenon, where two or more linked genes are always inherited together.

Sex Determination

xx-xy type

  • Females: XX (Homomorphic)
  • Males: XY (Heteromorphic)
  • e.g. insects including Drosophila and mammals

zz-zw type

  • Males: ZZ (homogamety)
  • Females: ZW (heterogamety)
  • Females: ZW (heterogamety)

xx-x0 type

  • Males: XO
  • Females: XX
  • e.g. round worms and some insects

Sex Determination In Humans

Human beings have 22 pairs of autosomes and one pair of sex chromosomes (XX/XY).


It is a sudden and inheritable change in the sequence of DNA.


It is the study of an inherited trait in a group of related individuals to determine the pattern and characteristics of the trait and mode of inheritance.

Mendelian Disorders

The disorders which are mainly determined by alteration or mutation in the single gene and are transmitted to the next generation. Examples : Hemophilia, Sickle cell anemia, Colour blindness, Phenylketonuria, Thalassemia, etc.

Hemophilia It is a sex linked recessive disease. It shows transmission from unaffected carrier mother to some of the male progeny.

Colour Blindness It is the defect in X chromosome. Since males have only one X chromosome and therefore about 80% males suffer while females are mostly carrier.

Thalassemia It is a autosome-linked recessive blood disorder. It arises due to mutation in the globin chains that makes hemoglobin.

Sickle Cell Anemia It is a autosome-linked recessive trait. The patient has sickle shaped RBCs with defective hemoglobin.

Chromosomal Disorders

Due to absence or excess or abnormal arrangement of one or more chromosomes due to non disjunction of chromosomes.

Aneuploidy Arises due to loss or gain of one or more chromosomes during gamete formation. Example: Down’s syndrome (47) and Turner’s syndrome (45).

Down’s Syndrome It is due to trisomy of 21st chromosome.

Klinefelter’s Syndrome It arises due to non-disjunction of X-chromosomes during ova formation.

Turner’s Syndrome It arises due to non-disjunction of X-chromosomes during ova formation. It is a chromosomal disorder in which a female is born with only one X chromosome.