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Principles of Inheritance and Variation

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Class 10 Class 12
Name one trait that does not blend.

Sex trait .

In man four types of blood groups A, B, AB and O are controlled by three alleles of a gene. What is the mechanism of inheritance of the blood groups ?

ABO blood types in human is a classic example of multiple allelism  and co-dominance where more than two allele forms are present for a trait and both A and B allele are dominant over i allele. Three different alleles IA, IB and i or IO  determine the phenotypes of the four blood groups. 

Both IA and IB are dominant and express themselves over IO or i. Iand IB show co-dominance and produce different forms of sugar wheras Ior i does not produce any sugar. The  Thus the phenotypes produced are -:

IAIA and IAIO Produces the phenotype of A blood group.
IBIB and IBIO produces blood group B.
IAIproduces Blood group AB .
IOIO produces Blood group O.




A black-colour cock when bred with a white coloured hen produced steel blue coloured offspring and when inbred, black, white and steel-blue coloured progeny were obtained:
(a) This result is genetically explained as.
(b) What will be the expected ratio and proportions of black, steel-blue and white progeny?

(a) Incomplete dominance 
(b) The expected ratio will be 1 : 2 : 1.
i.e. 1 black, 2 steel-blue and 1 white.


Write the various conclusions and hereditary principles drawn by Mendel from a monohybrid cross.

Conclusions and hereditary principles derived from monohybrid cross are-:

1. Principle of paired factors. 

2. Law of dominance. In a heterozygous individual only one allele is able to express its effect called dominant while the other remains hidden and is  termed recessive.

3. Principle of purity of gametes. A gamete receives only one of the two

4. Principle of segregation. The two factors of a character keep their identity in an individual and do not blend. They segregate (separate) during gamete formation and are passed on to the offspring randomly after fertilization.


List the main points of chromosome theory of inheritance.

Chromosome theory of inheritance staes that

1.Chromosomes and genes are found in pairs in diploid cells.

2. Both chromosomes and genes segregate in meiosis I.

3. Each gamete receives only one chromosome and one allele of the gene pair.

4. Paired condition of both is restored during fertilization.

5. Both chromosomes and genes retain their individuality throughout life cycle.


Who proposed the chromosome theory of inheritance ? What does the Chromosomal theory of inheritance.

Sutton and Boveri proposed the Chromosomal  theory of inheritance.

The Chromosomal theory of inheritance states that

1. Chromosomes as well as genes occur in pairs.
2. Two alleles of a gene pair are located on the homologous sites of homologous  chromosome
3. The  chromosome segregate and assort independently just like the genes during meiosis.
 4. They  recombine at the time of fertilisation in the zygote to re establish the parental chromosome number.