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Chromosomal Theory of Inheritance


Sutton (1903), while working on meiosis in grasshopper, and Boveri (1903), on sea urchin.
independently proposed the chromosomal theory of inheritance, according to which the inheritance is
due to the transfer of chromosomes from one generation to other. Thus, there is parallelism between
behaviour of chromosomes during meiosis and Mendelian traits.

  1. Similar to the separation of homologous chromosomes during Anaphase I of meiosis; the
    two alleles (e.g., D and d) for a trait in a hybrid (e.g., Dd FI) get separated into two
    gametes at the time of gamete formation (law of purity of gametes or law of segregation). This shows that Mendelian factors (genes) are located on chromosomes.
  2. Orientation of maternal and paternal choromosomes in a pair of homologous chromosomes is independent of the orientation of other homologous pairs of chromosomes, thus, to give 2” arrangement at the time of Metaphase I of meiosis (n = number of pairs of chromosomes), like the independent assortment of various traits where one pair of trait is independent of the pairs of traits as shown in dihybrid, trihybrid and so on crossos (law of independent assortment). Thus.



i) when one pair of trait/homologous chromosomes is considered, FI produces two types , of gametes (n = 1, 21 = 2) due to two types of orientations of chromosomes at Metaphase I,

(ii) when two pairs of traits are considered, FI gives tour types of gametes (n = 2, 22 = 4), due to four types of possible arrangements of chromosomes (Fig, 6,10)

(iii} when 3 pairs of traits are considered together, FJ gives eight types of gametes (n = 3, 23 =.8), and so on (Fig. 6,11).


Fig 6.10 Independent assortment of two pairs of alleles located on two pairs of homologous chromosomes.


Eight types of possible gametes produced by F1.

After self fertilization, give 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1 ratio in F2 generation.

Fig.6.11 Independent orientation or three pairs of homologous chromosomes at Metaphase I of meiosis.




  1. Why did Mendel use pea as the experimental material in his experiments. Give a brief outline of the experiments conducted by him.


Discuss Mendel’s laws of inheritance.


  1. Write short notes on:

i) Reciprocal crosses

ii) Back cross

iii) Test cross

iv) Incomplete dominance

  1. In human beings brown eyes (B) are dominant to blue eyes (b); and dark hair (R) dominant
    to red hair (r).

A man with brown eyes and red hair marries a woman with blue eyes and dark hair. They have two children, one with brown eyes and red hair and the other with blue eyes and dark hair.

Give the genotypes of the parents and children.

  1. Discuss the chromosomal theory of inheritance with the help of suitable sketches. Explain
    Mendel’s laws using chromosomes as the carriers of Mendelian factors.