Biology Class 12 NCERT Solutions: Chapter 5 Principles of Inheritance and Variation Part 4

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Q.8 Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in F1 generation for a Dihybrid cross?


  • Linkage is defined as the coexistence of two or more genes in the same chromosome. If the genes are situated on the same chromosome and lie close to each other, then they are inherited together and are said to be linked genes.

  • For example, a cross between yellow body and white eyes and wild type parent in a Drosophila will produce wild type and yellow white progenies.

  • It is because yellow bodied and white eyed genes are linked. Therefore, they are inherited together in progenies.

Image of Two Heterozygous Parents are Crossed

Image of Two Heterozygous Parents Are Crossed

Q.9 Briefly mention the contribution of T.H. Morgan in genetics.


  • Morgan’s work is based on fruit flies (Drosophila melanogaster). He formulated the chromosomal theory of linkage.

  • He defined linkage as the co-existence of two or more genes in the same chromosome and performed Dihybrid crosses in Drosophila to show that linked genes are inherited together and are located on X-chromosome.

  • His experiments have also proved that tightly linked genes show very low recombination while loosely linked genes show higher recombination.

Q.10 What is pedigree analysis? Suggest how such an analysis, can be useful.


  • Pedigree analysis is a record of occurrence of a trait in several generations of a family. It is based on the fact that certain characteristic features are heritable in a family, for example, eye colour, skin colour, hair form and colour, and other facial characteristics.

  • Along with these features, there are other genetic disorders such as Mendelian disorders that are inherited in a family, generation after generation.

  • Hence, by using pedigree analysis for the study of specific traits or disorders, generation after generation, it is possible to trace the pattern of inheritance.

  • In this analysis, the inheritance of a trait is represented as a tree, called family tree.

  • Genetic counselors use pedigree chart for analysis of various traits and diseases in a family and predict their inheritance patterns.

  • It is useful in preventing hemophilia, sickle cell anemia, and other genetic disorders in the future generations.

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