Biology Class 12 NCERT Solutions: Chapter 5 Principles of Inheritance and Variation Part 2

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Q.3 A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?


  • Locus is a fixed position on a chromosome, which is occupied by a single or more genes. Heterozygous organisms contain different alleles for an allelic pair.

  • Hence, a diploid organism, which is heterozygous at four loci, will have four different contrasting characters at four different loci.

  • For example, if an organism is heterozygous at four loci with four characters, say Aa, Bb, Cc, Dd, then during meiosis, it will segregate to form 8 separate gametes.

Image of Diploid Organism

Image of Diploid Organism

  • If the genes are not linked, then the diploid organism will produce 16 different gametes.

  • However, if the genes are linked, the gametes will reduce their number as the genes might be linked and the linked genes will be inherited together during the process of meiosis.

  • Law of dominance states that when a pair of alleles or allomorphs are brought together in F1 hybrid, then only one of them expresses itself, masking the expression of other completely

  • Monohybrid cross was made to study simultaneous inheritance of a single pair of Mendelian factors.

  • The cross in which only alternate forms of a single character are taken into consideration is called monohybrid cross.

  • The trait which appeared in the F1 generation was called dominant and the other which did not appear in the F1 population was called recessive.

  • Thus, when a pair of alleles is brought together in F1 hybrid, then only one of them expresses itself masking the expression of other completely.

  • In the above example, in Tt? F1 hybrid (tall) only? T? Expresses itself so dominant and 'f is masked so recessive. Thus, this proves and explains the law of dominance.

Image of Monohybrid Cross

Image of Monohybrid Cross

Q.4 Explain the Law of Dominance using a monohybrid cross.


  • Mendel’s law of dominance states that a dominant allele expresses itself in a monohybrid cross and suppresses the expression of recessive allele.

  • However, this recessive allele for a character is not lost and remains hidden or masked in the progenies of F1 generation and reappears in the next generation.

  • For example, when pea plants with round seeds (RR) are crossed with plants with wrinkled seeds (rr), all seeds in F1 generation were found to be round (Rr).

  • When these round seeds were self-fertilized, both the round and wrinkled seeds appeared in F2 generation in 3: 1 ratio.

  • Hence, in F1 generation, the dominant character (round seeds) appeared and the recessive character (wrinkled seeds) got suppressed, which reappeared in F2 generation.

Imag eof Law of Dominance

Imag Eof Law of Dominance

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