Heredity & Evolution
NCERT Exemplar Problems
Give the basic features of the mechanism of inheritance.
Answer: We know that each character is controlled by a gene. These genes are present on DNA. DNAs; in turn are present in the form of chromosomes. Every trait is present in pairs, i.e. alleles. Out of a pair of contrasting traits, one is present on a particular chromosome and another is present on another chromosome. This ensures that one trait goes to one gamete and another goes to another gamete; when gametes are formed. This forms the basis of the Law of Segregation; as proposed by Mendel.
Let us recall Mendel’s experiment on monohybrid cross in which he crossed a pure breeding tall plant with a pure breeding short plant. All the plants in F1 generation were tall. The gene governing the tallness dominated in F1 generation. However, in F2 generation some plants were tall while others were short. This could be possible because of recombination of gametes during fertilization.
In the following crosses write the characteristics of the progeny
(a) RRYY × RRYY (round yellow × round yellow)
(b) RrYy × RrYy (round yellow × round yellow)
(c) rryy × rryy (wrinkled green × wrinkled green)
(d) RRYY × rryy (round yellow × wrinkled green)
- Round yellow (RRYY)
- Round yellow (RRYY), Round yellow (RrYy), Round green (RRyy), Round green (Rryy), wrinkled yellow (rrYY), wrinkled yellow (rrYy), wrinkled green (rryy)
- wrinkled green (rryy)
- Round yellow (RrYy)
Study the following cross and showing self pollination in F1, fill in the blank and answer the question that follows
|round yellow||wrinkled green|
|RrYy (round yellow)||?|
Answer: RrYy (round yellow) refer to answer 'd' in previous question.
In the previous question, what are the combinations of character in the F2 progeny? What are their ratios?
Answer: Round yellow (RRYY), Round yellow (RrYy), Round green (RRyy), Round green (Rryy), wrinkled yellow (rrYY), wrinkled yellow (rrYy), wrinkled green (rryy). This can be shown by following Punette Square:
Give reasons for the appearance of new combinations of characters in the F2 progeny.
Answer: Appearance of new combination of characters in F2 progeny can be understood by illustrating dihybrid cross; as carried out by Mendel. Let us assume that Mendel selected pea plants with round green (RRyy) seeds, and plants with wrinkled yellow (rrYY) seeds. Let us assume that roundness and yellow colour were the dominant traits.
All the progenies in F1 generation produced round yellow seeds with genotype (RrYy) and showed dominant traits only. However, genes for recessive traits were present in all the plants. This happened because of Law of Independent Assortment. This facilitated the combination of the trait of roundness with the trait of yellow colour; although these traits came from different parents.
In the F2 generation, recombination of traits took place once again; in accordance with the Law of Independent Assortment.
During Mendel’s experiment with dihybrid cross, it was seen that all the plants in F1 generation produced round and yellow seeds. However, in F2 generation, phenotypes of different seeds were different. This shows new combination of characters in F2 generation.
Due to this, the F2 generation showed different combinations of genotype, viz. RRYY, RRYy, RrYY, RrYy, rrYY, rrYy, rryy. This can be shown by the Punette square used in previous answer.