Comprehension:
Principles of Inheritance
Read the following passage carefully and answer the given questions.
Gregor Mendel conducted hybridisation experiments on garden peas for seven contrasting characters. He crossed tall and dwarf pea plants to study the inheritance of characters. Mendel observed that all the F1 progeny plants were tall. Mendel then self-pollinated tall F1 plants and surprisingly observed the proportion of tall and dwarf plants in 3:1 ratio. Hence, the tall and dwarf traits were identical to their parental type and did not show any blending that is all the offspring were either tall or dwarf and none were of in between height. This study explains the Law of Segregation that the alleles do not show any blending and that both the characters are recovered as such in the F2 generation though one of these is not seen at the F1 generation. Further, Mendel observed the four different phenotypes from the cross of two contrasting characters (seed shape and seed color) which led to the conclusion that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters as stated by Law of Independent Assortment. Walter Sutton and Theodore Boveri noted that the behaviour of chromosomes was parallel to the behaviour of genes and used chromosome movement to explain Mendel's law. The important things to remember that chromosomes as well as genes occur in pairs. The two alleles of a gene pairs are located on homologous sites on homologous chromosomes.
According to the Mendelian theory of inheritance, the chromosome pairs align independently during:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Anaphase of meiosis-I
Mendel’s law of independent assortment says that one pair of characters separates independently of another pair. Sutton and Boveri explained it by the behaviour of chromosomes in meiosis. The two chromosome pairs behave independently of each other. So a chromosome of one pair can go to a pole together with either chromosome of the other pair, and all combinations are formed.
(1) Metaphase of meiosis-I — Incorrect. At metaphase I the chromosome pairs take their places on the plate. NCERT places the independent behaviour of the pairs in anaphase I, when the pairs separate.
(2) Metaphase of mitosis — Incorrect. The homologous chromosomes do not form pairs in mitosis. Each chromosome lines up alone, so there are no pairs to sort.
(3) Anaphase of meiosis-I — Correct. NCERT states that during anaphase of meiosis I the two chromosome pairs behave independently of each other. This gives the independent assortment of the characters.
(4) Anaphase of mitosis — Incorrect. In mitotic anaphase the sister chromatids separate. No chromosome pairs are sorted.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
Chromosomal Theory of Inheritance – …
