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.
Test cross refers to the cross between:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → F2 offspring with parent of recessive trait
A plant that shows a dominant trait may be pure (TT) or hybrid (Tt). Looking at it does not tell which. A test cross finds the genotype: the plant is crossed with the recessive parent (tt). If the tall plant is TT, all the offspring are tall (Tt). If it is Tt, half of the offspring are tall and half are dwarf (1 : 1).
(1) F1 offspring with parent of recessive trait — Incorrect. The F1 plants of a cross between two pure parents are already known to be heterozygous (Tt). There is no unknown genotype to find out.
(2) F1 offspring with parent of dominant trait — Incorrect. A cross with a dominant parent gives only plants with the dominant trait. It cannot show the genotype.
(3) F2 offspring with parent of recessive trait — Correct. To find the genotype of a tall plant of the F2, Mendel crossed it with a dwarf plant. This he called a test cross.
(4) F2 offspring with parent of dominant trait — Incorrect. The dominant parent hides the recessive allele. All the offspring show the dominant trait, so the cross tells nothing.
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:
Inheritance of One Gene – …
