Comprehension:
Mendelian genetics
Read the following passage carefully and answer the given questions.
In Mendel's monohybrid cross between pure tall (TT) and pure dwarf (tt) pea plants, the F1 generation consisted of all tall plants, including dominance of T over t. The tall and dwarf traits were identical to their parental type and did not show any blending. Selfing of F1 plants produced F2 generation in the ratio of 3 Tall: 1 dwarf, whereas genotypic ratio was 1TT : 2Tt : 1tt. Mendel concludes that alleles segregate during gamete formation and one allele from each parent is transmitted to offspring. Also, based on these observations, Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations. He called these things as ‘factors’. Now we call them as genes as unit of inheritance. The production of gametes by the parents, the formation of the zygotes, the F1 and F2 plants can be understood from a diagram called Punnett Square.
Cross between tall and dwarf pea plant showed 3:1 ratio when their first generation offsprings were self-crossed. This is due to:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Law of segregation
Note 1: The given answer is as per the official NTA Answer Key. However, the question can have more than one correct answer, i.e. both Option (1) and Option (3), because NCERT says that the law of dominance also explains the 3:1 proportion, as explained below.
In the cross of tall (TT) and dwarf (tt) plants, the F1 plants are all Tt and look tall. When they are selfed, the F2 plants are 3 tall : 1 dwarf. Two of Mendel’s laws work together here.
(1) Law of dominance — Also correct as per NCERT. In the pair Tt, the dominant factor T hides the recessive factor t. So TT and Tt plants are tall and only tt plants are dwarf. NCERT says that this law also explains the 3:1 proportion obtained in F2.
(2) Law of chromosomal inheritance — Incorrect. This is not a law of Mendel. The chromosomal theory of inheritance says that genes are located on chromosomes. It was put forward later to explain Mendel’s laws.
(3) Law of segregation — Correct. The two alleles of a pair separate during gamete formation, so each gamete receives only one of them. The recessive allele t is therefore not lost. It is recovered in the F2 and the dwarf trait appears again. This is the answer given by NTA.
(4) Law of independent assortment — Incorrect. This law applies when two pairs of traits are studied together, as in a dihybrid cross, which gives a 9:3:3:1 ratio. The cross here has one pair of traits.
Note 2: 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:
Law of Dominance – “… The law of dominance is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1 and the expression of both in the F2. It also explains the proportion of 3:1 obtained at the F2.”
Law of Segregation – “This law is based on the fact 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 stage. Though the parents contain two alleles during gamete formation, the factors or alleles of a pair segregate from each other such that a gamete receives only one of the two factors.”