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.
Why do dwarf plants appear in F2 generation?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Individuals with genotype tt are formed when recessive allele become homozygous.
Dwarfness is a recessive trait. It shows only when an individual has two copies of the recessive allele, that is, when it is homozygous recessive (tt). In the F1 plants (Tt) the dominant allele T hides it. In the F2 a plant can receive t from both parents, and it is then tt and dwarf.
(1) Individuals with genotype tt are formed when recessive allele become heterozygous. — Incorrect. A heterozygote has one recessive allele and one dominant allele. Its genotype is Tt, not tt.
(2) Individuals with genotype tt are formed when recessive allele become homozygous. — Correct. Two copies of the recessive allele give tt.
(3) Individuals with genotype TT are formed when recessive allele become heterozygous. — Incorrect. TT has two dominant alleles and no recessive allele.
(4) Individuals with genotype Tt are formed when recessive allele become homozygous. — Incorrect. Tt is heterozygous, so it does not have a homozygous recessive allele pair.
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 – …
