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Biology · 2026 · 5 marks
CBSE 2026 · Region 3 · Set 2 · Q31
(a) The following questions are based on the classic experiment conducted by Hershey and Chase. (i) Why were bacteriophages chosen by the scientists?…
The following questions are based on the classic experiment conducted by Hershey and Chase.
(i)
Why were bacteriophages chosen by the scientists ?
(ii)
Name the radioactive isotopes used and state the reason for using them.
(iii)
Describe the steps of the experiment.
(iv)
Why is this experiment considered a major breakthrough in the field of molecular biology ?
(i)
What are multiple alleles ? How do they differ from simple dominant-recessive allele pairs?
(ii)
Explain the concept of multiple alleles with the help of a human trait studied by you.
(iii)
Mention all possible genotypes with their corresponding phenotypes of the human trait as explained by you.
(iv)
Comment on the type of inheritance pattern involved in the example explained by you.
Marking-scheme solution
(i)
• The structure of bacteriophages consists of only DNA and protein
• Bacteriophage infects bacteria (E. coli) by inserting its genetic material.
(ii)
Radioactive phosphorous (32P) as phosphorous is present only in DNA and
not
in proteins.
Radioactive sulphur (35S) as sulphur is present only in proteins not in DNA.
(iii)
• Infection radioactive phage’s were allowed to attach to E. coli
bacteria
• Blending Viral coats were removed from the bacteria by blending
• Centrifugation virus particles were separated from bacteria by
spinning them in a centrifuge
(iv)
This established that DNA is Genetic material.
(i)
• When more than two alternative forms of a gene (alleles) exist for a
particular trait in a population.
• In multiple alleles a gene has more than two alternative forms where as in
simple dominant-recessive inheritance only two alleles are present.
(ii)
Human trait ABO Blood Group,
having Alleles $\displaystyle I^A$, $\displaystyle I^B$, $\displaystyle I^O$ / i
(iii)
| Blood group Phenotype | Genotype |
| A | $\displaystyle I^A I^A$ or $\displaystyle I^A i$ |
| B | $\displaystyle I^B I^B$ or $\displaystyle I^B i$ |
| AB | $\displaystyle I^A I^B$ |
| O | $\displaystyle ii$ |
(iv)
It shows Co- Dominance as when both dominant alleles $\displaystyle I^A$ and $\displaystyle I^B$ are present they show as AB blood group / $\displaystyle I^A$ and $\displaystyle I^B$ both express themselves this is called Co- dominance
All Molecular Basis of Inheritance questions2026 question paperNCERT solutions: Molecular Basis of InheritancePractice Molecular Basis of Inheritance →
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CBSE Class 12 Biology past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.