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Biology · 2026 · 2 marks
CBSE 2026 · Region 1 · Set 1 · Q20
(a) Analyse the following two cases of sex determination in different organisms: Case I: Males have (XO) sex chromosomes and females have two copies…
Analyse the following two cases of sex determination in different organisms:
Case I: Males have (XO) sex chromosomes and females have two copies of same sex chromosome (XX) .
Case II: Females have two different sex chromosomes (ZW) and males have two copies of same sex chromosome (ZZ) .
Identify the type of heterogamety in each case, giving one example of each.
(i)
Differentiate between a DNA and RNA nucleotide.
(ii)
Name the two types of bonds present within a single nucleotide.
Marking-scheme solution
Case $\displaystyle 1$ – Male heterogamety
Organism – Many insects / grasshopper / any correct example.
Case II – Female heterogamety
Organism – Birds / hen / any correct example.
(i)
| DNA nucleotide | RNA nucleotide |
| Has deoxyribose sugar | Has Ribose sugar |
| Types of nitrogen bases are A,G,C,T | Types of nitrogen bases are A,G,C,U |
(Any one difference)
(ii)
Bonds in a nucleotide
• N – Glycosidic linkage (between Sugar and Base)
• Phosphoester linkage – (Between Sugar and Phosphate)
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