(a) 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. [1+1]
OR
(b) (i) Differentiate between a DNA and RNA nucleotide. [1]
(ii) Name the two types of bonds present within a single nucleotide. [1]
(a) 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. [1+1]
OR
(b) (i) Differentiate between a DNA and RNA nucleotide. [1]
(ii) Name the two types of bonds present within a single nucleotide. [1]
The sex that produces two different kinds of gametes (w.r.t. sex chromosome) is the heterogametic sex — this decides the answer.
(a) PRIMARY answer — type of heterogamety:
Case I (Males XO, Females XX):
- Male heterogamety — of the XX–XO type.
- The male produces two kinds of gametes (some with X, some without any sex chromosome).
- Example: grasshopper (and many insects/roaches).
Case II (Females ZW, Males ZZ):
- Female heterogamety — of the ZZ–ZW type.
- Here the female is heterogametic (produces Z-bearing and W-bearing eggs).
- Example: birds (also several reptiles, moths and butterflies).
(OR alternative (b), briefly): A DNA nucleotide has deoxyribose sugar and thymine, whereas an RNA nucleotide has ribose sugar and uracil; the two bonds within one nucleotide are the N-glycosidic bond (sugar–base) and the phosphoester bond (sugar–phosphate).)
Marking Scheme
- 1½ mark: Case I — male heterogamety (XX-XO type).
- 2½ mark: Case I example — grasshopper/insect.
- 3½ mark: Case II — female heterogamety (ZZ-ZW type).
- 4½ mark: Case II example — birds (or moths/butterflies).
- 5OR (b): ½ DNA nucleotide has deoxyribose + thymine; ½ RNA nucleotide has ribose + uracil; ½+½ N-glycosidic bond and phosphoester (phosphodiester) bond within the nucleotide.
Hint
Heterogametic = the sex making two kinds of gametes. Match XO→grasshopper, ZW→birds.
Quick Oral Answer
In Case I the male is heterogametic — XX-XO type, seen in grasshoppers; in Case II the female is heterogametic — ZZ-ZW type, seen in birds; the heterogametic sex is simply the one that produces two different kinds of gametes.
Analysis & Explanation
Concept — heterogamety:
The heterogametic sex is the one producing two morphologically different gametes with respect to the sex chromosome; the homogametic sex produces only one kind.
Working out each case:
- Case I: Female XX makes only X eggs (homogametic). Male XO makes X-sperm and O-sperm (two kinds) → male heterogamety, XX–XO type, as in grasshopper. Humans (XX–XY) are also male-heterogametic, but the given case has no Y, so 'XO type' must be specified.
- Case II: Male ZZ makes only Z gametes (homogametic). Female ZW makes Z-eggs and W-eggs (two kinds) → female heterogamety, ZZ–ZW type, as in birds.
Exam trap:
- Students often answer 'male heterogamety' for both — read carefully: it is the sex with two different sex chromosomes/gametes that is heterogametic. In Case II the female is heterogametic.
- Give the correct example: grasshopper for XO, birds for ZW. Do not swap them.
Real-world link:
In birds, because the female is ZW, it is the mother's egg (Z or W) that determines the chick's sex — the reverse of the human situation.
Common Mistakes
- 1Calling both cases 'male heterogamety' — in the ZW/ZZ case it is the female that is heterogametic.
- 2Giving the wrong example (e.g. Drosophila for XO or humans for ZW) — grasshopper is the classic XO example and birds the classic ZW example.
- 3For the OR part, writing thymine for RNA or uracil for DNA, and confusing the glycosidic bond with the phosphoester bond.
Interesting Facts
In the XX-XO system of grasshoppers, males literally have one fewer chromosome than females; the 'O' means the absence of a chromosome, not a Y.
About 50% of grasshopper sperm carry no sex chromosome, so it is the father's sperm that decides sex — just as in humans, but without a Y chromosome.
In the ZW system of birds and butterflies the female is the heterogametic sex, exactly opposite to mammals, which is why sex-linked colour patterns are inherited differently in birds.
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Frequently Asked Questions
What is the difference between homogametic and heterogametic sex?
The homogametic sex produces only one kind of gamete with respect to the sex chromosome (e.g. XX females make only X eggs). The heterogametic sex produces two kinds (e.g. XO males make X and O sperm, or ZW females make Z and W eggs).
Why is the bird an example of female heterogamety?
In birds the female is ZW and the male is ZZ. The male makes only Z sperm, but the female makes both Z-bearing and W-bearing eggs, so the female is the heterogametic sex and the egg determines the chick's sex.
How is a DNA nucleotide different from an RNA nucleotide?
A DNA nucleotide contains deoxyribose sugar and the base thymine, while an RNA nucleotide contains ribose sugar and the base uracil (in place of thymine); both also contain a phosphate group.