Q.What is heterogamety ? Explain the mechanism of sex determination in Drosophila.

Step 1 — Set the genotypes (X-linked recessive inheritance).

Colourblindness is carried on the X chromosome; $X^{C}$ = normal allele, $X^{c}$ = colourblind allele.

• Colourblind man: a male is hemizygous — one X only — so he is $X^{c}Y$.
• Carrier woman: phenotypically normal but heterozygous, $X^{C}X^{c}$.

Step 2 — Gametes.

• Father: $X^{c}$ and $Y$
• Mother: $X^{C}$ and $X^{c}$

Step 3 — Punnett square.

Step 1 — What kind of genome HIV has.

HIV (Human Immunodeficiency Virus) is a retrovirus. Its genetic material is single-stranded RNA, not DNA. This single fact dictates the enzyme it must carry.

Step 2 — The infection cycle in a macrophage.

1. HIV enters a macrophage after gaining entry into the body through the blood/body fluids.
2. Inside the macrophage, the viral RNA genome is replicated to form viral DNA — this is the step the question is asking about, and it is catalysed by the virus's own enzyme reverse transcriptase.
3. This viral DNA gets incorporated into the host cell's DNA and directs the infected cell to produce virus particles. The macrophage thus continues to survive while acting as an HIV factory.
4. HIV then enters helper T-lymphocytes ($T_H$), replicates within them and progressively destroys them, causing the steady fall in $T_H$ count that defines AIDS.

Step 3 — Why only reverse transcriptase can do it.

Reverse transcriptase is an RNA-dependent DNA polymerase: it uniquely synthesises DNA using RNA as the template, reversing the usual direction of the central dogma:

Step 4 — Eliminate the others. …

1. Define the two reproductive whorls

A complete flower has four whorls, of which two are reproductive:

• Androecium — made of stamens; this is the male whorl (anther + filament, producing pollen).
• Gynoecium (pistil) — made of carpels; this is the female whorl (stigma, style, ovary, producing ovules).

2. The classification of flowers by sex

The question's flower has both stamens and carpels ⇒ bisexual.

3. Why the other options are wrong

• (A) Asexual — asexual means reproduction without gametes/fusion at all (budding, fragmentation, vegetative propagation). A flower is by definition a sexual reproductive structure, so "asexual flower" is a contradiction. …

Step 1 — Understand the terminology

• Homogametic sex → produces only one type of gamete with respect to the sex chromosome.
• Heterogametic sex → produces two different types of gamete.

Step 2 — Apply it to male heterogamety

Male heterogamety covers two systems:

In both, the male is the one making dissimilar gametes; the female ($XX$) makes only $X$-bearing eggs and is homogametic. The sex of the offspring is therefore decided by the sperm.

Step 3 — Eliminate the other options …

The concept here is gamety — whether a sex produces identical or different gametes with respect to sex chromosomes. "Homo" means same, "hetero" means different. The system is named after the sex that is heterogametic, because that's the one that determines the offspring's sex.

In the XO system (found in grasshoppers, for example), females have two X chromosomes (XX) and produce only X-bearing eggs — they are homogametic. Males have only one X (XO) and produce two types of sperm: half with X, half with no sex chromosome (O). That makes males heterogametic.

In the XY system (humans, Drosophila), females are XX and produce only X eggs — again homogametic. Males are XY and produce X-bearing and Y-bearing sperm — heterogametic.

So in both systems, the female is homogametic and the male is heterogametic. The question asks for the example these systems represent — they are both cases of male heterogamety.

Let's walk through the options:

1. Male Homogamety — This would mean males produce only one type of gamete. That's false here; males in both systems produce two types.
2. Male Heterogamety — Correct. Males produce two different gamete types (X and O, or X and Y). …

Step 1 — Recall the architecture of the lac operon.

Step 2 — What "constitutive" means.

A constitutive gene is one that is transcribed continuously and at a steady level, not switched on or off by the substrate. The i gene sits upstream with its own (weak) promoter that is not under the control of the operator. So the cell makes repressor mRNA — and hence repressor protein — all the time, whether or not lactose is in the medium.

Step 3 — Why that is biologically necessary. …

900 bases / 3 bases per codon = 300 codons total. The stem specifies there is 'no stop codon in between' — i.e., the stop codon (if any) is only at the very end of the reading frame, not interspersed within the coding sequence. Standard translation: one of the 300 codons at the end …

The key idea here is the sex determination system in organisms. In most mammals, including humans, males are heterogametic (XY) and females are homogametic (XX). But in some species — notably birds, some reptiles, and butterflies — the pattern is reversed: the female is the heterogametic sex. This system is denoted by the letters Z and W, where females are ZW and males are ZZ.

Let’s examine each option carefully.

1. Option (A): $XX - XY$

   This is the familiar mammalian system. Females are XX (homogametic — all eggs carry an X chromosome), and males are XY (heterogametic — sperm carry either X or Y). So here, male heterogamety is shown, not female.

2. Option (B): $ZZ - ZW$

   In this notation, males are ZZ (homogametic — all sperm carry Z), and females are ZW (heterogametic — eggs carry either Z or W). This is exactly female heterogamety. The female produces two types of gametes, determining the sex of the offspring.

3. Option (C): $XX - XO$

   This system is found in some insects like grasshoppers. Females are XX (homogametic), and males are XO (they have only one X chromosome, no second sex chromosome). Males produce sperm with either X or no sex chromosome — so again, male heterogamety.

4. Option (D): $XX - XXY$ …

Step 1 — The parts of the operon.

The lac operon of E. coli has a regulatory gene i, a promoter p, an operator o, and three structural genes: z (β-galactosidase), y (permease) and a (transacetylase).

Step 2 — The OFF state.

Gene i constitutively makes the repressor protein. The repressor binds the operator and physically blocks RNA polymerase from moving past it, so no mRNA is made. Note that RNA polymerase binds the promoter, never the operator — that alone eliminates option (B).

Step 3 — The switch (negative regulation, positive induction).

When lactose is supplied it is taken in by permease and converted to allolactose, which is the actual inducer. The inducer binds the repressor at its allosteric site:

$\text{repressor} + \text{inducer (lactose/allolactose)} \longrightarrow \text{repressor–inducer complex (inactive)}$ …