Sexual Reproduction in Flowering Plants Set-6

Q1. In a flowering plant with haploid chromosome number $n=12$, the primary endosperm nucleus is formed by fusion of two polar nuclei (each $n$) with one sperm nucleus ($n$) during double fertilization. What will be the chromosome number of the primary endosperm nucleus?

Q2. A flower produces $800$ pollen grains and has $50$ ovules. Of the pollen grains produced, $30\%$ land on the stigma and germinate; of those germinated only $4\%$ produce pollen tubes that successfully reach and fertilize ovules. Approximately how many seeds will develop per flower? (Round to the nearest whole number.)

Q3. In gametophytic self-incompatibility (GSI), compatibility is determined by matching S-alleles. Plant P1 is $S_1S_2$ and plant P2 is $S_2S_3$. If pollen from P1 is deposited on the stigma of P2, what fraction of P1's pollen grains is expected to be compatible with P2's pistil? Assume P1 produces equal numbers of pollen grains carrying $S_1$ and $S_2$ alleles.

Q4. In a large population of a hermaphroditic flowering plant three S-alleles ($S_1, S_2, S_3$) segregate equally and all individuals are heterozygotes (each individual carries two different alleles). Under gametophytic self-incompatibility (GSI), what is the probability that a randomly chosen pollen grain will be compatible with a randomly chosen pistil?

Q5. A diploid mother plant ($2n$) is pollinated by a tetraploid father ($4n$). Assume the diploid egg cell is $n$ and each polar nucleus is $n$, while the tetraploid pollen delivers an unreduced sperm nucleus of $2n$. What will be the ploidy of (i) the embryo and (ii) the primary endosperm nucleus, and what is the most likely outcome for seed viability?

Q6. A flowering plant has somatic chromosome number $2n=18$. A normal embryo sac is formed and double fertilization occurs. What will be the chromosome numbers in (i) the zygote (embryo) and (ii) the primary endosperm nucleus?

Q7. In a species with gametophytic self-incompatibility (GSI), plant A is genotype $S_{1}S_{2}$ and plant B is $S_{2}S_{3}$. If pollen from B pollinates A, what fraction of B’s pollen grains are expected to be compatible on A’s stigma?

Q8. Species X has $2n=16$ (diploid) and species Y is tetraploid with $4n=32$. For the reciprocal crosses X(female) × Y(male) and Y(female) × X(male), what are the chromosome numbers of the embryo and the primary endosperm nucleus respectively in each cross?

Q9. Two diploid species A ($2n=10$) and B ($2n=14$) are crossed to give a sterile F1 hybrid with $12$ chromosomes (n$_A$ + n$_B$). Colchicine treatment is applied to the sterile F1. Which of the following best describes the chromosome number after treatment and the primary reason fertility is restored?

Q10. In a diploid flowering plant allele $a$ is gametophytic lethal in pollen (pollen grains carrying $a$ are non-functional) but female gametophytes carrying $a$ are functional. A heterozygous plant $A/a$ self-pollinates. What is the expected genotypic composition among viable seeds (fractions of $AA: Aa: aa$)?

Q11. In a flowering plant with somatic chromosome number $2n = 24$, determine the chromosome numbers of (i) each sperm nucleus at fertilization, (ii) the primary endosperm nucleus formed after double fertilization, and (iii) the zygote (embryo).

Q12. A tetraploid female ($4x$) is crossed with a diploid male ($2x$). Assuming normal reductional meiosis in both parents and typical double fertilization, what will be the ploidy of the embryo and of the primary endosperm nucleus, and what is the most likely consequence for seed viability?

Q13. A gene $G$ has a dominant allele $G$ required in at least two copies in the triploid endosperm (maternal : paternal = 2 : 1) for normal endosperm development. A female plant with genotype $Gg$ is crossed with a male plant $gg$. Assuming a monosporic embryo sac (polar nuclei are genetically identical), what fraction of seeds will have normal endosperm?

Q14. Assertion (A): The ploidy of the primary endosperm nucleus varies with the type of embryo sac development (monosporic, bisporic, tetrasporic).
Reason (R): The number of maternal haploid genome copies contributed to the central cell equals the number of distinct megaspore‑derived nuclei that form the central cell; hence endosperm ploidy after fertilization can be expressed as $m + p$, where $m$ = number of maternal haploid genomes in the central cell and $p$ = haploid contribution of the fertilizing sperm.
Choose the correct option.

Q15. Some angiosperms shed pollen in bicellular condition (vegetative + generative cell) while others shed tricellular pollen (vegetative + two sperms). Considering timing of fertilization, pollen longevity and adaptive trade‑offs, which statement best explains the likely advantage of shedding tricellular pollen?