A child receives a tall beautiful plant as a birthday gift from his father with a quiz. The father asked her how she would verify whether this tall plant was the progeny of both the tall parents or one tall and one short parent plant. She could verify this through
- (a)cross-pollination
- (b)self-pollination
- (c)tissue culture
- (d)negative propagation
Correct — B, self-pollination. Height in the classic pea experiments is set by one gene with a dominant tall allele, so a tall plant can be either pure-breeding (TT) or a hybrid (Tt), and the two look identical. Selfing settles it in one generation. Pollinate the plant with its own pollen: if it is TT, every offspring gets a T from both sides and every offspring is tall. If it is Tt, a quarter of the offspring receive t from both sides and grow short. Short plants in the next generation therefore prove one grandparent was short; a whole crop of tall ones points to two tall parents. The gift plant is the only material the child has, and self-pollination is the one method on the list that needs nothing else and produces a segregating generation.
- (a)cross-pollination — Crossing needs a second plant, and the option does not say which. Cross it with another tall plant of unknown make-up and an all-tall result still proves nothing. The determinate version is the test cross, made specifically with a short plant — the option as printed does not say that.
- (c)tissue culture — Tissue culture raises clones from a piece of the parent. Every plantlet carries exactly the same alleles as the gift plant, so nothing hidden ever comes to light.
- (d)negative propagation — There is no such technique in plant breeding. It is a manufactured phrase put in to fill the fourth slot.
An organism's appearance does not fix its genetic make-up. Where one allele is dominant, the dominant trait shows in both the pure-breeding (TT) and the hybrid (Tt) plant, so the two are indistinguishable by eye. Breeders recover the hidden allele by forcing it into a homozygous combination in the next generation, either by selfing the plant or by crossing it to a plant carrying only the recessive allele.
Work it as Mendel's own second step. His F1 hybrids were all tall; selfing them produced the famous three-to-one ratio, and the short plants that reappeared were the proof that a recessive allele had been carried through unseen. The same logic answers the child's question. Note honestly that a candidate trained on 'test cross' may reach for cross-pollination, and against a short partner that would also work — the key rejects it because the printed option names no partner, and an unspecified cross can easily be uninformative. The stem also carries a printed slip worth noticing: the plant arrives 'from his father' and the next sentence has the father asking 'her'. The gender wobbles; the genetics does not.
- Tall is dominant to short in the garden pea, so TT and Tt plants are both tall.
- Selfing a Tt plant gives about three tall to one short in the next generation; selfing a TT plant gives only tall.
- A test cross pairs the unknown plant with a homozygous recessive one — half the offspring come out short if the unknown is a hybrid.
- The pea flower's structure makes it naturally self-pollinating, which is one reason Mendel chose it.
- Tissue culture is a cloning method and produces no genetic segregation.
The recessive allele only shows itself when two copies meet, which is why the answer is a method that lets the plant fertilise itself.
- Choosing cross-pollination on the strength of the test cross, without noticing that the option names no partner plant.
- Expecting the answer in the same generation — the hidden allele only surfaces in the offspring.
- Treating tissue culture as a breeding method; it copies, it does not recombine.
As an applied Mendel item — a dominant trait, an unknown make-up, and a choice of technique to expose it.
Different varieties of the same gene are called
- (a) Genotypes
- (b) Sib pairs
- (c) Alleles
- (d) Isomers
Answer(c) Alleles
The vocabulary this question runs on, tested by itself. The gift plant is tall because of the alleles it holds at one gene, and the whole puzzle is that two different allele combinations produce the same tall plant.
CDS_GK_2020_I_Q162020Which one of the following is the correct sequence of events during sexual reproduction in plants?
- (a) Seedling, formation of embryo, pollination, fertilization, division of zygote
- (b) Formation of embryo, seedling, pollination, fertilization, division of zygote
- (c) Pollination, fertilization, division of zygote, formation of embryo, seedling
- (d) Seedling, formation of embryo, division of zygote, pollination, fertilization
Answer(c) Pollination, fertilization, division of zygote, formation of embryo, seedling
The stage before this one. That item asks for the order of events in plant sexual reproduction, and pollination heads the list; this question turns on choosing which kind of pollination will expose a trait that is present but not showing.
- practice — not a real PYQ
A tall pea plant is crossed with a short pea plant and all the offspring are tall. The tall parent was
- (a)homozygous tall
- (b)heterozygous tall
- (c)homozygous short
- (d)impossible to decide
Answer(a) homozygous tall — a hybrid tall crossed with a short plant would have given about half short offspring, so an all-tall result marks the parent as pure-breeding.
- practice — not a real PYQ
In Mendel's experiments, self-pollination of the F1 tall pea plants produced tall and short plants in the ratio
- (a)1 : 1
- (b)2 : 1
- (c)3 : 1
- (d)9 : 3 : 3 : 1
Answer(c) 3 : 1 — three tall to one short, the monohybrid ratio that revealed the recessive allele hidden in the F1.