CDS (II) 2021 General Knowledge · Question 14 of 120

Correct — A, 1 : 1. Cross a pure tall pea plant, genotype TT, with a short one, tt, and every plant of the first generation is Tt — tall in appearance, because a single copy of T is enough to make the plant tall, but carrying a hidden t. Self-pollinate that generation and each parent contributes T or t with equal probability, so the second generation comes out in the genotype ratio 1 TT : 2 Tt : 1 tt. The question does not ask for tall against short, which would be the familiar 3 : 1 of appearance; it asks for pure tall against short. Pure tall means TT alone, since Tt plants are tall but not pure-breeding, and short means tt. One quarter of the offspring are TT and one quarter are tt, so the ratio is 1 : 1. The trap and the answer both live in the word 'pure', and the safest habit is to write out the four boxes of the cross before reading the options.

• (b)1 : 3 — Inverts the ratio of appearance and applies it to the wrong pair. Nothing in the second generation stands at one to three in this direction.
• (c)3 : 1 — The famous phenotypic ratio — tall plants of any genotype against short plants. It answers a question the stem did not ask, because two of the three tall plants are hybrids rather than pure tall.
• (d)2 : 1 — The count of hybrid tall plants against short ones. It would be the answer if the stem had asked for Tt against tt.

A gene may exist in more than one form, and the alternative forms are called alleles; in Mendel's height experiment T stands for the tall allele and t for the short one. An organism carrying two identical alleles is homozygous, or pure-breeding for that trait; one carrying two different alleles is heterozygous. A trait that shows itself even when only one copy is present is dominant, and one that appears only when both copies are present is recessive. The second generation of a single-character cross settles into 1 : 2 : 1 by genotype and 3 : 1 by appearance.

Every wrong option here is a real ratio from the same cross, attached to the wrong pair of categories, which is what makes the item harder than it looks. Working left to right through the cross keeps them apart. The first generation is uniformly Tt and shows nothing of the short parent. Its self-pollination produces TT, Tt, Tt and tt in equal numbers. From that single list you can read off 3 : 1 for tall against short, 1 : 2 : 1 for the genotypes, 1 : 1 for pure tall against short, and 2 : 1 for hybrid tall against short — four different answers to four different questions. Reading the stem twice is worth more marks here than any extra biology.

• In the second generation of a tall-by-short pea cross the genotypes appear in the ratio 1 TT : 2 Tt : 1 tt.
• Both TT and Tt plants are tall, while only tt is short, which gives the 3 : 1 ratio of appearance.
• Pure tall means the homozygous TT plants only; the heterozygous Tt plants are tall but not pure-breeding.
• One copy of the dominant allele is enough to produce the dominant trait; the recessive trait needs both copies.
• Mendel worked with the garden pea, Pisum sativum, which self-pollinates readily and shows several clear either-or characters.

• Answering 3 : 1 out of habit; that ratio compares appearance, not pure tall against short.
• Forgetting that the first generation is entirely hybrid, so the short character skips a generation and returns.
• Treating pure tall and tall as the same category — two of every three tall plants in the second generation are hybrids.

As a ratio question on a monohybrid cross with the category deliberately shifted, as a genotype-to-appearance match, or as a test-cross item asking how to identify a pure-breeding plant.

• practice — not a real PYQ

  In the second generation of a cross between a pure tall (TT) and a short (tt) pea plant, the ratio of tall plants to short plants is

  • (a)1 : 1
  • (b)3 : 1
  • (c)1 : 3
  • (d)2 : 1

  Answer(b) 3 : 1 — TT and both Tt plants are tall, and only tt is short.

• practice — not a real PYQ

  A tall pea plant is crossed with a short one and all the offspring are tall. The genotype of the tall parent must be

  • (a)tt
  • (b)Tt
  • (c)TT
  • (d)either TT or Tt

  Answer(c) TT — a heterozygous Tt parent crossed with tt would have produced short offspring as well.