Down syndrome is caused by an extra copy of which chromosome?

The correct answer is B, the garden pea, Pisum sativum. Mendel chose it because it breeds true, is easy to cross by hand, completes a generation in a single season and offers seven pairs of sharply contrasting characters such as tall against dwarf stem and round against wrinkled seed. Option A, maize, became an important material for later geneticists, above all for Barbara McClintock's work on transposable elements, but not for Mendel. Option C, the snapdragon or Antirrhinum, is the plant usually used to illustrate incomplete dominance, where a red and a white parent give pink offspring, so it appears in the same chapter but in a different place. Option D, the evening primrose or Oenothera, was the plant on which Hugo de Vries based his mutation theory. Keep the four pairings separate, because a question often supplies all of them as options.

The correct answer is B, 3 : 1. When two heterozygous plants of the first generation are crossed, three quarters of the offspring show the dominant character and one quarter the recessive one, because the recessive appears only when both factors in the pair are recessive. Option A, 1 : 2 : 1, is the genotypic ratio of the same cross, counting one homozygous dominant, two heterozygous and one homozygous recessive, and it is the commonest wrong answer because the question is often read carelessly. Option C, 9 : 3 : 3 : 1, is the phenotypic ratio of a dihybrid cross, where two pairs of characters are followed at once. Option D, 1 : 1, is the ratio obtained in a test cross, where a plant showing the dominant character is crossed with a homozygous recessive to find out whether it is pure or hybrid.

The correct answer is C, the law of independent assortment. In a dihybrid cross two pairs of characters are followed together, for example seed shape and seed colour, and the appearance of all four combinations in the proportion nine to three to three to one shows that the inheritance of one pair does not depend on the inheritance of the other. Option A, the law of dominance, is shown by the first generation, where only one of a pair of contrasting characters appears. Option B, the law of segregation or purity of gametes, is shown by the reappearance of the recessive character in the second generation of a monohybrid cross, which proves that the two factors separated cleanly at gamete formation. Option D is not Mendel's at all; use and disuse belongs to Lamarck's theory of evolution and is placed here as a distractor.

The correct answer is B, Wilhelm Johannsen, the Danish botanist who gave the words gene, genotype and phenotype, distinguishing the hereditary constitution of an organism from its visible expression. Option A, William Bateson, coined the word genetics itself for the new science of inheritance and championed Mendel's rediscovered work in the English-speaking world, so he is the obvious confusion here. Option C, Walter Sutton, with Theodor Boveri put forward the chromosome theory of inheritance in 1902, which located Mendel's factors on the chromosomes. Option D, Thomas Hunt Morgan, worked on the fruit fly Drosophila and established linkage and the mapping of genes along a chromosome, winning the Nobel Prize for it. The set to memorise is Bateson for genetics, Johannsen for gene, Sutton and Boveri for chromosomes and Morgan for linkage.

The correct answer is A, Watson and Crick. James Watson and Francis Crick published the double helix model in 1953, with two strands running in opposite directions and bases paired on the inside, using the X-ray diffraction photographs produced by Rosalind Franklin and Maurice Wilkins; Watson, Crick and Wilkins shared the Nobel Prize in 1962. Option B, Hershey and Chase, showed in 1952 by labelling a bacteriophage that DNA and not protein is the material that enters a bacterium and directs it. Option C, Avery and MacLeod, with Colin MacCarty, identified DNA as the transforming principle in the 1940s, building on Griffith's experiment of 1928. Option D, Miller and Urey, performed the experiment of 1953 in which amino acids formed from a mixture of gases and an electric discharge, which belongs to the origin of life and not to the structure of DNA.