Genetics and Evolution: Mendel, DNA and Darwin

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.

The correct answer is B, uracil. RNA carries adenine, guanine, cytosine and uracil, and the uracil occupies the place that thymine holds in DNA, pairing with adenine in the same way. Option A, thymine, is the reverse of the right answer: it is present in DNA and absent in RNA, and candidates in a hurry pick it because both words appear in the same sentence of the textbook. Option C, guanine, and option D, adenine, are purines and are present in both nucleic acids, so neither can be the answer. Two more points often asked with this one are that DNA has deoxyribose sugar where RNA has ribose, and that adenine pairs with thymine through two hydrogen bonds while guanine pairs with cytosine through three, which makes a region rich in guanine and cytosine harder to separate.

The correct answer is B, 22. A human body cell has 46 chromosomes arranged in 23 pairs, of which 22 pairs are autosomes, the chromosomes that are alike in both sexes, and the remaining pair is of sex chromosomes, XX in a female and XY in a male. Option A, 21, is placed to catch the candidate who is thinking of chromosome 21 and Down syndrome. Option C, 23, is the total number of pairs including the sex pair, and it is also the number of chromosomes in a human gamete, which is haploid; it is the answer most often given by mistake. Option D, 46, is the total number of chromosomes and not a number of pairs. The habit worth building is to notice whether a question asks for chromosomes or for pairs, and whether it asks for autosomes or for all chromosomes.

The correct answer is C, chromosome 21. A person with Down syndrome carries three copies of chromosome 21 instead of two, a condition called trisomy 21, so the total chromosome number is 47 rather than 46. It arises when the pair fails to separate during gamete formation. Option A, an extra chromosome 13, and option B, an extra chromosome 18, are also trisomies, named after Patau and Edwards respectively, and are offered here because the pattern of the answer looks the same. Option D, an extra X chromosome, produces Klinefelter syndrome in a male, with 47 chromosomes written as XXY; the related condition in which a female has a single X and a total of 45 is Turner syndrome. Keep the chromosomal disorders in one list and the gene disorders such as haemophilia and sickle cell anaemia in another.

The correct answer is B, 1859. Darwin set out in that book the theory of evolution by natural selection, drawing on observations made during his voyage on HMS Beagle, among them the finches of the Galapagos islands whose beaks differ with their diet. Option A, 1831, is the year the Beagle sailed, and it is the near miss most often chosen. Option C, 1866, is the year Mendel published his paper on the pea, which belongs to the other half of this chapter and was not noticed until the start of the next century. Option D, 1900, is the year in which three botanists independently rediscovered Mendel's work, so it is a real date but not Darwin's. Note also that Alfred Russel Wallace arrived at natural selection independently, and that the two men's ideas were first presented together in 1858.

The correct answer is B, Jean Baptiste Lamarck. He held that an organ used often becomes stronger and one left unused withers, and that characters so acquired in a lifetime are passed to the offspring, the giraffe's lengthening neck being his best known illustration. The theory is rejected because changes in body cells do not alter the DNA of the germ cells that form the next generation. Option A, Darwin, explained the same observations by natural selection acting on variation already present in a population. Option C, Hugo de Vries, proposed that evolution proceeds by sudden large mutations, working on the evening primrose. Option D, Wallace, arrived at natural selection independently of Darwin while working in the Malay archipelago. The pairing to fix is Lamarck with use and disuse, Darwin and Wallace with natural selection, and de Vries with mutation.

The correct answer is B, homologous organs. The same arrangement of bones, humerus, radius and ulna, carpals, metacarpals and phalanges, is used for walking in a man, for swimming in a whale, for flight in a bat and for running in a cheetah, which indicates descent from a common ancestor and is called divergent evolution. Option A, analogous organs, are the opposite case: the same function performed by structures of different origin, as in the wing of a bird and the wing of an insect, which shows convergent evolution. Option C, vestigial organs, are reduced remnants of organs that were functional in an ancestor, such as the vermiform appendix and the nictitating membrane of the human eye. Option D is not a standard category; atavism refers to the reappearance of an ancestral character in an individual, such as a human infant born with a short tail.

The correct answer is C, reptiles and birds. The fossil of Archaeopteryx carries feathers and wings like a bird together with teeth in the jaws, claws on the wings and a long tail with bones, all reptilian features, so it stands as the standard example of a connecting link in the fossil record. Option A is wrong: the lobe-finned fishes and the fossil Ichthyostega are the forms placed between fishes and amphibians, and the living lungfish is often cited in the same connection. Option B is wrong because Seymouria is the fossil usually given as the link between amphibians and reptiles. Option D is wrong because the egg-laying mammals, the duck-billed platypus and the spiny anteater, are the living forms that connect reptiles with mammals. Remember that a connecting link shows features of two groups at once, which is why such fossils are evidence of descent with modification.