Mendel and the Development of Genetics

1. 1 State [1 mark]

   State the organism that Mendel used in his experiments on inheritance.

   Show answerHide answer

   Model answer

   The pea plant.

   Mark scheme

   • Pea plant — 1 mark
2. 2 Give [2 marks]

   Give two reasons why pea plants were a good choice for Mendel's experiments.

   Show answerHide answer

   Model answer

   Any two from: they grow quickly; they have clear differences, such as tall or dwarf; they can be cross-pollinated by hand; they are cheap and easy to grow in large numbers.

   Mark scheme

   • First — 1 mark
   • Second — 1 mark
3. 3 Describe [3 marks]

   Describe what Mendel found when he crossed pure-breeding tall pea plants with pure-breeding dwarf pea plants, and then allowed the offspring to self-pollinate.

   Show answerHide answer

   Model answer

   All of the offspring in the first generation were tall. When these were self-pollinated, the second generation had about three tall plants to every one dwarf plant. The dwarf feature came back.

   Mark scheme

   • F1 all tall — 1 mark
   • F2 about 3 tall to 1 dwarf — 1 mark
   • Dwarf feature reappeared — 1 mark
4. 4 Calculate [3 marks]

   In another experiment, Mendel obtained 705 plants with purple flowers and 224 plants with white flowers. Calculate the ratio of purple to white, in the form x : 1, to 2 decimal places.

   Show answerHide answer

   Model answer

   705 ÷ 224 = 3.15, so the ratio is 3.15 : 1.

   Mark scheme

   • Division of 705 by 224 — 1 mark
   • 3.15 — 1 mark
   • In the form 3.15 : 1 — 1 mark
5. 5 Explain [3 marks]

   Explain why Mendel's work was not accepted by scientists when it was published.

   Show answerHide answer

   Model answer

   Most scientists believed in blending inheritance, which did not fit his results. Nobody knew about chromosomes or how they were passed on, so there was no mechanism to explain his factors. His use of mathematics in biology was unusual, and his paper was not widely read.

   Mark scheme

   • Blending inheritance was believed — 1 mark
   • No mechanism known — 1 mark
   • Unusual mathematics, or few readers — 1 mark
6. 6 Evaluate [6 marks]

   Evaluate the importance of Mendel's work, and of the later discoveries of chromosomes and DNA, to our understanding of inheritance.

   Show answerHide answer

   Model answer

   Mendel showed that features are controlled by separate factors that do not blend, and that one factor can be dominant over another, with a ratio of about 3 to 1 in the second generation. This was a major advance, because it replaced blending inheritance and used a careful, mathematical method. But he could not say what the factors were, or where they were, so his work was ignored. Later, scientists saw chromosomes, and Sutton and Boveri suggested that the factors were carried on them, which explained how they could be separated into gametes. The structure of DNA in 1953 showed what genes are made of and how information is copied, and the sequencing of the genome showed all of the genes. Each stage depended on the one before, and together they explain inheritance. Mendel's work was the foundation, but the later discoveries were needed to explain it.

   Mark scheme

   • Mendel showed separate, non-blending factors — 1 mark
   • Ratio and dominance — 1 mark
   • He could not explain what the factors were — 1 mark
   • Chromosomes carry the factors — 1 mark
   • DNA shows what genes are made of — 1 mark
   • A reasoned judgement of the importance — 1 mark

Quick check

1. 1

   Which organism did Mendel use?

   1. AThe fruit fly
   2. BThe pea plant
   3. CThe mouse
   4. DThe dog
   Show answerHide answer

   B: The pea plant

   The pea plant.

2. 2

   What did all of the F1 plants look like when tall and dwarf pure-breeding plants were crossed?

   1. ATall
   2. BDwarf
   3. CMedium
   4. DHalf tall and half dwarf
   Show answerHide answer

   A: Tall

   All were tall.

3. 3

   What was the approximate ratio of tall to dwarf plants in the F2 generation?

   1. A1 : 1
   2. B2 : 1
   3. C3 : 1
   4. D4 : 1
   Show answerHide answer

   C: 3 : 1

   About 3 : 1.

4. 4

   What is blending inheritance?

   1. AThe idea that genes are in chromosomes
   2. BThe idea that dominant factors hide recessive ones
   3. CThe idea that DNA is a double helix
   4. DThe idea that the features of the parents mix
   Show answerHide answer

   D: The idea that the features of the parents mix

   The idea that the features of the parents mix in the offspring.

5. 5

   What did Mendel call what we now call genes?

   1. AChromosomes
   2. BInherited factors
   3. CAlleles
   4. DCells
   Show answerHide answer

   B: Inherited factors

   Inherited factors.

6. 6

   Why was Mendel's work not accepted at first?

   1. AIt was wrong
   2. BHe did not publish it
   3. CIt did not fit the ideas of the time
   4. DHe used the wrong plant
   Show answerHide answer

   C: It did not fit the ideas of the time

   It did not fit blending inheritance, and no mechanism was known.

7. 7

   Who suggested that Mendel's factors were carried on chromosomes?

   1. ASutton and Boveri
   2. BWatson and Crick
   3. CDarwin and Wallace
   4. DHooke and Brown
   Show answerHide answer

   A: Sutton and Boveri

   Sutton and Boveri, in 1902.

8. 8

   Mendel obtained 750 tall and 250 dwarf plants. What percentage were tall?

   1. A25%
   2. B50%
   3. C66%
   4. D75%
   Show answerHide answer

   D: 75%

   \(750 \div 1000 \times 100 = 75\%\).

9. 9

   What did the reappearance of dwarf plants in the F2 generation show?

   1. AThe dwarf factor had been destroyed
   2. BThe dwarf factor had been hidden, not lost
   3. CThe plants had blended
   4. DThe tall factor was recessive
   Show answerHide answer

   B: The dwarf factor had been hidden, not lost

   The recessive factor was hidden, but not lost, in the F1 plants.