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15.1 The History of Genetics

GCSE Biology · AQA · Chapter 15: Genetics and Evolution

A free GCSE Biology lesson for AQA (8461) from OBA School: mendel’s pea plants, why his work was ignored, and the road to the structure of DNA. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe what Mendel did with pea plants and what he concluded
  • Explain why the importance of Mendel’s work was not recognised until after his death
  • Put the main steps in our understanding of inheritance in time order
  • Describe the double-helix structure of DNA and use base pairing to work out percentages
  • Calculate and interpret ratios from breeding results

Key words

  • Inheritance: The passing of characteristics from parents to offspring.
  • Inherited factor: Mendel’s name for the “unit” that carries a characteristic. We now call these units genes.
  • Gene: A small section of DNA on a chromosome that codes for a particular sequence of amino acids, to make a specific protein.
  • Chromosome: A long, coiled molecule of DNA, found in the nucleus, that carries many genes.
  • Dominant: A characteristic (allele) that shows up whenever it is present, even if only one copy is inherited.
  • Recessive: A characteristic (allele) that only shows up when two copies are inherited.
  • Double helix: The shape of a DNA molecule: two strands twisted around each other, joined by pairs of bases.

Gregor Mendel and his pea plants

Gregor Mendel was a monk who lived in Brno (now in the Czech Republic). In the 1850s and 1860s he grew and bred thousands of pea plants in the monastery garden. He was a careful scientist with training in maths and natural science.

  • He chose characteristics that had two clear forms, such as tall or dwarf stems, round or wrinkled seeds and purple or white flowers.
  • He controlled which plants bred together by transferring pollen himself with a small brush, and covering the flowers.
  • He started with pure-breeding plants that always produced offspring like themselves.
  • He used very large numbers of plants and counted the offspring of each type, so he could see patterns.

From his counts Mendel concluded that characteristics are controlled by separate inherited factors that are passed on unchanged from parents to offspring. Each offspring gets one factor from each parent. Some factors are dominant (tall) and hide the effect of others, which are recessive (dwarf).

Worked example: checking a ratio

A student repeats a Mendel-style cross. In the second generation she counts 928 tall plants and 312 dwarf plants.
Ratio tall : dwarf = 928 : 312. Divide both by 312: 928 ÷ 312 = 2.97 (to 3 s.f.).
So the ratio is 2.97 : 1, which is very close to 3 : 1.

Answer: about 3 : 1 – the pattern Mendel found

Why Mendel’s work was ignored

Mendel published his results in 1866, but very few scientists took notice. He died in 1884 and his work was only rediscovered in 1900. Reasons include:

  • Nobody had seen chromosomes or knew about genes or DNA, so there was no structure in the cell to explain his “units”.
  • He published in a little-known journal of a local natural history society, so few biologists read it.
  • He was a monk, not part of the well-known scientific community of the time.
  • He used maths and statistics, which most biologists of the time did not use or understand.
  • His ideas did not fit the popular belief that features of parents “blended” in their offspring.

Exam tip: In the exam, link each reason to the evidence available at the time: no chromosomes or genes had been observed, so there was no mechanism.

From “units” to genes

Better microscopes and stains allowed scientists to see what happens inside dividing cells. Step by step, Mendel’s ideas were linked to real structures.

  • 1866; What was found Mendel publishes his results on inheritance in pea plants.
  • Late 1800s; What was found Chromosomes are seen in cells. Scientists describe how they behave during cell division.
  • Early 1900s; What was found Scientists notice that chromosomes and Mendel’s units behave in similar ways. The idea grows that the units, now called genes, are carried on chromosomes.
  • Mid-1900s; What was found DNA is shown to be the genetic material. In 1953 its double-helix structure is worked out, and then how genes control the making of proteins.

The structure of DNA

In 1953 James Watson and Francis Crick built a model of DNA. They used X-ray images of DNA made by Rosalind Franklin and Maurice Wilkins, together with data showing that the amounts of certain bases are always equal.

  • DNA is a polymer made of two strands forming a double helix.
  • (Higher tier) Each strand is made of repeating units called nucleotides. Each nucleotide is a sugar and a phosphate group with one of four bases attached: A, C, G or T.
  • (Higher tier) The bases on the two strands pair up: A always pairs with T, and C always pairs with G. These are complementary base pairs.

Worked example: base percentages (Higher tier)

In a sample of DNA, 18% of the bases are G. What percentage are A?
G pairs with C, so C = 18%. G + C = 36%.
A + T = 100 − 36 = 64%, and A = T, so A = 64 ÷ 2 = 32%.

Answer: A = 32%

Common mistakes

  • Saying Mendel discovered genes or DNA. He described inherited “factors” or “units”; genes and DNA came later.
  • Thinking “dominant” means “more common” or “stronger”. It only means the characteristic shows when one copy is present.
  • Expecting a 3 : 1 ratio to give exactly 3 of every 4 offspring. Real counts only come close, so large numbers are needed.
  • Forgetting Franklin and Wilkins. Watson and Crick built the model, but they used X-ray data from Franklin and Wilkins.
  • Pairing A with G or C with T. The pairs are always A–T and C–G.

Revision questions

Try these, then check your answers below each one.

Question 1. Which organism did Gregor Mendel use in his breeding experiments? (Choose from: Pea plants, Fruit flies, Mice, Wheat plants.)

Answer: Pea plants

Working: Mendel bred thousands of pea plants and counted the offspring of each type.

Question 2. True or false? Mendel’s ideas about inheritance were widely accepted by scientists during his lifetime.

Answer: False

Working: False. He published in 1866, but his work was largely ignored until 1900, after his death.

Question 3. Which of these was not a reason why Mendel’s work was ignored at the time? (Choose from: Chromosomes and genes had not yet been discovered, He published in a little-known journal, He used maths that most biologists did not use, He only used a handful of plants in his experiments.)

Answer: He only used a handful of plants in his experiments

Working: Mendel used thousands of plants. The real reasons were: no known mechanism (no chromosomes or genes), an obscure journal, unfamiliar maths, and he was outside the scientific community.

Related lessons

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