Biology · Inheritance
Mendel and the Development of Genetics
Genetics began in a monastery garden, with a monk who counted peas. This lesson is for separate Biology only: Combined Science students do not need it. It describes the development of our understanding of genetics, including the work of Mendel, and how his ideas were later explained by the discovery of chromosomes and DNA.
Learning Objectives
- 1Describe the development of our understanding of genetics, including the work of Mendel.
- 2Describe how Mendel carried out his experiments on pea plants, and what his results showed.
- 3Explain why Mendel's work was not accepted in his lifetime.
- 4Explain how later discoveries about chromosomes and DNA supported his ideas.
Retrieval practice
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1
What is an allele?
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A different version of a gene.
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2
What does dominant mean?
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An allele that shows its effect when only one copy is present.
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3
What is a gamete?
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A sex cell with half the chromosomes of a body cell.
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4
What was the ratio of phenotypes in Tt x Tt?
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3 dominant : 1 recessive.
The monk and the peas
People have always noticed that children resemble their parents, but before the nineteenth century nobody could explain how. A common view was blending inheritance, the idea that the features of the parents mix in their children, like paint. Gregor Mendel, an Austrian monk, tested this idea. Between about 1856 and 1863 he grew and crossed thousands of pea plants in the monastery garden at Brno, and he counted the offspring. His results laid the foundation of modern genetics, although the importance of his work was not recognised until after his death.
Why Mendel used pea plants
Good choices made the experiments work.
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Easy to grow
Peas are cheap, grow quickly and take up little space, so many plants could be studied.
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Short life cycle
Several generations could be grown in a few years.
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Clear differences
The plants have features with two clear forms, such as tall or dwarf, with no in-between types.
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Controlled breeding
Pea flowers normally pollinate themselves, but Mendel could cross-pollinate them by hand, so he knew which plants were the parents.
Mendel's method
His careful method was as important as his results.
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Pure-breeding plants
He first let plants self-pollinate for several generations, until they always produced offspring like themselves.
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One feature at a time
He looked at seven features, including height, seed shape and flower colour, one at a time.
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Crossing
He cross-pollinated two pure-breeding plants with different forms, such as tall and dwarf. These plants were the parental, or P, generation.
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Counting
He counted the offspring in each generation, and used mathematics to look for ratios, which was very unusual in biology at the time.
Mendel's cross of tall and dwarf pea plants
The dwarf feature disappeared in the F1 generation, and came back in about a quarter of the F2 generation.
The tall and dwarf experiment
Mendel's results, in order.
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1
P generation
Pure-breeding tall plants were crossed with pure-breeding dwarf plants.
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2
F1 generation
All of the offspring were tall. There were no medium plants, so the features had not blended.
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3
Self-pollination
The F1 plants were allowed to pollinate themselves.
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4
F2 generation
Of 1064 plants, 787 were tall and 277 were dwarf, which is about 3 tall to 1 dwarf.
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5
Conclusion
The dwarf feature had not been lost. It was hidden in the F1 plants, and then reappeared unchanged.
Mendel's ideas
He explained the 3 : 1 ratio with ideas that match what we now know about genes.
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Inherited factors
Features are controlled by separate factors, which do not blend. We now call these factors genes.
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Pairs of factors
Each plant has two factors for a feature, one from each parent.
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Dominant and recessive
One factor can hide the other. The tall factor is dominant, and the dwarf factor is recessive.
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Gametes get one
The two factors separate when gametes are made, so each gamete carries only one.
Analysing Mendel's results
In the F2 generation there were 787 tall and 277 dwarf plants. Calculate the percentage that were tall, and the ratio of tall to dwarf in the form x : 1.
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- 1 Total \(787 + 277 = 1064\).
- 2 Percentage tall \(787 \div 1064 \times 100 = 74.0\%\).
- 3 Compare A 3 : 1 ratio predicts 75%, so 74.0% is very close.
- 4 Ratio \(787 \div 277 = 2.84\), so the ratio is 2.84 : 1.
Answer74.0% were tall, and the ratio is 2.84 : 1, which is close to 3 : 1.
Why Mendel was ignored
He published his findings in 1866, but they had little effect in his lifetime. He died in 1884, without recognition.
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The ideas did not fit
Most scientists believed in blending inheritance, and Mendel's results were in conflict with it.
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No mechanism
Chromosomes had not been linked to inheritance, and meiosis had not been described, so there was nothing to show how factors were passed on.
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Mathematics
His statistical approach was unusual in biology, and many biologists did not understand it.
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Few readers
He published in a local journal, and was not a well-known scientist.
How understanding developed
The mechanism that Mendel lacked was found gradually.
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1870s and 1880s
Scientists observed cell division, and saw chromosomes in the nucleus.
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1900
Three scientists repeated Mendel's experiments, and rediscovered his work, after 34 years.
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1902
Walter Sutton and Theodor Boveri suggested that Mendel's factors were carried on chromosomes.
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1953
Watson and Crick, using the work of Rosalind Franklin and Maurice Wilkins, described the structure of DNA, which showed what genes are made of.
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2003
The human genome was sequenced, which shows the order of the bases in all of our genes.
Case study
Blending against particles
Charles Darwin believed that inheritance involved blending, and this caused him difficulty with his theory of natural selection. If a useful new feature were blended with others in each generation, it would soon be diluted and disappear. Mendel's idea of separate, unchanging factors would have solved the problem, but Darwin never read it. It shows why scientists must share their work, and how the lack of an explanation of the mechanism held back biology for decades.
Summary and exam focus
- Mendel crossed pea plants and counted the offspring, in the 1850s and 1860s.
- All F1 offspring were tall, and the F2 had about three tall to one dwarf.
- He proposed separate inherited factors, which we now call genes.
- His work was not accepted because it did not fit blending inheritance, and no mechanism was known.
- Later discoveries about chromosomes and DNA supported his ideas.
Exam focus
Describe the work of Mendel, and explain why his ideas were not accepted until after his death. (6 marks) (6 marks)
Describe the pea plants, the cross of tall with dwarf, the F1 result of all tall and the F2 result of about 3 to 1, and what he concluded about inherited factors. Then give two reasons for the delay, such as blending inheritance being believed and chromosomes not having been linked to inheritance.
Key terms
The words this lesson expects you to use. Each one is linked from the first place it appears above.
- Blending inheritance
- The idea that the features of parents mix in their offspring.
- Pure-breeding
- Producing offspring that are always the same as the parent.
- Cross-pollination
- The transfer of pollen from one plant to the flower of another plant.
- P generation
- The parental generation in a cross.
- F1 generation
- The first generation of offspring from a cross.
- F2 generation
- The offspring from the cross of F1 individuals.
- Inherited factor
- Mendel's name for what we now call a gene.
- Chromosome
- A long molecule of DNA that carries genes.
- Genome
- The entire genetic material of an organism.
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