3.4 Inheritance

Making quantitative measurements with replicates to ensure reliability. Mendel’s genetic crosses with pea plants generated numerical data.

Gregor Mendel, the “Father of Genetics”, made his discoveries on inheritance by using the garden pea, Pisum sativum. Mendel’s experiments and data collection over eight years formed the foundation of theoretical genetics and were able to be used in diagnosing and explaining genetic diseases at the turn of he 20th-century.  Just as important as his discoveries, though, was his meticulous following of the scientific methods, illustrating perfectly that replicates in quantitative experiments allow for greater reliability in the conclusions.

The seven traits Mendel studied (Griffiths et al.)

Mendel worked with the seven traits outlined above and bred them for two years to establish pure, or homozygous, breeding strains. He then pollinated the parental flowers that showed variation in the trait – for example, crossing purple flowers with white flowers.  This produced in the F1 generation 100% purple flowers.  When these flowers were self-pollinated, Mendel noticed a curious relationship in the F2 offspring: a ratio of almost exactly 3:1 in the phenotype.

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The data of the F2 ratio from Mendel’s experiments, a total of over   18, 000 breeding experiments. (Griffiths et al.)

On the basis of this data, Mendel was able to draw key conclusions about the nature of inheritance.  These were:

  1. The existence of what we now know as genes.
  2. That these genes come in pairs
  3. Gene pairs segregate during the production of gametes
  4. Each gamete thus only contains one gene of a pair.
  5. Fertilisation is random

These statements were able to be tested by a new round of experiments, because they were based on quantitative data, had significant repetition and suggested certain patterns in inheritance.  His subsequent experiments provided confirmation of his analysis.  Not bad, considering the structure of DNA wouldn’t be determined for another century!

Mendel also did some interesting work on dihybrids, but that’s for a later topic!


Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000. Mendel’s experiments. Web. May 2, 2016.

Miko, I. Gregor Mendel and the principles of inheritance. Nature Education 1(1):134. 2008. Web. May 2, 2016.


10.3 Gene pools and Speciation

Looking for patterns, trends and discrepancies—patterns of chromosome number in some genera can be explained by speciation due to polyploidy.

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Polyploidy in a self-fertilising plant (Bioninja)

Polyploidy is a condition of possessing more than two sets of chromosomes.  This duplication typically occurs during meiosis and may result in triploid (3n), tetraploid (4n) or even hexaploid (6n) offspring. This sounds potentially devastating (think of the effects of just having one extra chromosome!) but is thought to be a key driving factor in speciation in some organisms. Up to 1 in 100,000 flowering plants are polyploid, showing a high degree of tolerance.  In fact, over 75% of flowering plant species may be recent polyploids! Common polyploids include potatoes, bananas, cotton, wheat and watermelon.

Although it also can occur in amphibians and fish, mammals and birds are typically less tolerant, with an estimate of 10% of spontaneous miscarriages in humans being due to polyploid embryos (Woodhouse et. al.).

The advantages of polyploidy include:

  • The extra sets of alleles mean that the offspring is less likely to be affected by recessive mutations
  • it promotes heterozygosity, which can make the hybrid offspring more fit than the parents
  • the extra sets of genes may evolve new functions over time
  • it seems to increase asexual reproductive ability, which could be advantageous under some conditions.

Potential disadvantages include:

  • the extra genetic material affects the surface area to volume ratio of the nucleus and the cell, disrupting many important processes
  • disruption to normal mitotic and meiotic divisions
  • negative effects on gene expression and interaction




n.a. Polyploidy. University of Texas at Austin. The Chen Laboratory. 2016. 28 August. 2018

n.a. Speciation. Bioninja. N. p., 2016. Web. 7 Apr. 2016.

Woodhouse, M., Burkart-Waco, D. & Comai, L. Polyploidy. Nature Education 2(1):1. 2009. Web. April 7, 2016.

3.3 Meiosis, 10.1 Gene Linkage and 10.2 Inheritance

Making careful observations—meiosis was discovered by microscope examination of dividing germ-line cells. (3.3 – Core)

Making careful observations—careful observation and record keeping turned up anomalous data that Mendel’s law of independent assortment could not account for. Thomas Hunt Morgan developed the notion of linked genes to account for the anomalies.  (10.1 – AHL)

Looking for patterns, trends and discrepancies—Mendel used observations of the natural world to find and explain patterns and trends. Since then, scientists have looked for discrepancies and asked questions based on further observations to show exceptions to the rules. For example, Morgan discovered non-Mendelian ratios in his experiments with Drosophila. (10.2 – AHL)

Both core and HL topics on meiosis focus on the importance of making observations and accurately interpreting them.  The HL NOS, with its references to Morgan, illustrates how this process helps expand and redefine our understanding of biology. Morgan is an interesting character and worth spending some class time on.

When Mendel’s genetic studies were rediscovered, scientists set about replicating and confirming them. However, one group (William Bateson, Edith Rebecca Saunders, and Reginald Punnett, he of the eponymous square) persistently found phenotypic ratios that were far more common than could be predicted based on Mendelian inheritance patterns. Examining pea plants during a dihybrid cross (link to AHL Topic 10.2), they received statistically significant deviations from the predicted, as seen below:

Characteristics of the F2 Generation (Bateson et al., 1905)
Characteristics of the F2 Generation (Bateson et al., 1905)

They surmised that the alleles must be coupled somehow, but could not explain how.  Enter Professor Thomas Hunt Morgan.  Five years later, 1910, he was experimenting on fruit flies, Drosophila melanogaster, and found a white-eyed male in one of his studies (fruit flies normally have red eyes). Further experimentation found that instead of an expected 1:1:1:1 ratio of red-eyed females, red-eyed males, white-eyed males, and white-eyed females, he observed the following phenotypes in his F2 generation: 2,459 red-eyed females, 1,011 red-eyed males and 782 white-eyed males. The lack of white-eyed females led him to hypothesise that the gene must be linked to the sex factor.  This later led to the concept of gene linkage: genes on the same chromosome do not assort independently.

Columbia University Fly Room - note bunches of bananas! © 2013 The American Philosophical Society
Columbia University Fly Room – note bunches of bananas!
© 2013 The American Philosophical Society

This was groundbreaking – it meant that genes were concrete, real objects that could be located on chromosomes  and their inheritance and behaviour mapped, predicted and analysed. Interestingly enough, Morgan had initially rejected the idea that genes were located on chromosomes, believing that data generated through passive observation could not be trusted; another instance of scientists having to reject old ideas in favour of new and compelling evidence. Morgan would go on to win the Nobel Prize in Physiology or Medicine  “for his discoveries concerning the role played by the chromosome in heredity” (Nobelprize.org; 2014).

We will discussing a lot more about Prof. Morgan in our next unit, Inheritance.

Extra: One of the prescribed TOK Essay Titles for May 2015 was: “There are only two ways in which humankind can produce knowledge: through passive observation or through active experiment.” To what extent do you agree with this statement?  You could consider drafting a short response using Morgan as a specific example.


Lobo, I. & Shaw, K. (2008) Discovery and types of genetic linkage. Nature Education 1(1):139. Web. 17 Mar 2015.

Miko, I. (2008) Thomas Hunt Morgan and sex linkage. Nature Education 1(1):143. Web. 17 Mar 2015.

“The Nobel Prize in Physiology or Medicine 1933”. Nobelprize.org. Nobel Media AB 2014. Web. 17 Mar 2015.


Bateson, W., et al. (1905) Characteristics of the F2 Generation. Reports to the Evolution Committee of the Royal Society. Nature Education. Web. 17 Mar 2015.

n.a. (1913). Columbia University Fly Room. The American Philosophical SocietyNature Education. Web. 17 Mar 2015.