Gregor Mendel was an Austrian monk that devoted nearly as much of his life to understanding the nature of heredity as he did in his fraternal duties. From his experiments with peas, he was able to determine several basic principles of how traits were passed from parents to offspring.
Pre-Mendelian Concepts on Inheritance
Blended inheritance hypothesis
Mendel wasn’t the first person to theorize how traits are inherited from generation to generation. Humans have probably done this since the dawn of time. For example, you hear family and friends comment on how a child is a perfect mix of father and mother, or how a baby has her father’s nose and her mother’s lips. During Mendel’s time, the most accepted hypothesis of inheritance of physical characteristics was blended inheritance. The blended inheritance hypothesis suggests that physical traits (or phenotypes) of offspring are an intermediate of the parents. For example if a tall man and a short woman have a child, this hypothesis predicts their child would have a height intermediate relative to her parents.
Inheritance of acquired characteristics hypothesis
Another (non-competing) hypothesis of inheritance was in vogue during Mendel’s lifetime: inheritance of acquired characteristics. This hypothesis was championed by Jean Baptiste de Lamarck. He used the fossil records of giraffes and their ancestors as the nexus of his argument. Early (now extinct) ancestors of giraffes had considerably shorter necks than modern day giraffes. Lamarck suggested that these early giraffes stretched their necks out to reach leaves in trees during times of drought, individually acquiring a longer neck. He suggested that the giraffes that were able to stretch their necks the most would be more likely to survive and reproduce (He coined “survival of the fittest,” not Darwin). Furthermore, Lamarck suggested that the offspring of the giraffes that stretched their necks would inherit this acquired characteristic. Lamarck argued that if this happened from generation to generation, we would see giraffe neck length increase through time. This prediction is supported in the fossil record.
Mendel's Monohybrid Cross
Creating pure lines
Mendel chose to study inheritance of the pea plant. While the pea is a fast-growing species (which makes it a good experimental subject), its most important characteristic is the pea can be self-fertilized. Self-fertilizing a plant is the process in which the sperm (pollen) from one plant is used to fertilize the eggs (ovules) of the same plant. Self fertilization creates pure lines, in which all offspring are exact copies (clones) of the self-fertilized plant.
From his controlled self-pollinations, Mendel germinated and grew the “pure line” seeds of plants with several different phenotypes: seed shape, seed color, pod shape, pod color, flower color and stem length. He collectively called pure line plants the P generation, the parent generation. The P generation served as the starting point for his inheritance experiments
Mendel mated peas representing two extreme “pure line” phenotypes from the P generation. The resulting offspring are the first filial (or F1) generation. We will focus on his experiment with different flower colors: purple and white.
Results of the F1 generation
Mendel’s results for all of his physical traits did not support the blended inheritance hypothesis. Rather, he found that one of the extreme traits appeared in a cross of different pure lines. He called these expressed phenotypes dominant, meaning that if there is a mix of two pure lines this phenotype will be expressed. For flower color, purple dominated over white, meaning if a pure-line, purple-flowered plant is mated with a pure-line, white-flowered plant, all of the resulting offspring have purple flowers. In contrast, the phenotype that is masked is known as the recessive phenotype. White flowers are recessive to purple flowers in pea plants.
Mendel pondered, “If one phenotype dominates over another, how can the recessive phenotype even exist in a population?.” This led him to conduct another controlled cross, this time between plants of the F1 generation. While the P generation was composed of pure line plants, he knew that the F1 generation was composed of half the genetic information from each plant in the P generation. What happens if the hybrids are crossed? The resulting generation is the F2 generation (hybrids of hybrids), and the results awaiting him were another surprise to Mendel.
For all the different phenotypes Mendel analyzed, the recessive characteristics reemerged in the F2 generation! And they did so with a predictable regularity. The ratio of dominant to recessive phenotypic ratio of all of the characteristics Mendel analyzed were all very close to 3 dominant: 1 recessive. In other words, in the F2 generation ¾ of the pea plants expressed the dominant phenotype, while ¼ expressed the recessive phenotype.
Mendel’s particulate inheritance hypothesis
Mendel’s elegantly simple experiment clearly disproved both the blending inheritance and the inheritance of acquired characteristics hypotheses. He proposed an alternate hypothesis, the particulate inheritance hypothesis. He predicted that the inherited phenotypes don’t blend from generation to generation. Rather, he suggested that the offspring inherit discrete ‘particles.’ If one of these particles is dominant, the dominant phenotype will be expressed. If both particles are recessive, the organism will express the recessive phenotype.
Alleles, genes and the genotype
Today, we call Mendel’s ‘particles’ alleles. In sexual reproduction, an allele from one parent combines with an allele from a second parent during fertilization. The combination of the two alleles is known as a gene. We now know that a gene is a segment of DNA that codes for a specific mRNA, which in turn codes for a specific protein. There are three possible combinations of genes. Two dominant alleles represent a genotype termed homozygous dominant, and are notated with two capital letters (e.g. AA). Two recessive alleles in a gene are said to be homozygous recessive, notated with two lowercase letters (e.g. aa). A genotype with both a dominant and recessive allele are heterozygous, and are expressed with a capital letter followed by a lower case letter (e.g. Aa).
Mendel's Principle of segregation
In order to explain the consistency of his F2 results (3 dominant: 1 recessive), Mendel suggested that the ‘particles’ (again, we call them alleles) must segregate during the production of gamete cells (i.e. sperm and egg). The resulting gametes therefore have only one allele from each parent. Further, he suggested that a gamete must have an equal probability (50%) of acquiring one allele vs. the other. This became known as the principle of segregation. Using the principle of segregation along with a simple technique developed by R. C. Punnett years after Mendel’s work, we can easily determine the ratios of phenotypes and genotypes predicted by Mendel’s particulate inheritance hypothesis resulting from controlled crosses starting from pure lines.
The Punnet square protocol
A cross of FF (purple-flowered peas) with ff (white-flowered peas) is shown as an example.
- Draw a box with lines bisecting it horizontally and vertically, extending the vertical line above the box and the horizontal line to the left of the box.
- Write the segregated alleles of one parent on the cells horizontally above the box. Write the segregated alleles of the other parent vertically to the left of the box.
- Pull the top alleles down into the boxes directly below them. Combine those alleles with the alleles from the other parent (represented on the vertical axis) by pulling the vertical alleles to the boxes to their right.
- In the case of a heterozygote, always write the dominant allele (represented by a capital letter) in front of the recessive allele (represented by the lower case letter).
- Each internal box represents the probability (25%) expected of that genotype in the next generation. In the case in which more than one box is represented by the same genotype, the probability increases by 25% for each additional box.
But wait...there’s more! Mendel’s monohybrid crosses established that inheritance of phenotypes happened by the interactions of discrete ‘particles’ (we now call alleles). Further, he discovered that those alleles segregate during the process of meiosis in the production of gametes, which recombine during fertilization. The result of these two insights can easily be predicted by a technique developed by R. C. Punnett. Once these principles were established, Mendel sought to understand how multiple phenotypes were related to each other during inheritance. He developed two alternate hypotheses:
Dependent assortment hypothesis
The dependent assortment hypothesis predicts that phenotypes are linked to each other. For example, a purple-flowered plant would always have green seeds (but never yellow seeds).
Independent assortment hypothesis
Alternatively, flower color and seed color could be independently assorted, in which purple-flowered plants could have green or yellow seeds, and a white-flowered plant could also have green or yellow seeds. In other words, flower color is independent of seed color.
Mendel’s dihybrid crosses
To test these alternate hypotheses, Mendel used the same approach as he did in his monohybrid crosses, except this time he analyzed two phenotypes at the same time. For our example, we will analyze his results of flower and seed color. He began his experiment by conducting several controlled crosses of one plant with two known dominant alleles generating a pure line: homozygote purple-flowered (FF) and yellow-seeded (SS). Collectively this is represented as FFSS. He conducted another controlled cross of another plant with two known recessive alleles, ffss.
The independent assortment hypothesis predicts that during meiosis, gametes can acquire any possible combination of alleles (e.g. FS, Fs, Fs, or fs). If this hypothesis were supported, Mendel expected to see all possible phenotype combination in the F2 generation: purple-flowered/green-seeded, purpled-flowered/yellow-seeded, white-flowered/green-seeded, and white-flowered/yellow-seeded. Furthermore, based on this understanding of segregation and recombination of alleles based on his monohybrid crosses, he could predict the relative abundance of each phenotype.