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Genetics 101: Mendelian Genetics

By Jennifer J | Wednesday, November 2, 2021

For centuries, humans have tried to understand the fundamentals of heredity with interesting theories on inheritance from Pythagoras to Darwin. In this series, titled Genetics 101, we will go over the fundamentals of genes. So let us begin by introducing the man who first discovered them, Gregor Mendel, known as the Father of Genetics.

Introduction

Father Gregor Mendel was a parish priest who worked as a substitute teacher because he could not pass his teacher’s qualifying exam in natural sciences (especially in Biology on topics of taxonomy and heredity which is quite ironic). In his defense, he struggled learning the classification system with no underlying logical mechanism of heredity. In his spare time, he tended to his pea garden growing seeds collected from his neighbors. He observed that all his peas plants differed in characteristics; some are were tall, some had purple flowers, etc.

In an effort to bring order to the chaos, he selected what we call pure lines or true breeding lines. This means that if a pea plant was tall in height, then all the plants arising from the seeds of that plant through self-fertilization must be tall, and plants from those pea plants must be tall and all successive generations of pea plants are tall.

Mendel picked seven distinct characteristics, each with 2 different traits (variants):

  1. Seed shape: Round and Wrinkled seeds
  2. Seed color: Yellow and Green seeds
  3. Flower color: Purple and White flowers
  4. Flower position in plants: Axial and Terminal flowers
  5. Plant height: Tall and Dwarf stems
  6. Pod shape: Smooth and Constricted pods
  7. Pod color: Green and Yellow pods

Pea plants can pollinate in two different ways : Self-pollination occurs when pollen from the anther (male part) of a flower is transferred to the stigma (female part) of the same flower or a different flower of the same plant leading to self-fertilization. The resulting plants will have the exact same features as their parent. Cross-pollination occurs when the pollen of one flower is transferred to another plant's flower leading to cross-fertilization. This will be referred to as a cross and the resulting plants will be called a hybrids.

Pea plants naturally reproduce by self-fertilization. Usually, pollination occurs before the flower opens because of a modified petal covering the reproductive structures. This makes cross-pollination (cross-fertilization) much more difficult.

To cross-pollinate, Mendel pried opened immature flowers and removed the anthers before they could produce pollen and self-fertilize. So now, these flowers could be pollinated by brushing the stigma with the pollen of another plant. This cross' resulting pea plant (progeny) would now be a hybrid.

Mendel’s Experiment

Mendel first crossed a pure-breeding plant with one distinct trait with another pure-breeding plant with an opposing trait; for example, he crossed (cross-fertilized) tall pure breeding plants with dwarf pure-breeding plants. This experiment was repeated for all 7 distinct characteristics in 7 different monohybrid crosses (a monohybrid cross is when only one characteristic is observed).

Let’s retrace Mendel's steps by using one monohybrid cross as an example; a cross between tall pure breeding plants and dwarf pure-breeding plants. This is also called the parental cross and the plants as parental plants.

Then Mendel collected the resultant seeds and planted them. These hybrids are called the F1 (Filial 1) generation. He observed the height of these plants and found that they all grew tall. Then he let these F1 plants self-fertilize and collected the resultant seeds. These seeds grew into F2 (Filial 2) plants. He observed that 787 plants were tall and 277 plants were dwarf, so the ratio is 2.84:1. Approximately 75% of the F2 plants were tall and 25% were dwarf. He did this kind of cross for all 7 characteristics of the pea plants and found that in each cross, all the F1 generation had the trait and the F2 generation had a 3:1 ratio.

Here is a table that summarizes his observations:

Mendel’s Proposals

From these experiments, Mendel put forth 2 proposals.

Law of Dominance

Traits can be either dominant or recessive. The trait of one character is dominant over another trait, and the trait that is masked by the dominant trait is called the recessive trait. For example, tall is a dominant trait, while dwarf is a recessive trait for plant height.

Law of Segregation

The second proposal he made was from the consistent observation of the ratios for each trait which suggests that the inheritance of traits is passed from one generation to the next by discrete unit factors that remain unchanged during this transfer. That is, the variants separate during gamete formation and do not remain together or get mixed with each other.

Modern Take on Mendel’s Proposals

Today, we call those unit factors described by Mendel genes and those different traits alleles. Although we tend to take this knowledge of genes for granted today, during the mid-19th century, his proposals were so radical that his work remained hidden for 30 years before it was rediscovered in 1900.

Genes/DNA/genetic material, as we know them today, are the blueprints that are transferred from parents to offspring that make up the characteristics of an individual. While alleles are the different variants of the gene, each parent contributes one allele to the progeny, so each gene has 2 alleles. Other variants of alleles are formed through mutations during the evolutionary process.

Let's take a hypothetical plant species A. This plant can only have red flowers (phenotype) with the genotype RR. Now, suppose there was a mutation in one of the alleles, resulting in a non-functional allele. However, because there is still one allele that can produce color, the plant's phenotype remains the same. After a few generations, both alleles can become mutated, where the gene can no longer make the red color, so now the plant produces white flowers, and the phenotype changed because its genotype changed (rr).

Let us look at the example of Plant height: Tall and Dwarf plants again.

Mendel crossed Tall pure lines to dwarf pure lines. He observed that the resulting plants were all tall. A pure line has homozygous alleles (when a gene has identical alleles repeated); here, let us denote TT for Tall pure breeding plants and tt for Dwarf plants.

Now we perform the parental cross: TT X tt. The resulting progeny will have the genotype Tt, whose phenotype Mendel observed as Tall, just like Tall pure breeding plants, i.e., T is dominant over t since it shows its phenotype. Now Mendel allowed them to self-fertilize, which means the cross would be: Tt X Tt.

The table (Punnet square) helps to summarize the different progenies:

Genotype: TT:Tt:tt =1:2:1 while the phenotype will be 3:1 as proposed by Mendel.

Legend: Bold font indicate Tall phenotype while the regular font indicates dwarf phenotype.

Mendel’s Second Set of Experiments

Mendel put forth another proposal with another experiment. Instead of looking at one character at a time, Mendel decided to look at two of them simultaneously through dihybrid crosses. So let us look at how he did this by looking at one such cross: seed shape and seed color.

Mendel took pure breeding plants with round and yellow seeds and crossed them with pure breeding plants with wrinkled and green seeds. He observed that in the F1 generation, all the seeds were round and yellow. He then self-fertilized the plants and observed 4 different traits in the seed shape and color. Most were round yellow seeds, some were round green seeds, some were wrinkled yellow seeds, and a few were wrinkled green seeds. He found them in the ratio of 9:3:3:1 (round yellow seeds: round green seeds: wrinkled yellow seeds: wrinkled green seeds)

Mendel’s Final Proposal

Mendel’s final proposal is the Law of Independent assortment. It states that the inheritance of one trait (such as yellow seeds) is independent of the inheritance of the other trait (such as wrinkled seeds).

Another Example

Here there are 2 genes, one for shape and another for color. The pure lines plants with round yellow seeds with a genotype of RRYY (R is dominant over r and Y is dominant over y) are crossed with plants with wrinkled green seeds (rryy). So the resulting F1 generation will have the genotype RrYy. This, when self-fertilized( RrYy X RrYy), will result in the F2 generation given below, whose phenotype ratio is 9:3:3:1

Legend: Here, bold font indicates the Round shape of the seeds, the regular font indicates the wrinkled shape of the seeds, and the text color indicates the color of the seeds of F2.

Conclusion

With these three proposals, Mendel proved that genetic material is passed from one generation to the next by discrete unit particles without changing. Coming from a non-scientific background, using statistics in biology, his scientific paper, along with his discovery, was forgotten till 1900 when three scientists independently rediscovered Mendel's laws of inheritance along with his forgotten research paper.

If you would like to learn more about this topic, I recommend the links I referenced below! If you would like to learn more about genetics, you can check out the new exciting biology courses at schoolhouse.world. Stay tuned for my next Genetic article where we explore genes that don't follow Mendelian rules!

Until next time, happy learning!

References :

  1. Books :
- Genetics : Analysis & Principles by Robert J. Brooker.


- The Gene : An intimate history by Siddhartha Mukherjee
  1. Links :
- https://www.sciencelearn.org.nz/resources/1999-mendel-s-experiments


- https://www.sciencelearn.org.nz/resources/2000-mendel-s-principles-of-inheritance

Thank you Sharon V for editing this article!

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