Mendel’s experiments

 Mendel’s work (1822-1884)

Gregor Mendel is well known as the father of genetics. His contribution to genetics was immense but he had a hard time through his work as the series of experiments conducted on pea plants went without being recognized for a very long time. He discovered the fundamental laws of inheritance by hybridizing the pea plants that had a mix of characters which he called traits. In his theories, Mendel stated that the genes come in pairs and these pairs are inherited as the distinct units. Both father and mother pass a pair of genes each to their offspring. Mendel was able to track how genes have segregated from parents and again reappear in the next generation. The appearance of traits was classified into 2 categories namely, the dominant and the recessive traits. He recognized the mathematical patterns of inheritance transmitted from one generation to the next. Mendel has chosen pea plants as his favourites to experiment. This is because pea plants are characterized by their wide variations such as tall, dwarf, light and dark colour, etc. He proposed the rules of inheritance which we call mendelian inheritance. Mendel found that the traits are present in pairs and such pair of contrasting characters are known as the alleles. Based on his experiments on pea plants, he published 3 main laws popularly known as Mendelian principles. This post takes you through the steps of Mendel's experiment. 

 

 

Mendel’s experiment on pea plants

Possible Reasons for  Mendel selecting Pea Plants for his experiment 

1. Pea plants are biennial I .e.they yield at least 2 generations of plants in a single year so, Mendel realized that he can have enough time to observe his findings.

2. Pea plants are also characterized by distinct contrasting traits such as round seed and wrinkled seeds, tall and dwarf, etc which enables to properly test both the dominant and recessive traits.

3. It’s easy to cross-pollinate pea flowers as they are bisexual in nature. In a pea plant,  male and female parts stay close to each other.

4. Pea plants have a short lifespan within which hey can produce a lot of seeds.

 

 

Steps of Mendel’s experiment

A. Mendel was a highly self-motivated and an inspired biologist. Yet, his contributions went unnoticed for a long time. He grew up watching plants and animals, as a result, he was fascinated by the way pea plants express the traits. He decided to conduct hybridization tests by taking into consideration a mix of traits. Mendel conducted hybridization experiments on the garden peas for about 7 years. After his prolonged trials, he published the laws of inheritance. 

B. In order to generalize and support his results, he had a large sampling size. In his experiments, he had investigated a mix of characters in the garden pea plants such as flower colour, flower position, seed colour, seed shape, pod shape and pod colour; all in their 2 contrasting forms.  The table below shows the contrasting pair of characteristics

C. To bring his ideas into actions he conducted artificial pollination(cross-pollination) experiments by using a number of true-breeding pea lines.  A true-breeding is when a breed has an ability to confer to a stable set of traits after having undergone continuous self-pollination,

D. Mendel selected 14 true-breeding pea plant varieties, ( i,e 7 pairs) with each pair having opposing characters  as shown in the table below.

 

SL.no

character

 

Dominant trait

 

Recessive trait

1.       

Flower color

Purple

 

White

2.       

Flower position

     Axial

 

Terminal

3.       

Seed color

 

Yellow

 

Green

4.       

Seed shape

 

Round

 

Wrinkled

5.       

Pod shape

 

Inflated

 

Constricted

6.       

Pod color

 

Green

Yellow

7.       

Height of plant

Tall

 

Short/dwarf

 

E. As a general rule, he studied only one character at a time to draw a proper conclusion .He used all the possible options to prevent the entry of undesirable pollen grains into the current generation. He conducted a Hybridization experiment with the help of emasculation (removal of anther) and transfer of pollen (pollination ) As shown in the picture below.

 

Picture showing the method of transferring pollen

 

 

Hybridization experiment

F. In order to allow the contact between male and female parts of a bisexual flower, he has done a procedure called emasculation. Emasculation was done by using a scissor, in which he carefully removed the anther of the flower and transferred the pollen to the other stamen of another flower as shown below.

 

 

Example of crossing to show the inheritance of one gene

In his experiment of hybridization, Mendel crossed tall and dwarf pea plants by transferring the pollen of as shown in the picture, then he collected the resulting plant’s seeds to culture them. The plants grown by such seed culture were called F1-first filial progeny. Mendel found that the resulting set of (F1 plants) progeny plants were all tall with none of them was dwarf.  On the other hand, he also made similar observations of other traits (seed shape, the colour of the flower, etc) and he found that the Falways resembled either one of the parents( either wrinkled or rounded seed / either, or either yellow or green seeded but not both) and that the trait of the other parent was not seen in them. Results of cross-pollination of both F1 and F2 are shown in the image below.

 

Mono-hybrid crossing

 

 

Results of the F1 generation ( phenotypically all are tall)

In the next stage, Mendel has self-pollinated (the same flower is used )  2 tall Fplants, resulted offsprings of Fg generation were that some of them were ‘dwarf ’ as the trait was not expressed in the F1 generation. At that stage, he has drawn a percentage chart of plants that were dwarf and tall. There were 25% (1 out of 3 as shown in the picture below) of the F2 plants were dwarf while the remaining 75% of the F2 plants were tall. The same results were observed with the other traits apart from the height that he studied. In general, only one of the parental traits was expressed in the F1 generation whereas in the F2 there were both features at the ratio of 3:1 in addition, contrasting traits were free from blending at either F1 or F2 stage.    The results of F2 are as shown in the image below.

 

 

Mendel`s conclusions 

Mendel felt that there is some factor or a force that passes through generations and which is almost unchanged. At the time, the real force that is being inherited was unknown hence he called them as factors instead of genes. Later on, after repeated research and investigations, the factor became invariably the gene. He explained that genes are the units of inheritance that transfer something to the next generation. Something here suggests the TRAITS. A trait can be tall or dwarf, fair or dark and so on. To indicate the traits, 2 types of letters were used, namely the capital letter that corresponds to the dominant trait for example, in the case of height, “T” refers to tall and “t” refers dwarf. So, “TT” “Tt “and “tt”.  Are the 3 combinations used to indicate dominant tall (genotypically tall), recessive tall (phenotypically tall but genotypically not purely tall) and dwarf used respectively. Because Mendel observed that both TT and Tt looks tall, they are classified as genotypically tall and phenotypically tall respectively, he also proposed that in a given pair of dissimilar factors (T and t) one will show dominance over the other, for instance, if you take height as the trait T (for tallness) is dominant over (for dwarfness), which is recessive. Whereas “tt” is purely dwarf both genotypically and phenotypically.

 

Questions

  1. What are the reasons behind the late recognition of Mendel`s findings?

  2. What are the traits? Explain the dominant and recessive traits with an example.

  3. What are monohybrid and dihybrid crossing? Distinguish between the 2.

  4. Explain the steps of Mendel`s experiments on pea plants

  5. Whay did Mendel chose pea plants for his expriments?

  6. What do you mean by genotypically tall and phenotypically tall?

  7. What is dominant and a recessive trait according to Mendel?

 

 

 



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