Mendel’s experiments

 Mendel’s work (1822-1884)

Gregor Mendel is known as the father of genetics. His contribution to genetics was crucial. He had a hard time through his work as the series of experiments conducted on pea plants went without recognition. He discovered the fundamental laws of inheritance by hybridizing the pea plants having 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 distinct units that come one from father and another from the mother. Mendel was able to track how genes get segregate 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 the ideal subjects for his experiment because of variations that they have (tall, dwarf, light and dark colour, etc.) and proposed the rules of inheritance. Mendel found that the traits or characteristics are present in pairs and such pair of contrasting characters is called an allele. He published 3 main laws popularly known as Mendelian principles.

Table of Contents

1. Mendel’s work (1822-1884)

2. Mendel’s experiment on pea plants

     2.1 Possible Reasons for  Mendel selecting Pea Plants for his             experiment 

     2.2 Steps of Mendel’s experiment

 3. Hybridization experiment

4. Example of crossing to show the inheritance of one gene

     4.1 Results of the F1 generation ( phenotypically all are tall)

     4.2  Results of crossing in the F2 generation (1:3)

5. Mendel`s conclusions 

 

Mendel’s experiment on pea plants

Possible Reasons for  Mendel selecting Pea Plants for his experiment 

1. Pea plants are biennial i.e. two generations of plants can be grown in a single year so, Mendel knew 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.

3. It’s easy to cross-pollinate pea flowers as they are bisexual in nature with 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 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 show a stable set of traits after having undergone continuous self-pollination,

D. Mendel selected 14 true-breeding pea plant varieties, as pairs (7 pairs) with each pair having opposing characters like 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 entering the generation. He conducted a Hybridization experiment that includes 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, he carefully removed the anther of the flower and transferred the pollen to the other stamen 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 these 2 plants as shown in the picture, then he collected the resulting plant’s seeds to culture them. The plants grown by such seed culture he called was F1Or filial progeny. Mendel found that the resulting set of (F1 plants) progeny plants were all tall and none of them were dwarf.  On the other hand, he also made similar observations of other traits (seed shape,  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.

Results of crossing in F2 generation (1:3)

 

Mendel`s conclusions 

Mendel felt that there is something that passes from each generation which is almost unchanged and such a factor was unknown at that point in time, so he called them as factors instead of genes. Later on after repeated research and investigations, the term factor turned out to be gene. He explained that genes are the units of inheritance that pass something to the next generation. Something refers to the TRAITS. A trait can be gall 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.

 



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