Mendel’s Law of Dominance

Mendel’s Law of Dominance can also be simply stated as:

“In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.”

What this means:

We all know that Mendel was breeding peas to observe the hereditary effects of different types of breeding.

In a cross of parents that are pure for contrasting traits:

Here pure means monohybrid whereas contrasting traits mean any two form a particular trait. For example, say height- Pea plant could be of two possible heights : either tall or short.

So, Mendel takes two pea plants: One short pea plant (thus having genes “tt” for expressing height) and one tall pea plant ( Having a genes “TT” which express or determine its height.)

Then he bred them by pollinating the flower of one monohybrid short pea plant with pollen of monohybrid tall pea plant.However, when he planted these seeds after harvesting, he found that all of the offspring plants were tall. Why was this so?

We all know that in sex gametes, genes break up and one of them forms a pair with an allele from the other cell. For pea plants, the two sets of gametes were TT and tt. Hence, during meiosis, the only possible combinations is Tt.

So why does only T expresses itself in a Tt pair? Why was a plant whose one parent was tall and another short expresses only the tall trait?

This anomalous behaviour was exhibited in various other crossbreeding experiments in which two contrasting trait exhibiting plants were used.

So Mendel thought, maybe one gene suppressed the other or prevented the other gene from expressing it. And thus he devised the law of dominance which states that:

“In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.”

So, if there exists two contrasting traits, one of the traits will always suppress the other, thereby expressing itself. T suppresses t, thus making the offspring plant tall.

Such a trait is known as a Dominating trait. The suppressed trait is known as Recessive trait. Also, the recessive trait freely expresses itself in the absence of the dominant state. And this is what Mendel’s Law of Dominance is all about.

So in the pea plant,
the seed color yellow always dominates seed color green. seed shape round always dominates seed shape wrinkled.So here, round (RR) and Green (GG) are the dominant traits whereas wrinkled (rr) and yellow (gg) are recessive trait.

12 Comments

  1. Vyom July 6, 2012 at 10:48 am #

    Hello everyone
    I wanted to ask that in case of antirhinum majus if (r) gene dosent is less efficient to produce the red color and the pathway is

    R -) enzyme 1 -) r -) enzyme 2 -) pink color

    Then how come RR will be able to express itself because both will produce enzyme 1) which converts substrate to pink color ?

    Reply

    • Srushti K July 12, 2012 at 2:49 am #

      Hey Vyom, we are just currently discussing basic Mendelian laws. However, the question you asked is far more complicated than that. I assume you are talking about flower coloring, am I right? Just email me at signa14@yahoo.co.in, and we shall discuss this further.

      Reply

    • Srushti Kumbhare July 12, 2012 at 11:04 pm #

      In A.majus, the red and pink color both is due to magenta anthocyanin pigmentation. Since r is less efficient to produce red color, that is the magenta anthocyanin pigmentation is in lesser concentration, giving the flower a pink color. However, in the case of RR, since there are double the amount of enzymes(enzyme1 +enzyme2), the magenta anthocyanin is in more concentration, thus giving the flower a crimson or red coloration. Hope that answers your question. :)

      Reply

  2. Bhaskar December 30, 2012 at 4:44 am #

    this information is really helpful. Thanks…

    Reply

  3. Chandan sharma February 12, 2013 at 9:20 pm #

    Hello every one
    I am chandan sharma and just i asked the one question how can be prepar the artificial gene of dominens and resesive

    Reply

  4. molly April 1, 2013 at 12:29 pm #

    wuz up :) :)

    Reply

  5. molly April 1, 2013 at 12:29 pm #

    wuz up.

    Reply

  6. Achyut June 8, 2013 at 12:14 am #

    Do you think law of dominanace is also a law that was generated by Mendel???
    It might be the principle of the experiment on which he deal with??

    Reply

    • Srushti Kumbhare August 26, 2013 at 2:55 am #

      These laws by Mendel are the result of over ten years of experimenting with pea plants. The principles were established by him ON the basis on the results and findings of these experiments. This experiment wasn’t a pre-designed one, rather the one he designed himself to satisfy his own curiosity – “How does this happen?”, the same question thousands of scientists had in their head before making a life changing discovery.

      Reply

  7. Abdullah collegiation November 14, 2013 at 1:05 am #

    hi everyone i need online lectures will u plez help me where i will i found it…..thanks

    Reply

  8. Shivam Kapadia February 2, 2014 at 2:31 am #

    why is it not universally applicable?
    is it because of the co-dominance and incomplete dominance?

    Reply

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