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How did Mendel derive the phenotypic ratio of the pea plant?

  1. Dec 17, 2015 #1
    i had doubt in hereditary and evolution chapter class 10 CBSE India
    So now I know that the phenotypic ratio of the pea ( with reference to dwarfness and tallness) is 3 : 1
    which can be interpreted as : for every 4 pea plants obtained by a peapod, there are 3 tall plants and 1 dwarf plant
    ( this is how ratios work ) so the statement is agreed
    But if you refer the image below,....
    You will see pea seeds .On average the number of peas per pod = 54 / 6 = 9 ( 54 peas within 6 pods )
    So, The average number of peas per pod is 9 peas per pod
    Suppose when Mendel crossed one flower of a pea and got one pea pod out of it with 9 pea seeds ( average number of peas per pod as calculated above ) how did he even come to the conclusion of the phenotypic ratio of the pea plant which is 3 : 1 experimentally ; when the total number of seeds is not even a factor of 4 ( i.e. 3 + 1 ; from the phenotypic ratio )
    experts please explain .... Peas_in_pods_-_Studio.jpg
  2. jcsd
  3. Dec 17, 2015 #2


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    Every plant has its own independent 3:1 probability ratio, if you have four plants it does not necessarily mean that you have 3 of one type and 1 of the other. You always have statistical fluctuations, but with an increasing sample size you can be more and more confident that the ratio is very close to 3:1. 4 plants are not sufficient, but if you make 99 plants (deliberately chosen to be not a multiple of 4) and get 77 of one type and 22 of the other, 3:1 is a reasonable guess. 4:1 is also possible, so you need more plants to narrow it down further. On the other hand, there is no model that would predict 4:1.
  4. Dec 17, 2015 #3

    jim mcnamara

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    Gregor Mendel was a monk - he spent time in food production for his monastery. These phenotypic observations are summaries from notebooks he kept over the years. Peas were and are a staple food in Northern Europe. For Mendel, this amounted to large numbers of pea plants. It is believed that he saw the trend in the larger production fields and then planted a small carefully maintained garden. Still lots and lots of pea pods and peas to keep track of. From which he took detailed notes. And worked out a concept which was completely new: dominant and recessive genes. All of modern Genetic traces its roots back to him.

    His work sat in some notebooks for thirty years until it was verified independently by several scientists in the early 1900's. Look up Hugo DeVries for example.

    Apropos of nothing special -
    IMO, OP's question is a very good one that reflects badly on Biology teaching. I had Intro Bio courses that dealt with the work of LaMarck, Mendel, Lister and many other early Biologists - at University we were required to take a History of Science, and another in the Philosophy of Science. You can make arguments as to why teaching only relevant current theory and practice is the only way to go, but sometimes it makes things hard to understand for someone new.

    In talking with a newly minted Bio PhD, I found out that she had never heard of Hans Zinsser ("Rats, Lice, and History") and why he is directly linked to Jared Diamond ("Guns, Germs, and Steel"). Biology went the way of Mathematics in terms of specialization. In Mathematics it is said that David Hilbert was the last person to understand all of the extant math in his own time. But Biology in its current guises all arise from a common history.
  5. Dec 18, 2015 #4


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    Except for chymeric plants you will have one plant that exhibits one phenotype. When you harvest the peas you will have one type of pea for the entire plant nearly all of the time.

    You still have the problem where your number of plants is not divisible by four but as mfb an jim explain statistical analysis will work out the details.


    PS: For the experts--are chymeric pea plants a thing?
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