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The phenotypic ratio of dihybrid cross 9:3:3:1 can be derived as a combination series 3 yellow: 1 green, with 3 round :1 wrinkled. This derivation can be writtenbas follows:

(3 round :1 wrinkled) (3 yellow: 1green)= 9 round,yellow: 3wrinkled, yellow: 3 round,green: 1 wrinkled, green

I only understand the phenotypic ratio but what is the above series mean?

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The characteristic(or trait) which is visible in an organism is called its phenotype. The ratio of the phenotypes of the progeny produced in a cross of trait(s) gives us the phenotypic ratio.

A dihybrid cross refers to a cross in which plants having two contrasting characters are crossed with each other. For example, crossing round and yellow seeds with wrinkled and green seeds is an example of a dihybrid cross where the contrasting characters are as follows:

1. Round and wrinkled seeds (contrasting on the basis of the seed shape)

2. Yellow and green seeds(contrasting on the basis of seed color)

enter image description here

The above image gives us the perfect example of a dihybrid cross where two heterozygous parents (RrYy and RrYy) crossed with each other to form four new progenies:

  1. Round and yellow (RRYY or RrYy or RRYy or RrYY)
  2. Round and green (RRyy or Rryy)
  3. Wrinkled and yellow (rrYy or rrYY)
  4. Wrinkled and green (rryy)

We will observe that for instance if 16 progenies are produced, then 9 will be round and yellow, 3 will round and green, 3 will be wrinkled and yellow and only one will be wrinkled and green.

This brings us to the conclusion that the phenotypic ratio of a dihybrid cross is always 9:3:3:1 regardless of the number of plants progenies produced. For instance if there are 60 progenies produced then :

  1. Number of round and yellow seeds will be - 9/16 of 60
  2. Number of round and green seeds will be - 3/16 of 60
  3. Number of wrinkled and yellow seeds will be - 3/16 of 60
  4. Number of wrinkled and green seeds will be - 1/16 of 60

You can reach this site for further clarification

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  • $\begingroup$ Great answer. It is important to mention that 9:3:3:1 are the odds of each phenotype occurring and not the actual fraction. We have statistical tests (like chi-squared) to see if a population is behaving in a mendelian form. $\endgroup$
    – Hachiloni
    Feb 3 at 21:26

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