How To Find Phenotypic Ratio: For Dihybrid and Trihybrid F2 Generations

Phenotype refers to the outwardly seen features of any organism without strictly considering the genetic makeup.

The phenotype is also dictated by the genotype of the organism, based on the gene composition. However, the phenotype does not distinguish between homogenous or heterogenous genes if they cannot be observed by the naked eye.

On how to find phenotypic ratio of a dihybrid or trihybrid cross the most commonly used methods is the Punnet square but as it gets more complex other methods such as Fork-line or probability methods are used instead

A phenotype is a set of observable characteristics about a person, such as height, eye colour, and blood type. The genotype is the genetic contribution to the phenotype. Some qualities are heavily influenced by genotype, whereas others are heavily influenced by environmental circumstances.

As a result in most cases of crossing dissimilar genotypes, the phenotypic ratio is simpler and different than the genotypic ratio.

Image showing the blood group classification in both phenotype and genotype
Image: Wikipedia

How to find phenotypic ratio of dihybrid cross?

A dihybrid cross involves the inheritance of two unrelated genes, that are both individually locate near the same locus or position in a gene.

A mating experiment between two animals that are identically hybrid for two features is referred to as a dihybrid cross. A heterozygous organism possesses two distinct alleles at the same genetic location or locus.

When both heterozygous parents are included, the primary cross is performed. We’ll use the Flower colour and Flower position to show.

ColourViolet WWWhite ww
PositionAxial AATerminal aa
Table showing the alleles used for determination of a dihybrid cross and how genes are inherited

Hence the dominant parent organism is written as “WWAA”, while the recessive homogenous parent is represented by “wwaa”.

In the first generation by crossing these parents we get only one single type of genotype, a hybrid organism represented as “WwAa”. The F2 generation is obtained by interbreeding of the hybrid genotypes we obtained in the F1 generation.

How the heterozygote is formed in the F1 generation
Image: Wikipedia

The crossing of WwAa X WwAa produces 4 different zygotic combinations namely:

  • WA
  • Wa
  • wA
  • wa

The following Punnet square shows the various genotypes produced in the F2 generation

  F2  WA  Wa  wA  wa
  WA  WWAA  WWAa  WwAA  WwAa
  Wa  WWAa  WWaa  WwAa  Wwaa
  wA  WwAA  WwAa  wwAA  wwAa
  wa  WWAa  Wwaa  wwAa  wwaa
Punnet square showing the genotypes of the plants obtained in the F2 generation in a dihybrid cross

The dihybrid cross has a complex genotypic ratio consisting of 9 different genotypes

  • WWAA: 1 (Violet and axial- Homogenous)
  • WWAa: 2 (Violet and axial- Hybrid 1)
  • WWaa: 1  (Violet and terminal- Hybrid 2)
  • WwAa: 4  (Violet and axial- Hybrid 3)
  • Wwaa: 2 (Violet and terminal- Hybrid 4 )
  • WwAA: 2 (Violet and axial- Hybrid 5)
  • wwAA: 1 (White and axial- Hybrid 6)
  • wwAa: 2  (White and axial- Hybrid 7)
  • wwaa: 1  (While and terminal- Homogenous)

However, the phenotypic ratio of the F2 generation of the dihybrid cross is much more simple.

  • Violet and axial flowers are 9
  • Violet and terminal flowers are 3
  • White and axial flowers are 3
  • White and terminal flower is 1

Hence the phenotypic ratio of the F2 generation of the dihybrid cross is 9:3:3:1 irrespective of their genetic constitution.

Image showing how segregation and independent assortment produce the different genotypes in a dihybrid cross
Image: Wikipedia

How to find phenotypic ratio of a trihybrid cross?

Just like a dihybrid cross as the name suggests a trihybrid cross shows how three unrelated genes that are found in the same locus are inherited from one generation to another.

Image showing all the features of a pea plant that can be used in the Mendellian experiments
Image: Wikipedia

It is a cross between two individuals of the same species to examine the inheritance of three sets of components or alleles from three separate genes. Each parent can create eight various types of gametes with three unrelated genes, resulting in 64 genotypic combinations.

Let us take 3 different features as the markers for this cross including – Plant height, Seed shape and seed colour.

  Features  Dominant gene    Recessive gene
  Plant height  Tall (T)  Dwarf (t)
  Seed shape  Round (R)  Wrinkled (r)
  Seed colour  Yellow (Y)  Green (y)
Table showing the alleles that determine the inheritance of genes in a trihybrid cross

Hence the dominant homogenous parent is represented as “TTRRYY” while the homozygous recessive parent is represented as “ttrryy”. When we perform the cross between these 2 parents we get a single genotype and phenotype, which is a hybrid represented as “TtRrYy”.

In the second or F2 generation, we cross two hybrid parents having the same genotype TtRrYy and we obtain a total of 8 different zygotic combinations:

  • TRY
  • TRy
  • TrY
  • Try
  • tRY
  • tRy
  • trY
  • try

On creating a Punnet square for the F2 generation we get:

F2 genTRYTRyTrYTrytRYtRytrYtry
Punnet square showing the genotype obtained in the second generation of a trihybrid cross

Though there are 27 different genotypes produced in the F2 generation of a trihybrid cross the phenotypes are only 8 including:

  1. Tall plant, Round and Yellow seeds– 27
  2. Tall plant Round and green seeds– 9
  3. Tall plant, wrinkled and yellow seeds– 9
  4. Dwarf plant, round and yellow seeds– 9
  5. Tall plant, wrinkled and green seeds– 3
  6. Dwarf plant, round and green seeds -3
  7. Dwarf plant, wrinkled and yellow seeds – 3
  8. Dwarf plant, wrinkled and green seeds -1

The phenotypic ratio of the F2 generation of a trihybrid cross is 27:9:9:9:3:3:3:1.

Hence we can see in all the cases although the phenotype is dependent on the genotype, the phenotypic ratio and genotypic ratio are not the same in any of the experiments performed by Mendel.

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