Dictionary > Genotypic ratio

Genotypic ratio

Genotypic ratio
n., plural: genotypic ratios
[ˈdʒɛnəʊˌˈtɪpɪk ˈɹeɪ.ʃoʊ]
Definition: Pattern of offspring distribution according to genotype

Genotypic Ratio Definition

To understand ‘Genotypic ratio’, let us first understand the terms: ‘Genotype‘ and ‘Phenotype‘. Genotype is the genetic constitution of an individual whereas phenotype is the physical appearance or expression of a specific trait. Essentially, both genotype and phenotype are attributed to the genetic combination present in an individual.


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So, what is a genotypic ratio? The genotypic ratio is the distribution pattern (expressed as a ratio) of the genes in the offspring obtained after a genetic cross.

Then, what is the phenotypic ratio? The phenotypic ratio is the distribution pattern (expressed as a ratio) of the physical characteristics in the offspring obtained after a genetic cross.

So, genotypic ratio and phenotypic ratio are the two types of genetic ratios used to express the genotype and the phenotype of offspring from a genetic cross.

The genotypic ratio and the phenotypic ratio may not necessarily be the same. Also, the genotypic ratio can not be predicted or distinguished based upon the phenotypic ratio. However, a genotypic ratio can indicate a phenotypic ratio.

The genotypic ratio is the outcome of Mendel’s law of segregation. According to Mendel’s law of segregation, two alleles segregate in the progeny, wherein half of the progeny inherits one of the alleles while the other half of the progeny inherits the second allele. This law explains the reason that the progeny is not the exact copy of their parents.

Biology definition:

The genotypic ratio is the ratio depicting the different genotypes of the offspring from a test cross. It represents the pattern of offspring distribution according to genotype, which is the genetic constitution determining the phenotype of an organism. It describes the number of times a genotype would appear in the offspring after a test cross.

For example, a test cross between two organisms with the same genotype, Rr, for a heterozygous dominant trait will result in offspring with genotypes: RR, Rr, and rr. In this example, the predicted genotypic ratio is 1:2:1.

Compare: phenotypic ratio


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Now let us understand the concept of the Genotypic Ratio with the help of an example.

Consider breeding between two homozygous plants, a RR (red flower) and rr (white flower).
[Capital letters signify the dominant trait, and small letters represent the recessive trait.]

  • RR – meaning a homozygous with both dominant alleles for a trait
  • rr – meaning- a homozygous with both recessive alleles for a trait

According to the law of segregation:

RR (Red)   X   rr (White) → Rr     (F1 generation)
  • Genotypic Ratio: All Rr
  • Phenotypic Ratio: All Rr

Suppose the Rr phenotype is pink, now, consider self-pollination has happened in plants with pink flowers (Rr), so what would be the progeny in such a case? Again, according to the law of segregation:

Rr (Pink)    X    Rr (Pink) → RR (Red) | Rr (Pink) | Rr (Pink) | rr (White)     (F2 generation)    

So, how to write a genotypic ratio for this case?

For writing the genotypic ratio, let us identify the number of red, white, and pink flowers in the progeny. As we can see from the above cross, there is one red flower, one white flower, and two pink flowers. So the predicted genotype of the progeny would be 1:2:1.

Accordingly,

  • Genotypic ratio- 1:2:1
  • Phenotypic ratio- 1:2:1

From the above example, we can define the genotypic ratio as the number of times a particular genotype appears after crossing over.

How to Find the Genotypic Ratio

In genetics, Punnett square is the most popular method of representing a breeding crossover to predict the genotype of the progeny. It is a square diagram named after its creator Reginald C. Punnett. Punnett square summarizes the maternal and paternal alleles along with all the probable genotypes of the progeny in a tabular form. Punnett square is also used as a genotypic ratio calculator. Calculate the number of squares having a specific allele combination and express it as the genotypic ratio.

1. Monohybrid cross

Monohybrid cross - Punnet Square example
Figure 1: Monohybrid cross: where R is a dominant trait and r is a recessive trait. Source: Dr. Amita Joshi of Biology Online.

To estimate the genotypic and phenotypic ratio, calculate the number of Punnett squares with each allele combination. So, in this example, one Punnett square for both RR and rr and two Punnett square boxes for Rr. Calculating Punnett square ratios as 1:2:1 will give the genotypic ratio. So, the monohybrid cross-ratios are as follows:

  • The genotypic ratio for monohybrid cross: 1:2:1 ratio
  • The phenotypic ratio would be 3:1 ratio.

This is based upon the rule of dominance, i.e., phenotypically, R being dominant trait, RR as well as Rr combinations would appear as the dominant trait. So phenotypic ratio would appear the same in 3 allele combinations resulting in a phenotypic ratio of 3:1.

See Figure 2 for a visual example. In this test cross, (Pp) male and (Pp) female parents would reproduce offspring that will bear either purple or white flowers. Of the four offspring, three of them will bear purple flowers and one, white flower.  Of the purple flowers, one of them is homozygous (PP) and two of them are heterozygous (Pp).

Thus, the genotypic ratio in this example is 1:2:1, which means:

  • 1 homozygous PP
  • 2 heterozygous Pp
  • 1 homozygous pp

This also indicates the phenotypic ratio is 3:1, which means:

  • 3 purple-flower-bearing offspring
  • 1 white-flower-bearing offspring
allele Punnett square - purple vs white flowers
Figure 2: Difference in a phenotypic and genotypic ratio based on the rule of dominance in a monohybrid cross. Image Source: Maria Victoria Gonzaga of Biology Online.

 

2. Dihybrid cross

Consider a case of breeding between two double heterozygous, having unlinked genes, i.e., dihybrid cross. Then, using this Punnett square:

Figure 2: Dihybrid cross: where B is for the dominant smooth seed trait, b is for the recessive wrinkled seed trait, A is for the dominant white seed trait, and a is for the recessive yellow seed trait. Source: Dr. Amita Joshi of Biology Online.

So, the dihybrid cross-ratios are:

  • Genotypic ratio of dihybrid cross- 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1
  • Phenotypic ratio would be 9:3:3:1

To calculate the genotypic ratios, one can color the Punnett squares having similar allele combinations and then calculate the number of Punnett squares with the same color to arrive at the genetic ratios

For a visual example, take a look at the figure below: a dihybrid cross of pea plants where two traits are considered:

  • Y, for yellow seeds (dominant seed color trait) vs. y for green seeds (recessive seed color trait)
  • R, for round seeds (dominant seed texture trait) vs. r for wrinkled seeds (recessive seed texture trait)
dihybrid cross example Punnett square
Figure 3: Using Punnett square for calculating the genetic ratios in a dihybrid cross. The filled-in squares of a Punnett square represent a specific allele combination that is used for calculating the genotypic ratio. Image Credit: OpenStax Biology

In this test cross, YyRr male and female parents (F1 generation) would reproduce offspring that will yield offspring showing this type of pattern:

The genotypic ratio in this example is  1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1, which means:

  • 1 YYRR
  • 2 YyRR
  • 1 yyRR
  • 2 YYRr
  • 4 YyRr
  • 2 yyRr
  • 1 YYrr
  • 2 Yyrr
  • 1 yyrr

This also indicates the phenotypic ratio is 9:3:3:1, which means:

  • 9 offspring producing yellow round seeds
  • 3 offspring producing green round seeds
  • 3 offspring producing yellow wrinkled seeds
  • 1  offspring producing green wrinkled seeds

For more complex cases like trihybrid cross, a large Punnett square is obtained, making it very complicated to calculate the genetic ratios. For such cases, the forked line method is used to find the genetic ratios. In this example below, the trihybrid cross genotypic ratio is 27:9:9:9:3:3:3:1.

forked line method
Figure 4: Forked-line method for a trihybrid cross. Image Credit: OpenStax Biology.

The video below explains how to do the forked line method.

 

Genotypic Ratio Example

Here are examples of genotypic ratios.

1. Dominant & Recessive

Case: Cross between Tt   X   Tt (where T: dominant tall; t: recessive short)

Dominant & Recessive Case: Cross between Tt X Tt
T t
T TT Tt
t Tt tt
Genotypic ratio: 1:2:1
Phenotypic ratio: 3:1

2. Incomplete dominance

Case: A cross between two medium heights (Tt X Tt), where the phenotypes, TT, for “tall”, Tt, for “medium”, and tt, for “short”.

Incomplete dominance Case: Cross between Tt X Tt
T t
T TT Tt
t Tt tt
Genotypic ratio: 1:2:1
Phenotypic ratio: 1:2:1

3. Co-dominance

Case: A cross between two tan skin colors (Aa  Aa), where phenotypes, AA, for “white”, Aa, for “tan”, aa, for “black”.

Co-dominance Case: Cross between two tan skin colors
A a
A AA Aa
a Aa aa
Genotypic ratio: 1:2:1
Phenotypic ratio: 1:2:1

 


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4. Sex chromosome-linked

Case: A cross between a male hemophilic carrier and a female hemophilic carrier (X / Y- Normal; X+- Hemophilia), where phenotypes…

  • X X+: Female Hemophilic Carrier
  • X+Y: Male Hemophilic Carrier
  • X+ X+: Hemophilic female
  • XY: Normal male
Sex chromosome-linked case:

A cross between a male hemophilic carrier and a female hemophilic carrier

X X+
A XX+ X+X+
Y XY X+Y
Genotypic ratio: 1:1:1:1
Phenotypic ratio: 1:1:1:1

 

5. Multiple alleles

Case: A cross between blood group AB X AO, where phenotypes…

  • AA: blood group A
  • AO: blood group A
  • AB: blood group AB
  • BO: blood group B
Multiple alleles case: A cross between blood group AB X AO
A O
A AA AO
B AB BO
Genotypic ratio: 1:1:1:1
Phenotypic ratio: 1:1:1:1

Further Reading:

References

  • Bakholdina V. (1983). Sootnoshenie chastot fenotipov grupp krovi ABO u nivkhov raznykh vozrastnykh grupp Ratio of the phenotype frequencies of ABO blood groups in Nivkhi in various age groups]. Nauchnye doklady vysshei shkoly. Biologicheskie nauki, (7), 51–53.
  • Baye, T. M., Abebe, T., & Wilke, R. A. (2011). Genotype-environment interactions and their translational implications. Personalized medicine, 8(1), 59–70. https://doi.org/10.2217/pme.10.75
  • Griffiths AJF, Gelbart WM, Miller JH, et al.(1999). Modern Genetic Analysis. New York: W. H. Freeman. Human Pedigree Analysis. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21257/
  • Griffiths AJF, Miller JH, Suzuki DT, et al. (2000). An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman. Using genetic ratios. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21812/

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