Phenotypic ratio
n., plural: phenotypic ratios
[fiːnə(ʊ)ˈtɪpɪk ˈreɪʃɪˌəʊ]
Definition: ratio based on the phenotypes of offspring and determined using tools such as Punnett Squares
Table of Contents
Phenotypic Ratio Definition
How would one define phenotypic ratio? The correlation between the amount of offspring that will obtain certain traits or a combination of traits is referred to as the phenotypic ratio. This ratio is usually acquired by performing a test cross and then using the information from that cross to note how often a trait or trait combination will be exhibited based on the offsprings’ genotype.
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See examples of test cross and phenotypes here: What is a Phenotypic Ratio?
A phenotypic ratio is a quantitative relation between phenotypes showing the number of times the frequency of one phenotype correlates with another. When a researcher would like to obtain the gene expression for generations of an organism, they use the phenotypic ratio obtained from a test cross.
A test cross is a method used in genetics to explore and obtain the possible phenotypes and genotypes of organisms’ offspring. The genotype of an organism is its genetic make-up; it shows the alleles and genes that the specific organism carries. When the genes and alleles are expressed in observable traits, it is considered the phenotype. Phenotypes can be eye color, height, or even hair texture. Genotypes can be used to find the phenotypes of an organisms’ offspring through a test cross and in turn, acquire the phenotypic ratio.
For instance, if a red bug and a blue bug mate, their offspring could be red, blue, or purple (a mixture of both colors). We would need to find the phenotypic ratio in order to determine the numbers of times a particular phenotype is observed as compared to another phenotype. In simpler terms, phenotypic ratios can help us determine the probability of a bug being blue, red, or purple.
Phenotypic ratio is the probability of an observable trait appearing for cross breedings. Phenotypic ratios will most easily be determined using Punnett Squares or a phenotypic ratio calculator. Compare: genotypic ratio
Important Genetic Terms
Before knowing how to find a phenotypic ratio, one should know certain genetic terms:
- Gene: This is what is inherited from a parent and passed on to its offspring.
- Allele: The variety of the gene that is received from one of two parents.
- Chromosome: The thread-like structure made up of nucleic acids and proteins that carry the gene.
- Locus: The specific location a gene has on a chromosome.
- Heterozygous: an offspring that obtains two different alleles of one gene in particular.
- Homozygous: An offspring that obtains the same alleles of a particular gene from both parents.
- Dominant Allele: the gene that will always be expressed as the phenotype even when it comes into contact with the recessive.
- Recessive Allele: the gene that will only be expressed as the phenotype when it comes into contact with another recessive allele.
- Monohybrid: This occurs when two parents are crossed and produce offspring with only one phenotype.
- Dihybrid: When two parents are crossed and produce offspring with combinations of the phenotypes of the parents.
- Trihybrid: When two parents are crossed and produce offspring that express a greater range of phenotypes than a dihybrid.
- Punnett Square: A square diagram is used as a tool to determine the genotype of offspring when specific parents are crossed.
How to Find Phenotypic Ratio
How to calculate a phenotypic ratio. To find a phenotypic ratio, we look at the alleles of the parent organisms and estimate how often those genes will be exhibited by the offspring. Most times, we know what the alleles will express and how they will look. Phenotypic ratios will most easily be determined using Punnett Squares or a phenotypic ratio calculator.
What is the phenotypic ratio formula? To use the phenotypic ratio formula, one must first use a frequency chart – this can be made if it does not exist prior to the information given. Identify each trait that is desired and organize them in columns. Then, tally the number of individuals with specific traits ensuring that an organism is accounted for only once. The frequencies will be ranked from smallest to largest. Each frequency will then be divided by the smallest possible frequency, and the answer will be noted to a different column in the table. These answers will be rounded off and used as the phenotypic ratio. For instance, in Table 1 below, the final phenotypic ratio would be 9:3:1 where 9 represents black hair, 3 represents brown hair and 1 reflects that of red hair.
Phenotypic Ratio Calculations For Cross Types
One can use either a phenotypic ratio calculator that is developed for specific crosses or using a Punnett square. Many times, calculations can be difficult since phenotypes are seen when numerous alleles are combined. However, the following examples will be done using a single allele which will produce only a single trait.
When you use these calculation methods, we can obtain results for phenotypes that will appear in the first filial generation (F1) of a crossing and for generations beyond. We can even determine different effects that might occur in those subsequent generations. Early horse and dog breeders learned how to produce animals with different traits even without knowing the details we know today about genetics. This type of selective breeding has brought the vast array of breeds of animals that we have today in our world.
Some phenotypic ratios can be simple.
What is a 1:1 phenotypic ratio? A 1:1 phenotypic ratio occurs when there are only two phenotype possibilities as outcomes when organisms are crossed and they both have a 50/50 chance of appearing. What does a 3:1 phenotypic ratio look like? This will occur when two heterozygous parents each give one allele to their offspring, creating two possible phenotypes even though there may be multiple genotypes. It is important to note that genotypic and phenotypic ratios will not always be the same. Figures 1 and 2 Showing 1:1 and 3:1 phenotypic ratio examples on Punnett Squares, respectively.
The next section will go through different phenotypic ratio examples, which are more complex. This will include monohybrid, dihybrid, and trihybrid crosses.
Phenotypic Ratio of a Monohybrid Cross
A monohybrid cross occurs between two parents who are both homozygous and so will produce only one phenotype in their offspring. It can also occur when the genotypes of both parents are completely dominant or completely recessive, which, for certain genetic traits, will produce the opposite phenotype. This can easily be determined by using a Punnett Square.
In Figure 3 below, we see a monohybrid cross. In this instance, AA, the male parent, possesses the phenotype of a tall tree, and aa, the female parent, possesses the phenotype of a short tree. A is a dominant trait, meaning that when it is present, the organism will always display its phenotype whether a recessive gene is present or not. The gene a is recessive, and will only be seen when it is paired with another a allele.
When the parents breed, they will each give their offspring one of their alleles to form its chromosome. Since the offspring must obtain one allele from each parent and because the parents are both homozygous, every offspring will be heterozygous. This makes every offspring they produce become a tall tree since their genotype will be Aa. So, because all 4 offspring are the same phenotype, the phenotypic ratio does not need to be measured. This is because despite there being two possible outcomes (tall or short tree), only one is an observable trait, so calculating the phenotypic ratio would be redundant. If the phenotypic ratio must be shown, it would be written as 4:0.
Phenotypic Ratio of a Dihybrid Cross
Dihybrid crosses come into play when there are two phenotypes involved. There is a reason though that breeders usually do not focus on only using one phenotype. If they do, they will never get to explore other possibilities and develop much more unique and promising features. Why breed larger pigs for more meat if it just inherits brain deficiencies from both parents. Hence, geneticists continue to look for and promote useful breeds and then avert breeding the less favorable ones. They can use a dihybrid cross calculator to obtain a phenotypic ratio.
In figure 4 above, we see the dihybrid cross between two yellow peas in an F1 Generation. The previous or parent generation of this cross consisted of two homozygous parents, one dominant (RR YY), one recessive (rr yy). The dominant RR exhibited round peas and YY, yellow color. The recessive rr exhibited wrinkled peas and yy, green color.
In the parent crossing, a yellow, round pea (RRYY) is crossed with a green, wrinkled pea (rryy). This then results in all their offspring being round and yellow, a monohybrid crossing. However, they carry the genes that will give yellow, green, round, or wrinkle alleles (RrYy) and so be heterozygous.
When two of these RrYy offspring are crossed, they produce different kinds of phenotypes. They carry alleles for both round (R) and wrinkled (r) and alleles for yellow (Y) and green (y).
Using a Punnett square to determine the phenotypes of the offspring is simple and gives a solid visual. Finding the phenotypic ratio is easily done using the dihybrid Punnett square calculator. Figure 5 below shows how easily the frequency of the genotypes can be tallied and a 9:3:3:1 ratio is obtained for this cross. This can be used for all types of phenotypes.
Phenotypic Ratio of a Trihybrid Cross
If another allele is added then genetic expression and possible phenotypic results expand even more. A trihybrid cross calculator would be used to calculate the phenotypic ratio for breeds such as this one. Trihybrid cross ratios can be very long because of the numerous possible outcomes that can be obtained from them.
Take for instance the following example:
Humans are known for having multiple types of hair. In a particular experiment, geneticists want to see what will happen if they cross humans based on hair length, color, and texture. The dominant A allele will give long hair and the recessive a will give short hair. For Black hair, the gene is represented as a dominant B, and brown hair is a recessive b. Finally, straight hair will be represented by a D and is the dominant allele whereas curly hair is the recessive allele represented by a d.
The scientists begin with a monohybrid cross of the hair length phenotype. They cross the one gene using two heterozygous parents. This can be seen in Figure 6 below. As seen previously in this article, this leaves us with a 3:1 phenotypic ratio, producing offspring with both long and short hair. Even though some of the offspring’s observable trait is long hair, they carry the recessive gene for short hair. The probability of the offspring having long hair is much higher than them having short hair.
A second cross is done, now including the gene for hair color. This dihybrid cross will bring out more than two phenotypic results since the two genes now give way to multiple phenotypic results. This brings the phenotypic ratio to 9:3:3:1 with the possibilities being long, black hair, long, brown hair, short, black hair, and short, brown hair respectively. We see as more genes are added to the breeding, we obtain greater and more complex phenotypes.
Finally, the third gene is added which contributes to the texture of the hair. This trihybrid cross-ratio can be obtained using a Punnett square calculator, just like with the monohybrid and dihybrid crosses. In this phenotypic ratio, there are a total of 8 possible visible traits. These are all combined in unique ways and produce special offspring.
The most popular phenotype will be a combination of all the dominant alleles – a black, long, straight-haired human offspring. This is because when the alleles are combined, the dominant allele will always take precedence once it is present. The other phenotypes seen will be:
- Long, straight and brown hair
- Long, curly and black hair
- long, curly and brown hair
- Short, straight and black hair
- Short, straight and brown hair
- Short, curly and black hair
- Short, curly and brown hair
As expected, the least common phenotype is the only one the includes all three recessive genes – aabbdd. This has the probability of occurring only once out of sixty-four (64) possible crossings.
These diagrams are shown as an example of phenotypic ratios involving one, two, and three genes, respectively. In reality, though, the inheritance of human hair traits is rather more complex as the phenotype is determined by the interaction of many genes (alleles) at many loci.
Watch the video below to understand more about phenotypic and genotypic ratios.
Try to answer the quiz below to check what you have learned so far about phenotypic ratios.
Further Reading
- Dominance – Dominant and Recessive Alleles (Biology Tutorial)
- Non-Mendelian Inheritance (Biology Tutorial)
References
- Nature Education. 2014. Test Cross. Genetics. https://www.nature.com/scitable/definition/test-cross-169/
- National Human Genome Research Institution. 2021. Phenotypes. About Genomics. https://www.genome.gov/genetics-glossary/Phenotype
- OpenStaxCollege. 2021. Laws of Inheritance. Mendel’s Experiment and Heredity. http://pressbooks-dev.oer.hawaii.edu/biology/chapter/laws-of-inheritance/#:~:text=For%20a%20trihybrid%20cross%2C%20the,%3A3%3A3%3A1.
- Slizewska, G. 2019. Dihybrid Crosses. Genetic Inheritance and Expression. https://www.expii.com/t/dihybrid-crosses-definition-examples-11020
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