Genotypic Ratio - Definition, Calculation, Monohybrid Dihybrid Trihybrid Cross, Significance and Important Terms

What is Genotypic Ratio?

Genotypic ratio is depiction of ratio of resulting patterns and frequencies of inherited genes after crossing in offspring - ratio of composition of genes in progenies.

• Genotype is genetic constitution or expression of organism - type or combination of different genes resulting in external appearance.
• Genes can be expressed dominant or remain suppressed without expression recessive.
• Expressed genes result in phenotypic characteristics.
• Particular phenotype can have different genotypes - e.g., Tt and TT are different genotypes but express same phenotype of tallness.

Calculation of Genotypic Ratio

Calculation is different for monohybrid, dihybrid, trihybrid crosses. Basic steps:

• Choosing characters to be crossed or under study.
• By applying Mendel's Laws of Inheritance, cross them by forming cross diagram or Punnett-square chart.
• Calculate number of specific genotypes (not phenotype) - tallness can be denoted by TT and Tt - one is pure and homozygous whereas other is hybrid and heterozygous.
• List genotypic outcomes distinctly and determine ratio by adjusting characters along with specific numbers as counted in Punnett chart - genotypes can be looked at diagonally which makes counting easier.

Genotypic Ratio in Monohybrid Cross

Single trait taken and studied - example height: homozygous tall father TT crossed with short mother tt where tallness is dominant.

Genotypes in second filial generation are:

• Pure Tall with homozygous allele TT - 1
• Hybrid Tall with heterozygous allele Tt - 2
• Pure Dwarf / Short with homozygous allele tt - 1

Genotypic ratio will be 1:2:1 - indicates if four offspring born, higher probability one pure tall, two phenotypically tall but hybrid.

Genotypic Ratio in Dihybrid Cross

Two characters taken - example height and color: tall white male TTWW crossed with short black female ttww where tallness and white are dominant.

Genotypes in second filial generation are tall and white:

• One pure tall and pure white - Homozygous alleles TTWW
• Two pure tall and hybrid white - TTWw
• One pure tall and pure black - TTww
• Two hybrid tall and pure white - TtWW
• Four hybrid tall and hybrid white - TtWw
• Two hybrid tall and pure black - Ttww
• One pure short and pure white - ttWW
• Two pure short and hybrid white - ttWw
• One pure short and pure black - ttww

Genotypic ratio will be 1:2:1:2:4:2:1:2:1 - indicates if 16 offspring born, higher probability only one pure tall white and one pure short black, all others hybrid.

Genotypic Ratio in Trihybrid Cross

Three characters taken - example height, eye, hair color: tall brown-eyed black hair TTBBWW crossed with short blue-eyed white hair ttbbww where T, B, W dominant over t, b, w.

Genotypes in second filial generation are total 27 in number:

• One pure tall, brown-eyed, black-haired - TTBBWW
• Two pure tall brown-eyed hybrid black-hair - TTBBWw
• One pure tall brown-eyed pure white-haired - TTBBww
• Two pure tall hybrid brown-eyed pure black-haired - TTBbWW
• Four pure tall hybrid brown-eyed & black-haired - TTBbWw
• Two pure tall hybrid brown-eyed pure white-haired - TTBbww
• One pure tall blue-eyed black-haired - TTbbWW
• Two pure tall blue-eyed hybrid black-haired - TTbbWw
• One pure tall blue-eyed white-haired - TTbbww
• Two hybrid tall pure brown-eyed black-haired - TtBBWW
• Four hybrid tall pure brown-eyed hybrid black-haired - TtBBWw
• Two hybrid tall pure brown-eyed white-haired - TtBBww
• Four hybrid tall brown-eyed black-haired - TtBbWW
• Eight hybrid tall brown-eyed black-haired - TtBbWw
• Four hybrid tall brown-eyed pure white-haired - TtBbww
• Two hybrid tall pure blue-eyed black-haired - TtbbWW
• Four hybrid tall pure blue-eyed hybrid black-haired - TtbbWw
• Two hybrid tall pure blue-eyed white-haired - Ttbbww
• One pure short brown-eyed black-haired - ttBBWW
• Two pure short brown-eyed hybrid black-haired - ttBBWw
• One pure short brown-eyed white-haired - ttBBww
• Two pure short hybrid brown-eyed pure black-haired - ttBbWW
• Four pure short hybrid brown-eyed black-haired - ttBbWw
• Two pure short hybrid brown-eyed pure white-haired - ttBbww
• One pure short blue-eyed black-haired - ttbbWW
• Two pure short blue-eyed hybrid black-haired - ttbbWw
• One pure short blue-eyed white-haired - ttbbww

Genotypic ratio will be 1:2:1:2:4:2:1:2:1:2:4:2:4:8:4:2:4:2:1:2:1:2:4:2:1:2:1 based on above data.

Genotypic Ratio Significances

• Genetic combinations necessary to identify alterations in genes in each generation.
• Calculation helps predict likelihood or chances of presence of particular genes in future generations.
• Aid in production of genetically engineered products as desired.

Important Terms Related to Genotypic Ratio

• Pure Line: Organisms arisen through self-fertilization repeatedly, remain homozygous for specific trait - transferred to offspring when mated to true breeding organism.
• Homozygous: Condition where individual carries two identical alleles for specific trait - e.g., TT is homozygous or pure allele indicating tallness.
• Heterozygous: When alleles for particular trait are different - e.g., Tt is heterozygous as both alleles different.
• Filial generation: Offspring obtained after cross between parents are called filial generation or first filial generation (F1 generation). When progenies self-fertilize and result in another generation then they are second filial generation (F2 generation).
• Punnett square: Illustration of all possible combinations of phenotypes in offspring after crossing specific traits - square-shaped table invented by R.C. Punnett in 1906 - male gametes arranged in top horizontal boxes, female in left vertical boxes.

Note: All genotypic ratios based on Mendel's laws where mutation not considered - characters in parents based on homozygous alleles - dominant allele written in capital and at first, recessive in small - recessive expressed only in homozygous condition - formula for number of genotypes is 3^n where n is number of genes - e.g., trihybrid cross no. of genes = 3, hence genotypes = 3^3 = 27.

References

• Keshari A.K. and Ghimire K.R. (2010) Mendel's Laws of Inheritance. Vidyarthi Pustak Bhandar. 4th ed. pg. 618-640.
• Genotypic Ratio. Accessed from: https://biologydictionary.net/genotypic-ratio/ Accessed on: 11/28/2022
• Ahluwalia, K.B. (2009) Genetics. 2nd ed. New Delhi. New Age International.
• Klug W.S. (2015). Concepts of Genetics. 11th ed. Boston. Lachance, Joseph. "A fundamental relationship between genotype frequencies and fitnesses." Genetics vol. 180,2 (2008): 1087-93.