Ratios, Ratios & More Ratios

By A.K. Matai

Like the title says, ratios, ratios and more ratios. But before we jump into it, there are a few concepts I would like to clarify.

A Gene Can Have Multiple Alleles 

For example, a gene coding for body color can have an allele coding for red body color (CR) and another allele coding for yellow body color (CY). And these alleles can interact in three ways: complete dominance, co-dominance and incomplete dominance.

Complete Dominance 

One allele is completely dominant to the other when one allele overwhelms the other. For example, if Cis completely dominant to CR then an individual that has the genotype CY Cwill have yellow colored body.

Co-Dominance 

One allele can be co-dominant to another allele when both alleles are expressed at the same time. For example, if Ccan be co-dominant with CR then an individual that has the genotype CY Cwill have red and yellow striped body.

Incomplete Dominance 

One allele can be incompletely dominant to another allele resulting in a blended phenotype. For example, if Cis incompletely dominant to CR then an individual that has the genotype CY Cwill have an orange colored body.

3:1 Ratio

1 Autosomal Gene 2 Alleles

Let’s say that you have two pure-breeding strains of dragons. One is blue and the other red. The autosomal gene responsible is called C and the alleles are CB and CR. Below is a mating scheme which will be used to go through the following cases.

Ratios 1.png

Case 1 – CB is completely dominant to CR

  • Parental Generation
    • Blue dragon mated to a red dragon.
  • F1 Generation
    • Blue dragons only. The F1 generation is self crossed to produced the F2 generation.
  • F2 Generation
    • The ratio for the F2 generation is 3 Blue : 1 Red.

Case 2 – CR is completely dominant to CB

  • Parental Generation
    • Blue dragon mated to a red dragon.
  • F1 Generation
    • Red dragons only. The F1 generation is self crossed to produced the F2 generation.
  • F2 Generation
    • The ratio for the F2 generation is 3 Red : 1 Blue.

Case 3 – Cis co-dominant with CR

  • Parental Generation
    • Blue dragon mated to a red dragon.
  • F1 Generation
    • Blue-red striped dragons only. The F1 generation is self crossed to produced the F2 generation.
  • F2 Generation
    • The ratio for the F2 generation is 2 Blue-Red Stripped: 1 Blue : 1 Red.  

Case 4 – CB is incompletely dominant to CR

  • Parental Generation
    • Blue dragon mated to a red dragon.
  • F1 Generation
    • Purple dragons only. The F1 generation is self crossed to produced the F2 generation.
  • F2 Generation
    • The ratio for the F2 generation is 2 Purple: 1 Blue : 1 Red.  

1 X-linked Gene 2 Alleles

Once again, you have two pure-breeding strains of dragons . One is blue and the other red. The gene responsible is called C and the alleles are CB and CR. BUT this time, the gene is on the x-chromosome.

SCENARIO 1 – Blue Mated to Red

Ratios 2

Case 1 – CB is completely dominant to CR

  • Parental Generation
    • Blue ♀ dragon mated to a red ♂ dragon.
  • F1 Generation
    • Blue dragons only. The F1 generation is self crossed to produced the F2 generation.
  • F2 Generation
    • The ratio for the F2 generation is 2 Blue ♀ : 1 Blue ♂ : 1 Red ♂.

Case 2 – CR is completely dominant to CB

  • Parental Generation
    • Blue ♀ dragon mated to a red ♂ dragon.
  • F1 Generation
    • Red ♀ dragons and blue ♂ dragons. The F1 generation is self crossed to produced the F2 generation.
  • F2 Generation
    • The ratio for the F2 generation is 1 Blue ♀ : 1 Red ♀: 1 Blue ♂ : 1 Red ♂.

Case 3 – Cis co-dominant with CR

  • Parental Generation
    • Blue ♀ dragon mated to a red ♂ dragon.
  • F1 Generation
    • Blue-red striped ♀ dragons and blue ♂ dragons. The F1 generation is self crossed to produced the F2 generation.
  • F2 Generation
    • The ratio for the F2 generation is 1 Blue ♀ : 1 Blue-red striped ♀: 1 Blue ♂ : 1 Red ♂.

Case 4 – CB is incompletely dominant to CR

  • Parental Generation
    • Blue ♀ dragon mated to a red ♂ dragon.
  • F1 Generation
    • Purple ♀ dragons and blue ♂ dragons. The F1 generation is self crossed to produced the F2 generation.
  • F2 Generation
    • The ratio for the F2 generation is 1 Blue ♀ : 1 purple ♀: 1 Blue ♂ : 1 Red ♂.

SCENARIO 2 – Red ♀ Mated to Blue ♂

Ratios 3

Case 1 – CB is completely dominant to CR

  • Parental Generation
    • Red ♀ dragon mated to a blue ♂ dragon.
  • F1 Generation
    • Blue ♀ dragons and red ♂ dragons. The F1 generation is self crossed to produced the F2 generation.
  • F2 Generation
    • The ratio for the F2 generation is 1 Blue ♀ : 1 Red ♀: 1 Blue ♂ : 1 Red ♂.

Case 2 – CR is completely dominant to CB

  • Parental Generation
    • Red ♀ dragon mated to a blue ♂ dragon.
  •  F1 Generation
    • Red ♀ dragons and red ♂ dragons. The F1 generation is self crossed to produced the F2 generation.
  • F2 Generation
    • The ratio for the F2 generation is 2 Red ♀: 1 Red ♂ : 1 Blue ♂.

Case 3 – Cis co-dominant with CR

  • Parental Generation
    • Red ♀ dragon mated to a blue ♂ dragon.
  • .F1 Generation
    • Blue-red striped ♀ dragons and red ♂ dragons. The F1 generation is self crossed to produced the F2 generation.
  • F2 Generation
    • The ratio for the F2 generation is 1 Red ♀ : 1 Blue-red striped ♀: 1 Blue ♂ : 1 Red ♂.

Case 4 – CB is incompletely dominant to CR

  • Parental Generation
    • Red ♀ dragon mated to a blue ♂ dragon.
  • F1 Generation
    • Purple ♀ dragons and red ♂ dragons. The F1 generation is self crossed to produced the F2 generation.
  • F2 Generation
    • The ratio for the F2 generation is 1 Red ♀ : 1 purple ♀: 1 Blue ♂ : 1 Red ♂.

9:3:3:1 Ratio – 2 Genes 4 Alleles 

Like the 3:1 ratio, there are variations on the 9:3:3:1 ratio. However, I will only cover a generic 9:3:3:1 ratio in this post and cover the variations in the Gene Interactions Post. So, here is your cheat sheet to the 9:3:3:1 ratio.

Ratios 4.png

Testcross Ratios  

A testcross is used to determine whether an individual is homozygous or heterzygous at a loci. Here is a genotypic cheat sheet for all the possible testcrosses that you  could possibly encounter.

Ratios 5

Questions, Comments or Concerns?

Feel free to post a your questions, comments or concerns down below!

References

Locke, John; Deyholos, Michael; Harrington, Michael; Canham, Lindsay; Kang, Min, “Open Genetics Lectures (OGL) Fall 2016”

Unknown. White Purple Flower Flowers Petunia Garden. 2016. http://maxpixel.freegreatpicture.com/White-Purple-Flower-Flowers-Petunia-Garden-963076. Web. 19 Mar. 2017.

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