Lesson 3 of 11 / Inheritance
From parents to F1 and F2
Where does the expected 3:1 phenotype ratio come from?
I can construct a complete-dominance cross through F1 and F2, distinguishing genotype and phenotype ratios.
I can predict 3:1 and 1:1 crosses with F1/F2 and zygosity terms.Syllabus
Syllabus K325, Topic 14(c). Predict 3:1 and 1:1 crosses with F1/F2 and zygosity terms.
I can solve monohybrid problems with genetic diagrams.Syllabus
Syllabus K325, Topic 14(e). Solve monohybrid problems with genetic diagrams.
About 6 min
Make a guess. You are not marked.
Two round-seeded plants, both Rr, are crossed. Can any offspring have wrinkled seeds?
Show the answer
Yes, about a quarter of them, rr.
Each parent can pass on r. An offspring getting r from both is rr and wrinkled, with probability 1/4.
The key idea
Crossing two heterozygotes gives an expected 1:2:1 genotype ratio and, under complete dominance, a 3:1 phenotype ratio.
Use R for the dominant round-seed allele and r for the recessive wrinkled-seed allele. The cross follows one gene: it is monohybrid.
| Gametes | R | r |
|---|---|---|
| R | ||
| r |
Selected: RR -> Round. Each displayed combination has probability 1/4. Each F1 parent supplies R or r. F2 genotypes are expected in a 1:2:1 ratio; grouping RR and Rr as round gives a 3:1 phenotype ratio.
Expected phenotypes: Round: 3/4; Wrinkled: 1/4
Rows: parent 1 gamete alleles. Columns: parent 2 gamete alleles. Boxes are equally likely combinations for the stated crosses, not a required sequence of offspring. Repeated genotypes can occur in different boxes. R = round; r = wrinkled, with complete dominance.
Explanation
Begin with homozygous parents RR and rr. The RR parent produces gametes carrying R, while the rr parent produces gametes carrying r. Gametes contain one allele of this gene, not both parental alleles.
The first offspring generation is the F1 generation. Every possible fertilisation in RR x rr gives Rr, so the F1 offspring are heterozygous and have the dominant round phenotype under this model.
Crossing two F1 heterozygotes, Rr x Rr, produces the F2 generation. Each parent can supply R or r with equal probability. A Punnett square combines each possible gamete from one parent with each from the other: RR, Rr, rR and rr. Rr and rR represent the same genotype.
The expected F2 genotype ratio is 1 RR : 2 Rr : 1 rr. Because RR and Rr are both round, the expected phenotype ratio is 3 round : 1 wrinkled. State which ratio is requested. These are probabilities, not a requirement that the next four seeds appear in this exact pattern.
Examiners mark a genetic diagram row by row, so set it out in full: parental phenotypes, parental genotypes, gametes, offspring genotypes, and offspring phenotypes with the ratio. Label each row and draw a circle around each gamete. Schemes deduct a mark when gametes are not circled, and they want the Punnett square spelled correctly if you name it.
Step by step
- 1
Key
R = round seed (dominant); r = wrinkled seed (recessive).
- 2
Parental phenotypes
Round x round (two F1 plants).
- 3
Parental genotypes
Rr x Rr
- 4
Gametes, each circled
(R) (r) from each parent.
- 5
Offspring genotypes
Use a Punnett square: RR, Rr, Rr, rr.
- 6
Offspring phenotypes and ratio
3 round : 1 wrinkled.
Worked example
Expected recessive offspring
For Rr x Rr, calculate the probability of a wrinkled offspring and the expected number among 80 offspring.
One way to explain it
Only rr is wrinkled, so the probability is 1/4. The expected number is 80 x 1/4 = 20, although the observed count may differ by chance.
Why this answer works
- Find the recessive genotype in one of four equally likely combinations.
- Multiply the probability by the number of offspring.
- Label the result expected rather than guaranteed.
Watch out for this
A student says: "The 3:1 ratio is the ratio of RR to rr genotypes." What is wrong with this?
Show the answer
The genotype ratio is 1 RR : 2 Rr : 1 rr. The 3:1 ratio groups both RR and Rr into the dominant phenotype.
Check your understanding
What is the expected phenotype ratio for Rr x Rr when R is completely dominant?
Show the answer
3 dominant : 1 recessive
RR, Rr and rR show the dominant phenotype; rr shows the recessive phenotype.
Quick recall
Where does the expected 3:1 phenotype ratio come from?