Lesson 4 of 10 / Inheritance
Explain a 1:1 cross
How can a recessive offspring reveal a hidden allele in a parent?
In this lesson: Use a heterozygote-by-recessive cross to predict outcomes and interpret evidence about genotype.
About 5 min
The key ideaRr x rr gives an expected 1 Rr : 1 rr ratio, so the dominant and recessive phenotypes each have probability 1/2.
| Gametes | r |
|---|---|
| R | |
| r |
Selected: Rr -> Round. Each displayed combination has probability 1/2. The heterozygote supplies R or r; rr supplies r only. Rr and rr are expected equally: a 1:1 phenotype ratio.
Expected phenotypes: Round: 1/2; Wrinkled: 1/2
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
In Rr x rr, the heterozygous parent produces R and r gametes with equal probability. The homozygous recessive parent supplies only r. Fertilisation can therefore produce Rr or rr, each with probability 1/2.
Because Rr is round and rr is wrinkled under complete dominance, the expected phenotype ratio is 1 round : 1 wrinkled. A diagram must show how the gametes produce this result; a memorised ratio alone does not identify the correct parental genotypes.
A dominant-phenotype individual may be RR or Rr. Crossing it with rr is a test cross. If a recessive rr offspring is produced, the dominant-phenotype parent must have supplied r, so it is heterozygous under the stated model.
If all observed offspring show the dominant phenotype, that supports RR but does not prove it from a small sample: a heterozygote could produce only dominant offspring by chance. Use the strength and limits of the evidence, assuming the stated single-gene model and accurate observations.
Step by step
- 1
List the gametes
Rr supplies R or r; rr supplies r only.
- 2
Build the offspring genotypes
Combine one allele from each parent, giving Rr or rr.
- 3
Interpret evidence cautiously
A recessive offspring reveals a recessive allele in both parents; absence in a small sample is weaker evidence.
Worked example
A recessive seed appears
A round-seeded plant is crossed with rr and produces a wrinkled seed. What does this reveal about the round parent?
One way to explain it
The wrinkled seed is rr, so each parent supplied r. Because the round parent also expresses the dominant phenotype, its genotype must be Rr under the stated complete-dominance model.
Why this answer works
- Work backwards from the offspring genotype.
- The rr parent supplies one r; the other must come from the round parent.
- Use the round phenotype to establish the presence of R as well.
Is this true? "One dominant offspring proves that a dominant parent is RR."
Both RR and Rr parents can produce a dominant offspring in a test cross. More offspring and their full pattern provide stronger evidence.