To determine whether the curl allele is dominant or recessive, you would design and analyze specific matings, then look at the phenotypes of the offspring across generations.

Core idea in simple terms

  • If cats need only one copy of the curl allele to have curled ears, the curl allele is dominant.
  • If cats need two copies of the curl allele to show curled ears, the curl allele is recessive.

Everything you do experimentally is aimed at figuring out which of those two patterns fits the data.

Step‑by‑step experimental strategy

Imagine you have the very first curl‑eared cat (the original mutant). You want to know how that curl allele behaves.

1. Cross curl cat × normal cat

  1. Mate the original curl‑eared cat with a cat that has normal (non‑curl) ears and that comes from a long line of normal‑eared cats.
    • Those normal cats are very likely to be homozygous for the normal allele.
  1. Look at the F₁ (first‑generation) kittens:
    • If all or almost all F₁ kittens have curled ears, that strongly suggests the curl allele is dominant over the normal allele.
 * If **all** F₁ kittens have normal ears, that suggests the curl allele is **recessive** (the original curl cat is likely heterozygous, and normal allele masks curl in the heterozygote).

In many textbook versions of this curl‑ear story, the data fit a dominant curl allele: curl × normal gives mostly curl offspring.

2. Intercross curl offspring (F₁ × F₁)

Once you have curl‑eared F₁ cats, mate two of them:

  • Expected patterns if curl is dominant :
    • F₁ parents are typically Cc (C = curl, c = normal).
    • Punnett square: Cc × Cc → 1 CC : 2 Cc : 1 cc.
* Phenotypes:
  * 3/4 with curl ears (CC and Cc)
  * 1/4 with normal ears (cc)
* So you’d expect **about 3 curl : 1 normal** among the kittens if the curl allele is dominant and simple Mendelian.
  • Expected patterns if curl is recessive :
    • F₁ from curl × normal would all be carriers (Nn), but look normal.
* Nn × Nn → 1 NN : 2 Nn : 1 nn
* Phenotypes:
  * 3/4 normal
  * 1/4 curl
* You’d see **about 1 curl : 3 normal** among kittens if curl is recessive.

Compare your actual litter ratios to these expected ratios; that ratio is a powerful clue.

3. Test crosses to confirm

To really nail down whether curl is dominant and to find genotypes, do test crosses :

  1. Take a curl‑eared cat and mate it with a known homozygous normal (normal‑eared animal from a true‑breeding normal line).
  1. Interpret the offspring:
    • If the curl parent is homozygous (CC) and curl is dominant:
      • All offspring will get C from the curl parent and c from the normal parent → all Cc → all curl.
 * If the curl parent is **heterozygous** (Cc):
   * Offspring genotypes: 1/2 Cc (curl), 1/2 cc (normal) → about **half curl, half normal**.

So:

  • “All curl” offspring ⇒ suggests the curl parent is CC under a dominant model.
  • “Half curl, half normal” ⇒ curl parent is Cc, again supporting a dominant curl allele.

If curl were recessive (cc), then:

  • cc (curl) × CC (normal) gives all Cc, all phenotypically normal , so you’d see no curl at all in that test cross.

How you’d phrase the answer for an exam

In genetics‑problem style, an answer would look like this:

To determine whether the curl allele is dominant or recessive, first mate the original curl‑eared cat with a true‑breeding normal‑eared cat and observe the F₁ phenotypes.

  • If all F₁ are curl‑eared, curl is likely dominant.
  • If all F₁ are normal‑eared, curl is likely recessive.
    Next, intercross F₁ cats and examine the F₂ ratio. A 3:1 curl:normal ratio supports a dominant curl allele, whereas a 1:3 curl:normal ratio supports a recessive curl allele. Finally, test‑cross curl‑eared cats with true‑breeding normal‑eared cats: the presence of both curl and normal offspring indicates a heterozygous dominant curl allele, while only normal offspring would be expected if curl were recessive.

That logic directly answers “how would you determine whether the curl allele is dominant or recessive?” using classic Mendelian crosses and phenotype ratios.

TL;DR:
Use controlled matings:

  • Curl × true‑breeding normal → see whether F₁ shows curl or normal.
  • F₁ × F₁ → look for ~3:1 vs ~1:3 ratio.
  • Test cross curl cats with true‑breeding normal to confirm genotypes.

Information gathered from public forums or data available on the internet and portrayed here.