Question

Cats may have straight or curly ears. A cat with curly ears mated with a straight-eared cat. All the kittens had curly ears. Are curly ears a dominant or
 recessive trait? Explain your answer.

Answer 1

You have to assume that this is complete dominance and one of these traits is dominant. To get the recessive characteristic, the alleles (versions of the gene) both have to be recessive (both lower case). But to have the dominant phenotype, either both alleles could be dominant (upper case) or you could have one of each (heterozygous, where you have one upper case and one lower case). 

So there are two possibilities for genotype of the cat with the dominant characteristic. Since you don't know (yet) which it is, just use the letters "E" and "e" and set up the two possible Punnett squares: 

......|...e...|...e...| 
..E..|..Ee..|..Ee..| 
..E..|..Ee..|..Ee..| 

......|...e...|...e...| 
..E..|..Ee..|..Ee..| 
..e...|..ee..|..ee..| 


In the first Punnett square, all the offspring have the same genotype, so they would also have the same phenotype. In the second square, you have offspring with two genotypes, one of which would produce the dominant phenotype (Ee) and one of which would produce the recessive phenotype (ee). Since your problem states that all the offspring have the same phenotype, the correct Punnett square is the first one where all the kittens are heterozygous (Ee). Since the dominant allele (E) masks the recessive allele (e), then curly ears has to be the dominant trait, since that's the phenotype of all the kittens.

Sorry if this is confusing... D: 

Answer 2

I believe that curly ears are dominant because all the kittens had curly ears.