Based on this information, what is Kevin's phenotype for the Marfan syndrome trait?

 - having Marfan syndrome
 - not having Marfan syndrome
Kevin's genotype for the Marfan syndrome gene is mm. Kevin's genotype of mm has only m alleles. The m allele is for not having Marfan syndrome. So, Kevin's phenotype for the Marfan syndrome trait must be not having Marfan syndrome.
To check this answer, consider whether Kevin's alleles are dominant or recessive. The allele for not having Marfan syndrome (m) is recessive to the allele for having Marfan syndrome (M). This means M is a dominant allele, and m is a recessive allele.
Kevin's genotype of mm has only recessive alleles. An organism with only recessive alleles for a gene will have the recessive allele's version of the trait. So, Kevin's phenotype for the Marfan syndrome trait must be not having Marfan syndrome.