Marfan syndrome is a genetic problem which affects the connective tissue in the body. The trait for this disease is autosomal dominant which means even one abnormal copy of this gene in the offspring or individual is sufficient to cause this syndrome.
Let S is the allele that is dominant for this syndrome and s is recessive. So if a cross between homozygous dominant(SS) and heterozygous individual (Ss) occurs than all the offspring would have this syndrome.
S s
S SS Ss
S SS Ss
Therefore all 4 offspring would have at least one dominant allele which is sufficient to cause this syndrome. So the phenotype ratio would be 4:0.
