In order for a male to express a recessive sex-linked trait that is located on x chromosomes, he must have:

an additional recessive allele on his y chromosome

two recessive autosomal alleles

an inactive allele

one recessive allele on his x chromosome
!

the nitrogen cycle provides nitrogen to the ecosystem from the atmosphere, ground and oceans. nitrogen is an important component of complex molecules such as amino acids and nucleotides, which lead to the creation of proteins and dna, the building blocks of all life. plants absorb nitrogen in the form of ammonium, nitrate ion and, on rare occasions, as amino acids. animals receive nitrogen necessary for biological processes from feeding on living or dead organic matter.