Answer: X-linked recessive inheritance. Because it is a mutation in a gene on the X chromosome which causes the phenotype to be expressed in males who only have one X chromosome.
Explanation:
X-linked recessive inheritance refers to a mutation where both alleles in sex chromosomes are recessive. Males only have one X chromosome, then there is no need for two recessive alleles (one on each chromosome) as there is only one chromosome. Thereby, if a baby boy inherits a recessive allele from his mother, if this recessive allele is found in the X chromosome then the boy will develop the trait. The other sex chromosome from the father is the Y chromosome which only has genes for sexual development. It is not a homologous of the X chromosome. So under this circunstances, there is a 100% of chances of expressing the phenotype.
If it were a recessive allele on an autosomal chromosome (which are all the chromosomes that are not the sexual ones) then we would have to consider that allele inherited by the father, that can be recessive or dominant. If it is also recessive, the phenotype is expressed. If it is dominant, it is not expressed because that allele masks the effects of the recessive allele. So there's a 50% chances of expressing the phenotype.
