How can mendel's principles be used to study human traits?

mendel's principles apply to peas as they apply to human chromosomes.
His first law was the law of segregation and it states that the two alleles of and individual are segregated (separate) and passed on to the next generation singly.
this is exactly what happens to chromosomes at the end of meiosis 2.Gametes are haploid (we have 23 Chromosomes from each parent).
the second law is the principle of independent assortment which is really a extension of the first it states that when the alleles separate and gametes form, their separation is independent os the separation of alleles at other loci. (one allele passes to each gamete).
  A person that is homozygous recessive not only will be a carrier for the genetic disease but it will most probably express the disease.
even though he is recessive for the disease if he is homozygous he will not have any other gene to mask the recessive so he will have a 75% chance of phenotypically expressing the disease and 100% he will carry it.
You can makea punnet square crossing a homozygous recessive and a heterozygous person for the same disease and it will become clear.