<P> In independent assortment, the chromosomes that result are randomly sorted from all possible maternal and paternal chromosomes . Because zygotes end up with a random mix instead of a pre-defined "set" from either parent, chromosomes are therefore considered assorted independently . As such, the zygote can end up with any combination of paternal or maternal chromosomes . Any of the possible variants of a zygote formed from maternal and paternal chromosomes will occur with equal frequency . For human gametes, with 23 pairs of chromosomes, the number of possibilities is 2 or 8,388,608 possible combinations . The zygote will normally end up with 23 chromosomes pairs, but the origin of any particular chromosome will be randomly selected from paternal or maternal chromosomes . This contributes to the genetic variability of progeny . </P> <P> Mendel's Law of Dominance states that recessive alleles will always be masked by dominant alleles . Therefore, a cross between a homozygous dominant and a homozygous recessive will always express the dominant phenotype, while still having a heterozygous genotype . The Law of Dominance can be explained easily with the help of a mono hybrid cross experiment: - In a cross between two organisms pure for any pair (or pairs) of contrasting traits (characters), the character that appears in the F1 generation is called "dominant" and the one which is suppressed (not expressed) is called "recessive ." Each character is controlled by a pair of dissimilar factors . Only one of the characters expresses . The one which expresses in the F1 generation is called Dominant . However, the law of dominance is not universally applicable . </P> <P> A Mendelian trait is one that is controlled by a single locus in an inheritance pattern . In such cases, a mutation in a single gene can cause a disease that is inherited according to Mendel's laws . Examples include sickle - cell anemia, Tay - Sachs disease, cystic fibrosis and xeroderma pigmentosa . A disease controlled by a single gene contrasts with a multi-factorial disease, like arthritis, which is affected by several loci (and the environment) as well as those diseases inherited in a non-Mendelian fashion . </P> <P> Mendel explained inheritance in terms of discrete factors--genes--that are passed along from generation to generation according to the rules of probability . Mendel's laws are valid for all sexually reproducing organisms, including garden peas and human beings . However, Mendel's laws stop short of explaining some patterns of genetic inheritance . For most sexually reproducing organisms, cases where Mendel's laws can strictly account for the patterns of inheritance are relatively rare . Often, the inheritance patterns are more complex . </P>

Most genetic disorders are covered by single genes inherited in mendelian fashion