Answer:
Non-mendelian inheritance can also manifest as pleiotropy, where one gene can affect multiple characteristics which can be seemingly unrelated. Pleiotropy can be observed in human genetic disorders such as a hereditary disorder called Marfan Sydrome. Symptoms include thin fingers and toes, heart problems, dislocation of the lense of the eye and an unusually tall height. This is caused by a mutation in a gene responsible for the production of elastic fibrils that provide flexibility and support to connective tissue and serve as storage houses for growth hormones. The amount of functional protein produced is reduced resulting in fewer fibrils due to this mutation. This leads to malfunctions in heart and eye development, as well as an overabundance of free growth hormones in the blood stream resulting in tall height.
Combinations of genes are often needed to promote the survival of organisms. If an allele that contributes to a gene is not expressed it can lead to harmful activity. This is known as lethality. A classic example of an allele that affects survival is the lethal yellow allele, a spontaneous mutation in mice that makes their coats yellow. Mice that are homozygous die early in development. Although this particular allele is dominant, lethal alleles can be dominant or recessive, and can be expressed in homozygous or heterozygous conditions.
