Pleiotropy is the phenomenon when one gene influences two or more phenotypic traits that may seem completely unrelated. One example of pleiotropy is the disease phenylketonuria, which is also known as PKU. This disease can cause brain damage, seizures, and intellectual disability. It is caused by a large number of mutations in a single gene. Pleiotropy is more complicated than Mendelian genetics as it involves one gene affecting more than two phenotypic traits, whereas in Mendelian genetics, one gene affects only one phenotypic trait.

Epigenetics literally means above genetics. Epigenetics is the study of chemical changes and heritable changes in gene activity, but these changes do not effect the DNA sequence. Instead, these modifications affect how cells “read” the genes. An example of epigenetic change is the process of cellular differentiation. During morphogenesis, less specialized cells become fully differentiated cells. So when a single fertilized zygote continues to divide into daughter cells, they begin to differentiate in types of cells, some being neurons cells, blood vessels, muscle cells, etc. The differences between the different cell types are activated by specific chemicals. Epigenetics is more complicated than Mendelian genetics as epigenetics does not actually affect the gene, it changes its surrounding environment. Mendel’s laws only focused on genes and their alleles. Mendel never considered or realised that there are other factors on how genes are perceived and read.

Penetrance is the percentage of individuals carrying a mutant gene that exhibits clinical symptoms of that disorder. So if a mutation in the gene responsible for a certain autosomal dominant disorder has 85% penetrance, then 85% of people who have that