Some plants do cross-fertilize in nature. Mendel tracked the observable traits to determine if there were certain patterns of inheritance. Because Mendel was a mathematician, he used probability theory to predict the possible outcomes of a cross. This is shown by using Punnet squares. Huntington's disease Occurs in 1 out of 20,000 births neurodegenerative disorder leading to severe muscle spasms and personality disorders, usually not apparent until middle age.
Affected individuals lack the enzyme needed to breakdown the amino acid phenylalanine.
Instead, phenylketone accumulates and leads to mental retardation. If a trait is governed by 2 or more sets of alleles, the effects are additive, this is called polygenetic inheritance. The distribution of phenotypes from this type of inheritance resembles a bell-shaped curve, these include height and skin color. Sometimes multiple alleles exist for the same gene. An example of this is our blood types.