Demonstrate your understanding of the phenotypic expression of genes based on modes of inheritance. Explain the evolutionary significance of different modes of inheritance; for example an autosomal dominant allele which is lethal will not be passed on to future generations. The mode of inheritance, together with the properties and location of the gene, will determine how alleles are passed on to future generations.

For example, the gene for Huntington's Disease is a genetic mutation of an autosomal gene. The allele is maintained in the population because the disease occurs late, at the end of a person's reproductive years. While the mutation is lethal, it is persistent in the population. A test for the Huntington gene mutation is now available and this might lead to a significant reduction of the rate of Huntington's Disease in the population. The inheritance of lethal autosomal gene mutations is rare because, in most cases, mutations will lead to early death and will not be passed on to future generations.