Recessiveness and Expressivity Exercise

This exercise is designed to show you how different mutations in the same gene result in a variation protein activity and therefore, variability of the resulting phenotype.

Oculocutaneous albinism is a recessively inherited genetic disorder that is caused by mutations in the gene encoding tyrosinase. This enzyme converts tyrosine to DOPA, which is then converted to melanin.

tyrosinase

tyrosine à DOPA à à à melanin

Albinism is most commonly observed as absence of pigmentation. However, variability in the phenotype is observed. For example, one organism might exhibit severe pigment deficiency and the involvement of the eyes, while another organism might exhibit moderate pigment deficiency. The “native”cookie forms are tyrosine, while the “open” cookie forms are DOPA.

enzyme – your job is to split open as many cookies as you can in 15 seconds

time keeper – your job is to start the reaction and stop it at 15 seconds, count each second aloud

recorder – your job is to keep count of the number of cookies opened at 5, 10, and 15 seconds

grapher – your job is to record and graph the results

mutation chooser – pick a mutation card and read the mutation and directions aloud

If 10 units of DOPA/15 seconds (open cookies) are required to make enough pigment for a “normal” phenotype, which mutations will result in a phenotype? Remember, that you have two alleles.

Which mutations result in the most severe phenotype? Why?

If we have a compound heterozygote, how will this affect the severity of phenotype?