1. Your roommate purchased a “direct-to-consumer” genetic test and now wants you

Question

1. Your roommate purchased a “direct-to-consumer” genetic test and now wants your help understanding the results. (These genetic tests analyze DNA in consumer’s saliva to determine their genotypes for certain diseases. The consumer then receives a printout of the results.)

You roommate’s results indicate homozygous recessive for a gene related to caffeine metabolism. The wild-type gene codes for enzymes that metabolize caffeine; therefore, your roommate may have “caffeine sensitivity” due to slow metabolic rates.   Explain to your roommate how genes affect the production of enzymes and how mutations can affect enzyme function.

Your roommate’s results also indicate polydactyly phenotype, but he/she has only 5 fingers on each hand. Explain why a genotype does not always result in the predicted phenotype. Additionally, explain what phenotype could be expected in his/her offspring.

Lastly, your roommate’s results are positive for a loss-of-function mutation in the retinoblastoma gene, which acts as a tumor suppressor. Only one copy of this gene is mutated. Explain to your roommate why this mutation increases his/her chances of developing cancer.  

Explanation / Answer

Polydactyly is inherited by a dominant allele. A person can express the polydactyly phenotype only if the gene is homozygous dominant i.e; Polydactyly is seen in person with PP allele. The person with heterozygous dominant Pp would be carrier for polydactyly phenotype without showing polydactyly in the phenotype. Here the person is a carrier of polydactyly phenotype (Pp). If he/she marries a person normal person, pp, the 50% of the progeny would be carriers while 50% would be normal.

Pp x pp

Pp (carrier) pp (normal) Pp (carrier) pp (Normal)

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