IB Biology II / Revised 2011-12

Lab: Asparagus? Urine for a Surprise!

Adapted from Genetics Laboratory Investigations by Mertens &

Hammersmith. Macmillan: NY. © 1991.

Introduction

Routine urinalysis is an important, commonly used screening test for urinary and systemic pathologies. Freshly voided urine is nearly odorless. (If allowed to stand, it develops an ammonia odor due to bacterial metabolism of its urea.) After eating and metabolizing asparagus, everyone makes and ultimately excretes volatile, malodorous sulfur compounds. Some people (“smellers”) can detect the strong odor of these excretion compounds after consuming just a few stalks of asparagus. Other people (“nonsmellers”) lack the ability to smell these sulfur-containing compounds. The chemicals in asparagus that are the source of the odorous compounds apparently remain unknown.

Nucleotides on homologues / What the Study Found
A A / Substantially higher odds of being a smeller
A G / Moderately higher odds of being a smeller
G G / Typical odds of being a smeller

A recent study (Eriksson et al, 2010) of 4,737 individuals of European ancestry identified a number of genetic variants associated with the ability to detect the scent of asparagus metabolites. Many of these variants were in a region of the genome that contains genes encoding olfactory receptors. The strongest association was found at a SNP (remember those!?!) on human chromosome 1. Each copy of an “A” at this SNP increased a person’s odds of being able to smell asparagus in their urine, compared to people with two “G’s” as shown above.

Materials

Fresh or canned asparagus (3 or 4 spears per person). Note: Do not eat asparagus if you are allergic to it!

Procedure & Analysis

1. Note the time at which you consume the asparagus. For the next 24 hours, each time you urinate record whether or not you smell the distinctive odors of the sulfur compounds in your urine.

2. Identify yourself as a smeller or nonsmeller.

3. From the sample pedigree (Fig. 25.2) at the right, postulate the

mode of inheritance of this trait. Be sure to explain how you know!

4. Record class data for both periods of IB Bio Year 2.

5. Use our data to calculate smeller and nonsmeller allele &

genotype frequencies. We’ll assume our population is in Hardy-Weinberg equilibrium with respect to this trait.

6. List as completely as possible the various body

pathways the asparagus and the sulfur-containing compounds traversed to get from your mouth to your urine!