C1015 ’04 -- Mini-Exam #1                                                     


Each question or part is worth 5 points. If it's multiple choice, the answer = 2 and explanation = 3.


Fructose is a simple (small) sugar that is broken down to provide energy. Fructose can be broken down as follows:


  enzyme 1    enzyme 2      
Fructose -----------> A ----------->  X ----> ----> ----> complete breakdown in usual way


There is a rare genetic condition called fructosuria (meaning fructose in the urine) that is caused by a lack of enzyme 1. Fructosuria is inherited in a standard Mendelian fashion. (No sex linkage or other complications.) This condition is relatively benign, but if you eat too much fructose, you get fructose in your urine.


1. You would expect fructosuria to be inherited as a (dominant) (recessive) (either way). Explain why. 


 2. Answer the questions below assuming that your answer to question 1 is correct. (If you said “either way” pick one of the alternatives, and continue.)

A. What do you expect for the parents of a person with fructosuria? (Both should be normal in phenotype) (Both should have fructosuria) (One should be normal and one should have fructosuria) (At least one should have fructosuria) (Any of these are equally likely). Give the expected genotypes and phenotypes of the parents and child, and explain briefly.

                B. Suppose a person with fructosuria marries a person without it who has no family history of the condition. Remember that the condition is rare. The couple have a child. Do you expect the child to have fructosuria? (yes) (no) (it’s about 50-50) (can’t predict without more info). Give genotypes and phenotypes and explain briefly.


3. Suppose a woman with fructosuria marries a man who is homozygous normal. Both parents and the child have the normal number of chromosomes. The child has fructosuria. How is this possible?

A.  This could happen if nondisjunction occurred in (the normal parent) (the parent with fructosuria) (both parents) (neither parent – this result is normal) (none of these cases – something else must have happened here, such as mistaken paternity). Explain briefly.



B. Suppose you look at the karyotypes (with banded chromosomes) in the ordinary body cells of the two parents and the kid. What would you expect? (All 3 karyotypes should be significantly different) (All 3 karyotypes should be essentially the same) (The kid and his mother will have the same karyotype but it will be diff. from his father’s) (The kid and his father will have the same karyotype but it will be diff. from his mother’s) (The kid’s karyotype will be the same as his father’s or as his mother’s -- but can’t predict which).

Background information for the questions below:

Fructose is found in many fruits. It is also part of ordinary table sugar (sucrose).

Sucrose is a dimer of two simple sugars, glucose and fructose. In other words, sucrose = glucose + fructose.

Lactose, the sugar found in milk, is a dimer of two simple sugars, glucose and galactose. In other words, lactose = glucose + galactose.

When you eat the dimers, they are broken into the simple sugars, which are then broken down further (metabolized) to get energy.


4. Suppose you have fructosuria. You are most likely to have symptoms -- fructose in your urine --  if you (put lots of sugar on your oatmeal) (put a lot of milk on your oatmeal) (put lots of both on your oatmeal – neither one alone will do it) (eat your oatmeal plain) (however you eat your oatmeal – it should make no difference – it’s an inherited disease and diet should have no effect) (beats me – and besides I hate oatmeal). Explain your reasoning. Note: Plain oatmeal has little or no sugar of any kind.


 5. There is a second way to break down fructose, as follows:


  enzyme 3    enzyme 2      
Fructose -----------> B ----------->  X ----> ----> ----> complete breakdown in usual way


Note that the same enzyme (enzyme 2) is used here and in the pathway shown earlier. There is a second genetic condition that affects fructose metabolism. In condition #2 it is enzyme 2 that is missing. This is a very serious condition that can be fatal. Now that you know about both pathways

A. Why is the condition #2 (lack of enzyme 2) so much worse than condition #1 (lack of enzyme 1)? Explain briefly.


                B. Which of the following is most likely to be the best advice to a person with condition #2? (Eat lots of fructose) (Avoid fructose completely) (Take it easy on the fructose, but be sure to eat a little) (Eat whatever you want – diet won’t make any difference). Explain briefly.