Solution-Example of co-dominance, 1. In humans, sickle-cell

1. In humans, sickle-cell anemia is an example of co-dominance, when considered at the molecular level. People who are homozygous dominant produce all normal-shaped hemoglobin molecules. Hemoglobin is a protein found in red blood cells that carries oxygen. People who are homozygous recessive produce all abnormally-shaped hemoglobin, which cannot carry oxygen very well. People who are heterozygotes produce some normal and some abnormal hemoglobin. If a person who is homozygous dominant for this allele produces children with someone who is heterozygous, will any of their children produce only abnormal hemoglobin? What percentage of their offspring will be homozygous versus heterozygous? 

2. In humans, the ability to roll up the sides of the tongue into a U-shape is controlled by one gene. Ability to roll the tongue is dominant to the inability to roll it.  If two people who can roll their tongues each had one parent who could not, what is the probability that they will have a child who cannot roll his/her tongue? 

3. Suppose you had to figure out a tri-hybrid cross (dealing with three traits at once)!  You will not be asked to do a Punnett Square for this, but you should be able to set it up.  Suppose one parent organism is heterozygous for gene A, heterozygous for gene B, and homozygous recessive for gene C.  Genes A, B, and C are each located on different chromosomes. Write that parent organism’s genotype:  List all the gametes that this parent could produce:  

4. Suppose an organism’s genotype is: AA  Bb  CC  ID  Ee (That’s considering 5 genes at once-assume each is on a different chromosome) Which of the following are gametes that it could not produce? For each, explain why not. 

A B C d E 

a B C D e  

A B C D E         

A B C D d E  

A C d e  

5. In humans, the allele for normal blood clotting is dominant to the allele for a type of hemophilia. This gene is located on the X chromosome. Two parents produced daughters who are all carriers of hemophilia and sons who are all normal. What are the probable genotypes of each parent?

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