Genetics Lab


Answer the questions for this lab on a separate piece of paper. When you hand it in, attach your prelab table.

Part One. Dominant and Recessive Alleles.

An introduction or review of the concepts of genes, alleles, and dominance will lead into these questions. Remember, showing a dominant allele means that you are carrying one for sure, two maybe. You can only show the recessive allele if you are carrying two of them.


1. If both of 2 parents show the dominant allele for a trait we'll label "A," show: what alleles each may be carrying, and what cross combinations you can get; and what offspring combinations you could get from each parental combination.


2. Answer like #1, but for both parents showing the recessive allele for trait "A."


3. Answer like #1, but for one parent showing dominant and the other showing recessive.


4. A couple you meet both have brown eyes, but their child has blue eyes. Explain in terms of alleles.



Part Two. Using the Results from the Prelab Tables.

1. In light of the results from Part One, if the tables contain dominant/recessive traits, which would you expect to be in the majority, the dominant or the recessive ones?


2. The entire results from the class' tables will be tallied on the board. What sort of proportions or ratios do you find among the pairs? Do you think that you can predict which traits are dominant?




3. The actual dominant traits (known from how they behave when inherited) will be circled on the board. How often is the lower number circled?


4. When surveying a population for traits like these, you could actually predict numbers for how often the traits should be seen, but to do that what important details must you know first?



Part Three. Genetics of Human Blood Types.

An introduction into the biology and genetics of human blood types will lead into these questions. Remember, "A" and "B" alleles are dominant over "o," but not over each other. Rh "+" allele is dominant over Rh "-" and inherited completely separately from the ABO alleles.


1. Someone with B+ blood marries someone with A- blood. Show what alleles might be present in this combination.



2. For the combination in question 1, tell whether the following blood types could show up in their children and explain!

      a) AB+? 

      b) A+? 

      c) B-? 

      d) O-?


3. In an old court case, a woman with O- blood has accused a man with AB+ of blood of being the father of her B- child. Could he be?  Why or why not?


4. In the mother-child circumstance above, what blood type could the father not be?



5. These are the blood types of the McDarby family: dad, A+; mom, B+; son, A+. For each of the three (a 3-part answer), tell what blood type alleles, ABO and Rh, might possibly be present.

      Dad -

      Mom -

      Son -


Part Four. Other Forms of Inheritance.

1. After an explanation of blended / intermediate/ codominant inheritance, explain the following situation by showing the alleles involved: Breeding grey gerbils gives you offspring that are all white, all black, or all grey. Why?



2. Some apparently simple traits inherit in amazingly complex and unpredictable ways. What is happening on the gene level?




Part Five. Chromosomes, Linkage. Sex Linkage.

1. What purpose is served by chromosomes? What are homologous chromosomes?



2. How do chromosomes in a single organism differ, usually?



3. What is meant by genetic linkage?  



4.  Briefly explain how genetic linkage is used to find where in a person's chromosomes a particular gene is?



5. What are the various ways that gender can be determined in organisms?



6. How are genes normally expressed?  How is this affected by extra chromosomes?



7.  Why is this a problem with genes on the sex chromosomes?



8. What adaptations could "get around" the problem of unequal expression of sex chromosomes?



9. What is a Barr body? Why do they exist?



10. In mammals, including humans, which gender is most likely to show a sex-linked trait? Why?



11. A color-blind man marries a woman with no history of color blindness in her family. What are the chances that:

a) Their sons will be color-blind?

b) Their daughters will be color-blind?

c) Their daughters will carry the color-blind allele?


12. As explained leading into question 6 above, having extra chromosomes usually leads to major problems. However, having extra sex chromosomes (being XXX, or XYY, or XXY, or XXXX, even XXYY) is usually a minor problem only...

a) What keeps an extra X chromosome from being really dangerous?

b) What keeps an extra Y chromosome from being really dangerous?



Original Version 1987;  Web Version 2004.  Last revision 2010.  M. McDarby.


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