LA 4 – Part 1: Pedigrees

Mendelian traits can easily be traced and "predicted" using pedigrees and Punnett squares, and you will do just that in Part 1 of this lab activity! You are going to play the role of a genetic counselor, and help several expectant parents determine their risks for passing on genetic disease.


Use the family information provided below for each “Case Study” couple to draw a pedigree diagram and answer the questions provided below. You may draw your pedigrees by hand and upload photos of your work when you submit the assignment, or draw/embed them directly into the worksheet.  Be sure to label your pedigrees with their associated Case Study ID (A, B, C).  To receive full credit for this portion of the lab, each pedigree diagram must:

  1. Include all family members whose trait information is provided in the Case Study
  • Identify the genotypes for all individuals, using the letters specified in the Case Study
  • Identify all “affected” individuals (i.e. those who have the trait) with a shaded symbol

Case Study A

Maria and Arjun are having twins, a boy and a girl.  Although neither Maria nor Arjun have sickle cell anemia, the condition runs in both of their families.  The couple is aware that sickle cell anemia, like other autosomal recessive traits, can “skip” generations, and so they are looking for information about their “risk” of passing the condition on to their twins.  After speaking to the couple about their family history, you are given the following information to help you create a pedigree and better assess their likelihood of passing sickle cell anemia onto their children:

Maria comes from a family of four, including herself.  Maria and her mother are carriers for the sickle cell trait, and her only sibling, a brother, has the condition.  To her knowledge, the only other family member to have the condition is her paternal grandfather (her dad’s dad). 

Arjun is also one of two children in his family.  He, his brother, and his mother do not have sickle cell anemia, although his father does.  Arjun knows that his mother is a carrier for the sickle cell trait, because her father, Arjun’s grandfather, also had the condition.  

Complete your pedigree of Maria and Arjun’s families, and include everyone’s genotype for the sickle cell trait using a capital “A” for the dominant allele, and a lowercase “a” for the recessive allele.

  1. Add Maria and Arjun’s genotypes for the sickle cell trait to the Punnett square below and cross their alleles to determine the possible genotypes for their children. 
  • Based on your Punnett square, what is the likelihood that Maria and Arjun will have a child with sickle cell anemia?  Will male or female children be more likely to inherit the disease?  Why or why not?


Case Study B

Lionel and Hui Yin are thinking about starting a family.  Lionel recently discovered that Huntington’s disease, an autosomal dominant condition that affects the brain, runs in his family. Lionel is too young for symptoms of the disease to be apparent and is apprehensive about undergoing genetic screening.  He and Hui Yin are seeking genetic counseling to find a less invasive way to assess their risk of producing a child with Huntington’s disease, and have agreed upon pedigree analysis.  They provide the following information about their family history:

Lionel was adopted as a small baby, and recently became reunited with his biological mother and sister.  His sister has Huntington’s disease, but they are certain it was not inherited from their mother, who is 55 years old and has never had any symptoms. The condition must have come from the siblings’ father, who is now deceased. He got the condition from his father, Lionels’ grandfather.

Hui Yin is an only child, with no known family history of Huntington’s disease.

Complete your pedigree of Lionel and Hui Yin’s families.  Assume that Lionel is heterozygous for Huntington’s disease, and include all genotypes for this trait using a capital “H” for the dominant allele, and a lowercase “h” for the recessive allele.

  • Cross Lionel’s genotype with Hui Yin’s in the Punnett square below, assuming that Lionel is heterozygous for Huntington’s disease.
  • Based on your Punnett square, what is the likelihood that Lionel and Hui Yin will produce a child that will NOTbe at risk for developing Huntington’s disease as an adult?

Case Study C

Paula and David are pregnant, and just learned that their baby has two X chromosomes.  They visit a genetic counselor with concerns about passing hemophilia to their baby, as this X-linked recessive condition is present in both of their families. They decide to pursuit pedigree analysis to assess their child’s risk before undergoing prenatal testing to screen for the genetic variants associated with the condition.  They provide you with the following information about their family histories:

Paula is one of three siblings, and is a carrier for hemophilia.  Her mother and sister are also carriers, but her father is completely unaffected.  The only person in her family to actually have the condition is her brother. 

David has two sisters, one with hemophilia and one without.   Although David and his mother do not have the condition, his father did.  In addition to the baby on the way, Paula and David have two sons, both of whom were diagnosed with hemophilia shortly after birth.  

Complete your pedigree for Paula and David’s families, and record everyone’s genotype for heemophilia using a capital “B” for the dominant allele, and a lowercase “b” for the recessive allele.  Because hemophilia is an X-linked trait, you will need to include the sex chromosomes in the genotypes (e.g. XBXB, XBXb, XbXb, XBy, XbY)

  • Cross Paula and David’s genotypes for hemophilia in the Punnett square below.  Be sure that you add the sex chromosomes to your genotypes, and cross them along with their alleles. 
  • What is the likelihood that Paula and David will pass hemophilia to their unborn daughter?  Would the chances be the same if she was male?  Why or why not?   

LA 4 – Part 2: Evolutionary Forces

There are four evolutionary forces that shape which traits are present within a population, and which get passed to future generations.  These forces include mutation, gene flow, genetic drift, and natural selection.


You recently reviewed a video called “The Five Fingers of Evolution” that described the different forces that act on populations to cause genetic change over time.  Use information provided in the video to answer the following questions.

  • Which finger(s) in “The Five Fingers of Evolution” do you think represent genetic drift?  Be sure to explain your answer.
  • What is gene flow, and what effects can it have on a population’s gene pool?
  • What specific advantage(s) must a genetic trait provide in order to be “favored” by natural selection?