Genetics Worksheet
BIOL211 Summer 2012
40 points
Monohybrid Cross
1. Fold your hands together so your thumbs cross over, and look at your thumbs. Which thumb feels most
‘comfortable’ on top is actually controlled by a gene. The left thumb folding over the right thumb is a dominant
trait, whereas the right thumb folding over the left thumb is a recessive trait. Let’s call the dominant form of this
gene “L,” and the recessive form “l.”
a. What is your phenotype? _______________
b. What is your genotype? (Assume heterozygote if you are dominant) ____________
c. What possible gametes can you produce? _______________
Suppose that you had children with someone whose right thumb folded over their left thumb.
d. What is their genotype?_____________________
Complete the Punnet Square below to find the possible genotypes of your children
e. Genotype ratio: ______________________
f.
Phenotype ratio: _____________________
g. What is the probability your first child will fold their left thumb over their right? _______________
2. In dogs, a type of hereditary deafness is caused by a recessive gene that we will call “d.” A kennel owner has a
male dog that she wants to use for breeding purposes. The dog can hear, so the owner knows the genotype is
either DD or Dd. If the dog is a carrier for the deafness gene, the owner does not want to use him for breeding so
the deafness gene will not be passed on. This can be tested by breeding the dog to a deaf female (dd).
Draw the two Punnett squares to illustrate these two possible crosses.
Punnet square 1: DD xdd
a. How many of the offspring would be hearing? ____________________
b. How many of the offspring would be deaf? ______________________
Punnet square 2: Dd xdd
c. How many of the offspring would be hearing? ____________________
d. How many of the offspring would be deaf? _______________________
e. Using this information, how could you tell the genotype of the male dog?
_________________________________________________________________
Using a Punnet square, show how two hearing dogs could produce deaf offspring
f.
What is/are the possible genotype(s) of the two dogs?
_____________________________________________________________
Dihybrid Cross
3. In addition to the gene for deafness, there is also in dogs a recessive gene that causes the dog to have a wiry
coat with hard hair follicles. This is opposed to the dominant trait of a soft, fluffy coat. Imagine two dogs, as cute
as you want. Both dogs are carriers for the deafness gene and also carriers for the wiry coat.
a. What is the genotype of the two dogs? (Pick any letters/symbols) _____________________
b. What is the phenotype of the two dogs? __________________________________________
c. What are the possible gametes of the two dogs? ____________________________________
d. What were to happen if you breed these two dogs together? Draw your Punnet square.
e. What are your genotype ratios?
_________________________________________________________________________________
f.
What are your phenotype ratios?
_________________________________________________________________________________
Codominance
Note: The alleles for blood types are written in a rather unique fashion. We write the allele for blood type A as IA,
blood type B as IB, and blood type O as i.
4. In humans, blood type is expressed in a codominant fashion. The possible human phenotypes for blood group
are type A, type B, type AB, and type O. Type A and B blood are codominant, and if inherited together both will
be expressed. On the other hand, type O blood is completely recessive to both. Type A and B individuals can be
either homozygous (IAIA or IBIB, respectively), or heterozygous (IAi or IBi, respectively), whereas type O blood can
only be homozygous recessive (ii).
Someone who is a heterozygote for type A is crossed with someone who is a heterozygote for type B blood.
a. What is the genotype of the type A person? _______________________
b. What gametes can this person produce? __________________________
c. What is the genotype of the type B person? _______________________
d. What gametes can this person produce? __________________________
Complete the Punnet Square
e. Genotype ratios: __________________________
f.
Phenotype ratios: __________________________
Codominance plus a dihybrid cross
5. Rh is another way to measure blood type. Originally discovered in Rhesus macaque monkeys, the Rh factor is
a transmembrane ion channel. The Rh gene can encode for two possibilities: the presence or absence of the ion
channel. Someone who has the channel is called Rh positive, also written as Rh+, while someone without the
channel is Rh-. A patient with type AB blood is Rh+. This person has children with someone who also has type
AB blood, but is Rh-.
a. What is the genotype of the first person? ____________________
b. What gametes can this first person produce? __________________
c. What is the genotype of the second person? __________________
d. What gametes can this second person produce? _________________
Complete the Punnet Square
e. Genotype ratios: ________________________________________________________________
f.
Phenotype ratios: _______________________________________________________________
Incomplete dominance
6. In humans, curly hair is an example of incomplete dominance. Someone who is homozygous for straight hair
will have stick-straight hair, someone who is homozygous for curly hair will have ringlets, and someone who is
heterozygous will have the intermediate phenotype, which is wavy hair.
a. What is your phenotype? (Best guess):_________________________________
b. What is/are your possible genotypes?:__________________________________
c. What is your mother’s phenotype? (Best guess):__________________________
d. What is/are your mother’s possible genotypes?:__________________________
e. What is your father’s phenotype? (Best guess):___________________________
f.
What is/are your father’s possible genotypes?:___________________________
Epistasis
7. In humans, some forms of eye color are epistatic traits, and controlled by multiple genes. Let us assume eye
color is controlled by two genes. The first gene controls whether a brown pigment is expressed. Homozygous
recessive individuals for this gene will have no pigment expressed, whereas heterozygous and homozygous
dominant individuals will express a brown pigment. The second gene controls the expression of blue or green
color. Green is dominant over blue.
a. What letters will you assign to brown, green, and blue? ______________
b. Is this a monohybrid or dihybrid cross? _________________
c. Someone has green eyes. What is/are their possible genotype(s)? ________________________________
d. Someone has blue eyes. What is/are their possible genotype(s)?__________________________________
e. Someone has brown eyes. What additional information do you need to determine their genotype?
_____________________________________________________________________________________
Sex-linked trait
8. A mother and father with normal color vision produce six male children, two of whom exhibit red-green
colorblindness. Their five female children exhibit normal color vision.
a. What is the father’s genotype? __________________
b. What is the mother’s genotype? _________________
Use the Punnet square below to prove your answers