Download Name: Date: ______ GENETICS TEST STUDY GUIDE How to do

Name: _______________________________ Date: _____________
GENETICS TEST STUDY GUIDE
How to do Monohybrid Cross (Complete Dominance) Problems
1. First, know your vocabulary! You need to know these terms and what they mean.
a. Homozygous Dominant (HD) – two dominant forms of a gene (2 capital letters)
b. Heterozygous (Het) – one dominant and one recessive form of a gene (1 capital letter and 1
lower case letter)
c. Homozygous Recessive (HR) – two recessive forms of a gene (2 lower case letters)
2. Second, make a KEY. Dominant traits are represented by a capital letter and recessive traits are
represented by a lower case letter.
3. Third, write out the parental cross (ex: parent genotype x parent genotype).
4. Plug parent genotypes into a punnett square and solve.
5. Determine probabilities of all possible genotypes and phenotypes of offspring (# out of 4 = %).
Example:
Purple flowers are dominant over white flowers. A heterozygous purple-flowered plant is crossed with a whiteflowered plant. Give the genotype and phenotype ratios (percentages) of this cross.
KEY
Parental Cross: ________________________________
Genotypes
Phenotypes
Monohybrid Cross Practice Problems
1. In guinea pigs, black coat color is dominant over white coat color. What type of offspring could result
from a cross between a homozygous black guinea pig and a white guinea pig? Use a Punnett Square to
obtain your results. Include genotype and phenotype ratios for F1 and F2 generations.
2. In fruit flies, normal wings are dominant over vestigial wings. A cross is done involving a homozygous
normal-winged fruit fly and a vestigial-winged fruit fly? What is the chance that an offspring will have
vestigial wings? Show with a Punnett Square.
3. In pea plants, tall stems are dominant over short stems. Can short-stemmed offspring result from a cross
between a heterozygous tall plant and another heterozygous pea plant? Use a Punnett Square to obtain
your results.
4. A brown-eyed man, whose mother was blue-eyed, marries a blue-eyed woman. Use a Punnett Square
with genotype and phenotype ratios to show what color eyes their children could have. Remember:
brown eyes are dominant over blue eyes.
How to do Incomplete Dominance and Co-dominance Problems
These problems are done the same way. The difference would be in the way you would see them in person.
Neither trait is dominant, so your key will be different. You will still use a capital and lower case letter for
Incomplete dominance problems and two DIFFERENT capital letters for codominance. Incomplete
dominance shows a blend of both traits if both are present (heterozygous). Co-dominance shows both traits if
they are present (heterozygous).
1. Make your key for the traits.
2. Write out parental cross.
3. Complete your Punnett Square just as you would complete a monohybrid cross problem.
4. Determine probabilities of all possible genotypes and phenotypes of offspring (# out of 4 = %).
Example:
In Four o’clock flowers the alleles for flower color are both equal therefore neither dominates over the other
(co-dominant). A red four o’clock flower (RR) is crossed with a white flower (WW). Since there is no
dominant trait we use two different letters for the genotype.
COMPLETE THE PRACTICE PROBLEMS ON THE BACK OF THIS PAGE
Incomplete and Co-dominance Practice Problems
1. In humans straight hair (SS) and curly hair (ss) are traits that have incomplete dominance, that result in
hybrids who have wavy hair (Ss). Cross a curly hair female with a wavy haired male. Provide the
genotypes and phenotypes of the offspring.
2. In cattle, neither red (RR) nor white (WW) hair dominates. Instead, both show up if present to make a
“color” called roan (RW). Show the cross between a roan bull and a roan cow. Provide the genotypes
and phenotypes of the offspring.
3. In andalusian chickens, gray is a blend of pure black with pure white. Cross a gray rooster with a gray hen.
Give the genotypes and phenotypes of their offspring.
4. In flowers, red and blue petals are codominant. Show the cross between a red flower and a blue flower.
Provide the genotypes and phenotypes of the offspring.
How to do Sex-linked Trait Problems
In humans sex is determined by the twenty third pair of chromosomes known as “sex chromosomes”. If you
have two x-shaped (XX) chromosomes you are destined to be a female. If you have an x and a Y-shaped (XY)
chromosomes you are destined to be a male. Since the X and Y chromosomes carry different information, any
genes found on the X chromosomes are referred to as sex-linked genes. Therefore, women will have two
alleles for these genes because they have two (XX) chromosomes. On the other hand, men have only one allele
for each of these genes because they have only one X chromosome (XY).
1. Make your key. This will be a “normal” key where the capital letter is the dominant trait and the lower
case is the recessive trait.
2. Since this is a sex-linked trait problem, you must show the traits and the XX (female) or XY (male)
chromosomes as well. (Example: XRXr) Write the parent genotypes.
3. Set up your Punnett Square and complete it. (4 boxes)
4. Write your genotype ratio and the numbers that go with it – you will be listing exactly what is in your
boxes.
5. Write your phenotype ratio (it will be your traits from the key with male and with female) and the number
of each.
Example:
In fruit flies, the gene for eye color is carried on the X chromosome, which is a sex chromosome
(sex-linked). The allele for red eyes is dominant over the allele for white eyes. If a white-eyed
female fruit fly is mated with a red-eyed male, predict the possible offspring.
COMPLETE THE PRACTICE PROBLEMS ON THE BACK OF THIS PAGE.
Sex-linked traits practice problems
1. Hemophilia is a sex-linked trait. A person with hemophilia is lacking certain proteins that are necessary for
normal blood clotting. Hemophilia is caused by a recessive allele so use “N” for normal and “n” for
hemophilia. Since hemophilia is sex-linked, remember a woman will have two alleles (NN or Nn or nn)
but a man will have only one allele (N or n). A woman who is heterozygous (a carrier) for hemophilia
marries a normal man. Give the genotype and phenotype ratios along with your Punnett Square.
2. In canaries, the green variety with black eyes is dependent upon a dominant sex-linked factor (B), the
cinnamon variety with brown eyes is due to its recessive allele (b). A cinnamon male is mated to a
homozygous green female. Show all work and ratios.
3. Colorblindness is due to a recessive sex-linked factor. Normal vision is dominant. Show the Punnett
Squares and ratios for a carrier female and a colorblind male.
How to do Blood Type (Multiple Alleles) Problems
In humans, there are four types of blood; type A, type B, type AB, and type O. The alleles A and
B are co-dominant to each other and the O allele is recessive to both A and B alleles.
Blood Type (Phenotype)
Type A
Type B
Type AB
Type O
Genotype
IAIA or IAi
IBIB or IBi
IAIB
ii
1. Follow the steps for completing a monohybrid cross. You do not need a key for these Punnett Squares.
Write out the parents and complete the Punnett Square.
2. Write your genotype ratio by listing what is in your boxes and the number of each.
3. Write your phenotype ratio by listing the blood types you have present and the number of each.
Example:
A man with blood type AB marries a woman with type O blood. Show the possible genotypes and phenotypes
for their children.
COMPLETE THE PRACTICE PROBLEMS ON THE BACK OF THIS PAGE.
Blood Typing practice problems
1. An extremely rich elderly couple died in a car accident. They had no known living relatives. A man shows
up claiming to be their long lost son. The elderly couple both had Type AB blood. The man claiming to
be their son has Type O blood. Does this man have a claim to their fortune? Use a Punnett Square to
support your answer.
2. A woman with Type A blood and a man with Type AB blood have a baby. The woman is suing the man
for child support, but he is refusing to pay because he says the baby (who has Type O blood) is not his.
Is he right or should he be paying child support? Use a Punnett Square to prove your answer.
3. Is it possible for a man with Type A blood and a woman with Type B blood to have a child with Type O
blood? Use a Punnett Square to support your answer.
4. If both of your parents had Type O blood, what blood type must you be? Use a Punnett Square to prove
your answer.
How to do Pedigree Problems
In genetics, traits can be traced over several generations similar to a family tree. This family tree is called a
Pedigree chart. Pedigree charts are useful in gathering background genetic information that can be used for
medical reasons. When interpreting pedigree charts remember squares are male and circles are females.
Horizontal lines indicate mating, vertical lines indicate offspring. Shaded is affected and not shaded is
unaffected.
1. Number the generations down the left-hand side with Roman numerals.
2. Number each person above them using Arabic Numerals, 1, 2, 3, etc.
3. Find two parents that are the same (both shaded or both not shaded). This can be anywhere in your
pedigree.
4. Once you have found these parents, look at their children. If they have a mixture of shaded and unshaded
children, the (parents) HAVE TO be heterozygous (Aa). Put these letters under the people.
5. Make your key. If you said the parents were heterozygous (Aa) and they were shaded, then make shaded
dominant (A = affected (shaded) a = unaffected). If they are heterozygous (Aa) and they are NOT
shaded, then make shaded recessive (A = unaffected a = affected (shaded)).
6. Use the parents and children to figure out everyone else’s genotype (letters). If you can’t tell with 100%
certainty that someone is Aa then you need to put A__. Do not put AA.
Example:
COMPLETE THE PRACTICE PROBLEMS ON THE BACK OF THIS PAGE.
Pedigree Practice Problems
Label the following pedigree completely.
Answer the following questions according to this pedigree:
1. How is person II-1 related to I-2?
2. How many children did II-5 have? __________________
3. Is this trait dominant or recessive? EXPLAIN WHY.
Draw a pedigree that matches the following description: One couple has a son and a daughter with normal
pigmentation. Another couple has one son and two daughters with normal pigmentation. The daughter from the
first couple has three children with the son of the second couple. Their son and one daughter have albinism;
their other daughter has normal pigmentation. (Having albinism = affected)