STUDY GUIDE MENDELIAN GENETICS.
Make sure you know the following concepts. Then, check your answers with those
at the end of this study guide.
1. What is genetics?
2. Who was Mendel?
3. What did Mendel discover?
4. What are Mendel’s Laws?
5. What is a gene?
6. What are alleles?
7. Why are genes expressed with TWO letters?
8. Difference between P Generation, F1 and F2?
9. Difference between a genotype and phenotype.
10. Difference between Dominant and Recessive Alleles.
11. Difference between homozygous and heterozygous.
12. Difference between a purebred and a hybrid.
13. What is probability?
14. How to make a Punnett Square?
15. Difference between Incomplete Dominance, Codominance, Dominant/Recessive,
and sex-linked traits.
Problems:
1. Figure out the genotypes and the phenotypes from a cross between a purebred longtailed cat and a purebred short-tailed cat.
2. In wasps, brown eyes are dominant over blue eyes. Determine the genotypes and
phenotypes of the offspring from a cross between a purebred brown-eyed individual
and a hybrid brown-eyed individual.
3. Determine the genotypes and phenotypes of a cross between a hybrid brown-eyed
individual and a purebred blue-eyed individual.
4. In seals, the gene for the length of the whiskers has two alleles. The dominant allele
(W) codes long whiskers & the recessive allele (w) codes for short whiskers.
a. What percentage of offspring would be expected to have short whiskers from the
cross of two long-whiskered seals, one parent is homozygous dominant and the
other one is heterozygous?
b. If one parent seal is pure long-whiskered and the other is short-whiskered, what
percent of offspring would have short whiskers?
5. In dogs, there is a hereditary deafness caused by a recessive gene, “d.” A kennel owner
has a male dog that she wants to use for breeding purposes if possible. The dog can hear,
so the owner knows his genotype is either DD or Dd. If the dog’s genotype is Dd, the
owner does not wish to use him for breeding so that the deafness gene will not be passed
on. This can be tested by breeding the dog to a deaf female (dd).
a. Draw the Punnett squares to illustrate these two possible crosses. In each case,
what percentage/how many of the offspring would be expected to be hearing?
Deaf?
b. How could you tell the genotype of this male dog?
c. Using Punnett square(s), show how two hearing dogs could produce deaf
offspring.
6. In dogs, erect ears (E) are dominant over droopy ears. What are the results if two
heterozygous dogs have a litter of puppies?
7. The ability to roll the tongue (R) is determined by a dominant gene while the recessive
gene results in the inability to roll the tongue (r). A man and his wife can both roll their
tongues and are surprised to find that their son cannot.
Explain this by showing the genotypes of all three persons.
8. In humans, wavy hair results by the incomplete dominance situation of curly hair (C)
and straight hair (S). What are the possible results if a curly-haired man and wavyhaired woman have children?
9. In crocus flowers, white (W) and purple (P) colors are co-dominant, and result in a
purple and white striped flower when both genes are present.
What are the possible results from the cross-pollination of a striped crocus with a white
crocus?
10. Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW =
red & white hairs together).
a. Predict the phenotypic ratios of offspring when a homozygous white cow is
crossed with a roan bull.
b. What should the genotypes & phenotypes for parent cattle be if a farmer
wanted only cattle with red fur?
11. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur
together).
a. What pattern of inheritance does this illustrate?
b. What percent of kittens would have tan fur if a tabby cat is crossed with a black
cat?
12. A cross between a blue bird and a white bird produces offspring that are silver, an
intermediate color between the two parents. The color of these birds is determined by
just two alleles.
a. What are the genotypes of the parent birds in the original cross?
b. What is/are the genotype(s) of the silver offspring?
c. What would be the phenotypic ratios of offspring produced by two silver birds?
ANSWERS:
1. What is genetics?
The science that studies how traits are inherited.
2. Who was Mendel? Known as the Father of Genetics. Mendel was a Monk that
crossed pea plants and discovered how traits are passed from one generation to the
next.
3. What did Mendel discover? That traits are passed in a different way from
generation to generation. For example, a tall plant crossed with another tall plant
may give small plants in the F1. In this case, both tall plants carry the gene for
short height, but it is hidden by the dominant trait (tall height).
4. What are Mendel’s Laws?
a. Organisms have two alleles (forms of a gene) per characteristic, but you
pass only one to your children.
b. Alleles are either dominant or recessive. Dominant shows over the
recessive. The recessive trait needs to be inherited from Mom and Dad to be
expressed (showed).
5. What is a gene? A sequence of DNA that codes for a specific characteristic. It
may be slightly different in each person, but it is in the same section of the DNA.
6. What are alleles? Different forms of the same gene.
Example: the gene for height in pea plants has two alleles: short and tall. The gene
for Widows Peak has two alleles: with widows peak and without it.
7. Why are genes expressed with TWO letters?
Because each letter represents one allele. You get one allele from each of your
parent.
8. Difference between P Generation, F1 and F2?
P is the parent generation. F1 is the first generation of CHILDREN!!!, and F2 is the
second generation of children.
9. Difference between a genotype and phenotype.
A genotype is the genes inside the organism. They are expressed with two letters.
A phenotype is the way those genes are expressed, in other words, the way they
look. You use words.
10. Difference between Dominant and Recessive Alleles.
A dominant allele is showed over the recessive. Dominant is expressed with a
Capital letter, and Recessive is expressed with the lower case of the SAME letter.
Examples: B= dominant for black fur, b= recessive for white.
11. Difference between homozygous and heterozygous.
Homozygous traits have the same type of alleles. They are represented with the
same two letter, either two dominants or two recessives. Example: B= black,
b=white. BB, or bb.
Heterozygous traits have two types of alleles, but it is still the same gene. One is the
dominant trait and the other will be the recessive trait. Example: Bb.
12. Difference between a purebred and a hybrid.
A purebred is a homozygous.
A hybrid is a heterozygous.
13. What is probability? A number that describe how likely is it that an event will
occur.
14. How to make a Punnett Square? PLEASE PRACTICE AND PRACTICE!!! See
problems below.
15. Difference between Incomplete Dominance, Codominance,
Dominant/Recessive, and sex-linked traits.
Dominant/Recessive: follows Mendel’s laws. The dominant will always show its
genotype. Example: Chef chin (dominant) is showed either with homozygous
dominant or heterozygous (CC or Cc) but not with homozygous recessive (cc)
Incomplete Dominance: You get an intermediate characteristic with the
heterozygous: Black fur is dominant (BB), white fur is recessive (bb) and gray fur is
the heterozygous phenotype (Bb).
Codominance: Both alleles show. Please remember that they are written
differently. Example: White Flower (FWFW), Red Flower (FRFR). The heterozygous
is spotted white and red (FWFR).
Sex-linked Traits: They are related to the X chromosome (sex chromosome) and
are usually recessive. Example: Hemophilia. If a man suffers from hemophilia his
genotype for that trait is XhY. If a woman suffers hemophilia, her genotype is XhXh.
Notice that she has the trait in both genes, because it is a recessive trait, but men
only have it in one gene (the X), since they only have one X.
PROBLEMS:
1. SAMPLE PROBLEM
In cats, long tails are dominant over short tails. Predict the genotypes and
phenotypes of a cross between two hybrid long-tailed cats.
This is a Dominant/Recessive problem.
Step 1. Choose a letter to represent the genes in the cross.
T = Long tails
t = Short tails
Step 2. Write the genotype of the parents.
Tt
x
Tt
Step 3. Write the possible genes that each parent
Could pass on to the offspring.
T
t
Step 4. Write the possible genes at the top and side of the Punnett square.
T
t
T
t
T
T
Step 5. Complete the Punnett square by writing the gene combinations in the appropriate
boxes.
T
t
T
TT
Tt
T
TT
Tt
Step 6. Write the genotypes and phenotypes of the offspring in percentages. (each box =
25%)
Genotypes: TT= 50%, Tt=50%
Phenotypes: Long tail=100%
2. Brown eyes are dominant over blue eyes. Determine the genotypes and
phenotypes of the offspring from a cross between a purebred brown-eyed individual
and a hybrid brown-eyed individual.
B
B
BB
B
BB
b
Bb
Bb
Offspring’s Genotype: BB= 50%, Bb= 50%
Offspring’s Phenotype: Black eyes =100%
3. Determine the genotypes and phenotypes of a cross between a hybrid browneyed individual and a purebred blue-eyed individual.
B
B
b
Bb
Bb
b
Bb
Bb
Offspring’s Genotype: Bb= 75%, bb= 25%
Offspring’s Phenotype: Black eyes =75%, Blue eyes =25%
8. In humans, wavy hair results by the incomplete dominance situation of curly hair
(C) and straight hair (S). What are the possible results if a curly-haired man and
wavy-haired woman have children?
C
C
C
CC
CC
c
Cc
Cc
Offspring phenotype: 50% will have straight hair, 50% will have curly hair.
10. Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan
(RW = red & white hairs together).
CW
CW
CR
CR CW
CR CW
CW
CW CW
CW CW
Phenotypes: 50% White and 50% Roan
To get only Red fur, the parent cattle should be CR CR
12. A cross between a blue bird and a white bird produces offspring that are silver,
an intermediate color between the two parents. The color of these birds is
determined by just two alleles.
a. What are the genotypes of the parent birds in the original cross? BB and bb
b. What is/are the genotype(s) of the silver offspring? Bb
c. What would be the phenotypic ratios of offspring produced by two silver birds?
75% Silver and 25% white.
B
b
B
Bb
Bb
b
Bb
bb