Performance Benchmark L.12.A.5
Students know how to predict patterns of inheritance. E/S
A gene is the unit of heredity in living organisms. Genes are encoded in an organism's genome,
composed of DNA or RNA, and direct the physical development and behavior of the organism.
Multiple versions can exist for each gene. Different forms of a gene are called alleles. For
example, a one allele can code for blue eyes and another brown. While some genes have only
two alleles, many genes have three or more alleles. Diploid organisms contain two alleles for
every gene.
The father of genetics is Gregor Mendel who conducted extensive studies on the heredity of pea
plants. His research resulted in the Law of Segregation and the Law of Independent Assortment.
The Law of Segregation first states that each diploid organism inherits two genes for each trait,
one from each parent. It also states that the two alleles are separated during gamete formation.
The Law of Independent Assortment states that every trait is inherited independently of another,
thus creating new combination of genes unique to that individual. We now know that some
traits are linked because they are located on the same chromosome.
Mendel also concluded that some alleles will be expressed when present and others can be
suppressed. The form of the gene that is always expressed when present is called dominant
while the gene that is suppressed in the presence of the dominant form is called recessive. An
identical pair of alleles for a trait is called homozygous (dominant or recessive) or and a mixed
pair is called heterozygous. A heterozygous pair will always express the dominant trait. The
pair of genes for each trait is called a genotype and the expression of that trait is a phenotype.
A Punnett square is a biological tool used to calculate the mathematical probability of inheriting
a specific trait. It was named for an English geneticist, Reginald Punnett. A Punnett square
uses a system of letters to represent the alleles involved in the cross. A simple single trait
problem is called a monohybrid cross. A capital letter represents the dominant trait and a lower
case letter represents the recessive allele.
You would use the following procedure to set up a cross.
1. Determine the genotype of the parents.
2. Segregate the alleles to determine the gametes
3. Construct the Punnett square
4. Complete the Punnett square
5. Calculate the genotypic and phenotypic probabilities of each possible offspring.
For example: In squash, yellow (Y) fruit is dominant to green (y). A homozygous yellow plant
is crossed with a green plant. Calculate the genotypic and phenotypic ratios of the offspring.
1. Parents’ genotypes: Homozygous yellow = YY Homozygous green yy
YY x yy
2. Segregate the alleles
Y
Y
x
y
y
3. Construct the Punnett Square
y
y
Y
Y
4. Complete the Punnett square
y
y
Y
Yy
Yy
Y
Yy
Yy
5. Calculate genotypic and phenotypic ratios
Genotypic Ratio
Phenotypic Ratio
Yy 4/4 100%
Yellow 4/4
100%
Other types of Punnett square problems:
1. Dihybrid Cross – A cross of two traits together. A Punnett square is used to determine the
gametes that would be formed through independent assortment.
Problem: A homozygous round, homozygous yellow plant is crossed with a wrinkled, green
seeded plant.
Figure 1: an example of a dihybird cross. (from http://fig.cox.miami.edu/~cmallery/150/mendel/heredity.htm)
2. Codominance: Both alleles of a gene are expressed equally. A common example is blood
types in humans. A person with blood type AB has the alleles for blood type A and B. Two
different capital letters are used to represent the codominent relationship.
Codominant Trait
Figure 2: Human blood groups are controlled by multiple alleles two of which are codominant (A
and B) (from http://www.gdn.edu/Faculty/pjen/Biology%201111K%20%20lecture%205%20notes.htm)
Problem: “Roan” coloring in cows also shows a codominent relationship. Red (R) hair is
codominent to white (W). The heterozygous condition (RW) results in a roan color. A roan cow
is crossed with a red bull. What is the probability of a roan offspring?
Parents:
RW x
RR
R
W
R
RR
RW
R
RR
RW
Figure 3: A Roan color pattern is expressed as a
red coat with white splotches. (from
http://www.ccs.k12.in.us/chsTeachers/BYost/Biol
ogy%20Notes/CH11notescoincompletedom.htm)
Genotypic Ratio:
Phenotypic Ratio:
RR
RW
Red 2/4
Roan 2/4
2/4
2/4
50%
50%
Answer: 50% chance of a roan offspring
red 50%
roan 50%
3. Incomplete Dominance – This is sometimes called partial dominance. The heterozygous
condition results in an intermediate (third) blended phenotype. A capital letter is used to
represent one allele and the same capital letter prime represents the other allele.
Figure 4: An example of incomplete dominance showing how red and white
blends to form pink.
http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookgeninteract.html
Problem: Red (R) flower color in snap dragons shows an incomplete dominance relationship to
white (R`). What is the probability of a pink flower from the cross of two pink flowered plants?
Parents:
RR`
x
RR`
R
R`
R
RR
RR`
R`
RR`
R`R`
Genotypic Ratio
Phenotypic Ratio
RR 1/4 25%
RR` 2/4 50%
R`R` 1/4 25%
Red 1/4 25%
Pink 2/4 50%
White 1/4 25%
Answer: 50% chance of producing a pink flowered plant.
Performance Benchmark L.12.A.5
Students know how to predict patterns of inheritance. E/S
Common misconceptions associated with this benchmark:
1. Students do not understand both parents contribute genes for each characteristic. They
believe that one parent contribute genes for some characteristics, while the other features
come from the other parent.
Mendelian inheritance is the mode of inheritance for nearly all multicellular organisms.
Inheritance is controlled by genes, which are passed on to the offspring in the same form as they
were inherited from the previous generation. At each locus, the location on a chromosome of a
particular gene, an individual has two genes one inherited from its father and the other from its
mother. The two genes are represented in equal proportions in its gametes.
More details about gamete formation visit:
http://www.biology.arizona.edu/CELL_BIO/tutorials/meiosis/main.html).
2. Students believe that inherited traits are blended.
Blending inheritance is the erroneous idea that organisms contain a blend of their parent
hereditary factors and pass that blend onto their offspring. Inheritance of certain traits like hair
color, eye color, and skin tone are all controlled by multiple alleles therefore allowing for
variation in color. Incomplete dominance results in an intermediate (third) phenotype. Although
these traits appear to be a blend of inherited traits they are not. Each trait is still determined by
the form of the gene (allele) inherited from parent to offspring.
3. Dominant alleles are generally the most frequently occurring alleles in a population.
Dominance refers to expression of an allele within an individual organism. Prevalence in the
population is determined by fitness and natural selection. A good example to use to demonstrate
this is polydactylism, an anatomic variation where an individual has more than the usual number
of digits on the hand or feet. Polydactylism is a dominant trait yet it is not the most frequently
occurring trait in the human population.
For more details about population genetics, visit:
http://naturalsciences.sdsu.edu/ta/classes/lab2.4/altern.html
4. Dominant alleles are the most desirable ones.
Once again, dominance refers to expression of an allele within an individual organism. The
word “dominant” in layman terms leads to this misconception. The expression, whether
dominant or recessive, that is selective for by the environment is the most desirable. A good
example to use to demonstrate this is polydactylism, an anatomic variation where an individual
has more than the usual number of digits on the hand or feet. Polydactylism is a dominant trait
yet it is not desirable.
Performance Benchmark L.12.A.5
Students know how to predict patterns of inheritance. E/S
Sample Test Questions
Parents Genotypes
Ff (freckled) x
Ff (freckled)
F
f
1
2
3
4
F
f
1. In humans, having freckles (F) is dominant to not having freckles (f). The child represented
in Box “1” in the above Punnett Square would
a. be homozygous for freckles
b. have an extra freckles chromosome
c. be heterozygous for freckles
d. not have freckles
2. Different forms of a gene are known as
a. chromosomes
b. alleles
c. purebred
d. hybrid
3. If an individual possessed two recessive alleles for the same trait, the individual is said to be
a. homozygous for the trait
b. haploid for the trait
c. heterozygous for the trait
d. mutated for the trait
4. A cross of a red cow with a white bull produces all roan offspring. This type of inheritance is
known as
a. codominance
b. incomplete dominance
c. multiple alleles
d. recessive
5. Which statement describes how two organisms may show the same trait, yet have different
genotypes for that phenotype?
a. One is homozygous dominant and the other heterozygous
b. One is homozygous dominant and the other homozygous recessive
c. Both are heterozygous for the dominant trait
d. Both are homozygous for the dominant trait.
6. In a certain species of wasp, the gene for long mandibles (M) is dominant over the gene for
short mandibles (m). If a biologist wants to produce wasps with long mandibles only, which
wasps should be crossed?
a. heterozygous long-mandibled wasps with heterozygous long-mandibled wasps
b. homozygous short-mandibled wasps with homozygous short-mandibled wasps
c. heterozygous long-mandibled wasps with homozygous short-mandibled wasps
d. homozygous long-mandibled wasps with heterozygous long-mandibled wasps
7. In certain rats, black fur is dominant over white fur. If two rats, both heterozygous for fur
color, are mated, their offspring would be expected to have
a. three different genotypes and three different colors
b. four different genotypes and two different colors
c. two different genotypes and three different colors
d. three different genotypes and two different colors
8. In humans polydactylism, having more fingers or toes than usual, is a dominant trait. Why is
it that most individuals have five fingers and toes which is the recessive trait?
a. Having five fingers and toes is selected for by the environment, therefore, the trait
is found in a higher frequency.
b. Polydactyl is the dominant trait, therefore, polydactylism will have the highest
frequency.
c. The dominant trait will always be more frequent.
d. Recessive traits are weaker than the dominant trait.
Performance Benchmark L.12.A.5
Students know how to predict patterns of inheritance. E/S
Answers to Sample Test Questions
1.
2.
3.
4.
5.
6.
7.
8.
(a)
(b)
(a)
(a)
(a)
(d)
(d)
(a)
Performance Benchmark L.12.A.5
Students know how to predict patterns of inheritance. E/S
Intervention Strategies and Resources
The following is a list of intervention strategies and resources that will facilitate student
understanding of this benchmark.
1. Lessons, quizzes and activities on genetics:
“This is a website for high school science students. You'll find a variety of interactive quizzes,
games, and puzzles to practice what you're learning.” The homepage www.zerobio.com is a
portal to interactive tutorials on many topics in high school biology. The “Java Genetics” has a
variety of interactive Mendellian genetics problems. The “Drag and Drop Genetics” program
gives students the opportunity to drag and drop alleles to complete monohybrid crosses. It also
allows students to check their own answers.
The following link is available and specific to this benchmark:
http://www.zerobio.com/drag_gr11/mono.htm
2.
Genetics practice problems:
“The following links will take you to pages containing genetics practice problems. Students who
already understand the material can use these questions to review. Students who are having
trouble understanding how to work all or just some of the problems are provided with hints that
will take you stepwise through the problem. Additionally, you can use these pages for
vocabulary review. As you go through the problems, if you are unsure of the meaning one or
more of the terms highlighted in blue, just click on it and you will be taken to a definition page.”
http://www.ksu.edu/biology/pob/genetics/intro.htm
3. Punnett Squares and online manipulation of alleles:
This website has wonderfully interactive web labs on “Mendel’s Peas”, “Incomplete Dominance
and Codominance”, and “Punnett Squares”. Students can even explore the hereditary of
imaginary dragons by manipulating alleles.
http://www2.edc.org/weblabs/WebLabDirectory1.html
4. Human blood type web tutorial:
This is a human blood type web based tutorial. Articles are provided to teach/reinforce the
classroom content on human blood type genotypes, phenotypes and the concept of multiple
alleles. It also has a blood type calculator where students can enter child/parent or parent/parent
blood types to see if the combination is possible. Lastly, the web tutorial has a “problem set that
is based on questions received from real people who have real questions about their parents or
about the father of their child or grandchild.”
http://www.biology.arizona.edu/human_bio/problem_sets/blood_types/Intro.html
5. The following websites are good articles about Gregor Mendel to promote content area
literacy
http://www.mnsu.edu/emuseum/information/biography/klmno/mendel_gregor.html
http://www.ndsu.nodak.edu/instruct/mcclean/plsc431/mendel/mendel1.htm
http://anthro.palomar.edu/mendel/mendel_1.htm
http://www.zephyrus.co.uk/gregormendel.html
For higher level readers,
http://astro4.ast.vill.edu/mendel/gregor.htm