Content Benchmark L.12.A.5
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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

3. Construct the Punnett Square

4. Complete the Punnett square

5. Calculate genotypic and phenotypic ratios.

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.

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?

Answer: 50% chance of a roan offspring

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`

Genotypic Ratio RR 1/4 25% RR` 2/4 50% R`R` 1/4 25%	Phenotypic Ratio Red 1/4 25% Pink 2/4 50%
White 1/4 25% Answer: 50% chance of producing a pink flowered plant.

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Content 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.

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Content Benchmark L.12.A.5

Students know how to predict patterns of inheritance. E/S

1st Item Specification: Explain how reproduction is responsible for genetic variation.

Depth of Knowledge Level 1

1. Mendel’s second principle states that the inheritance of one characteristic will not affect the inheritance of another characteristic.  What is the principle called?
1. Mutation
2. Fertilization
3. Crossing over
4. Independent assortment

2. Which statement describes how two organisms may show the same trait, yet have different genotypes for that phenotype?
   1. One is homozygous dominant and the other is heterozygous.
   2. One is homozygous dominant and the other is homozygous recessive.
   3. Both are heterozygous for the dominant trait
   4. Both are homozygous for the dominant trait.

3. Sexual reproduction contributes to
1. less genetic variation within a population.
2. more identical genotypes within a population.
3. greater genetic variation within a population.
4. greater mutation rate within a population.

Depth of Knowledge Level 2

4. Which of the following genotypes has the potential for the greatest genetic variation in the offspring?
1. Homozygous dominant crossed with homozygous recessive.
2. Heterozygous crossed with homozygous recessive.
3. Homozygous dominant crossed with homozygous dominant.
4. Heterozygous crossed with another heterozygous individual.

5. In pea plants, yellow pea pods are dominant to green pea pods and round-shaped pods are dominant to wrinkled pods. How do the offspring of two plants that are heterozygous for yellow, round pods result in four different phenotypes? Is it the result of
1. random alignment of chromosomes during meiosis.
2. random fertilization during sexual reproduction.
3. crossing over between chromosomes during meiosis.
4. mutation in the DNA of the gametes.

2nd Item Specification: Use a Punnett Square to predict the proportion of specific genetic traits in offspring.

Depth of Knowledge Level 1

6. In pea plants, yellow is a dominant trait and green is a recessive trait. In a genetic cross of two plants that are heterozygous for the pea color trait, what percent of the offspring should have yellow peas?
1. 100%
2. 75%
3. 50%
4. 25%

7. Use the Punnett Square below to answer the following question.

In humans, having freckles is dominant to no freckles. Which of the following statements BEST describes the child represented in box 1 in the above Punnett Square?

1. Homozygous for freckles.
2. Homozygous for extra freckles.
3. Heterozygous for freckles.
4. Heterozygous for no freckles.

8. In humans, having freckles is dominant to no freckles. In a genetic cross of a homozygous dominant parent with a homozygous recessive parent, what percent of the offspring should have freckles?

1. 100%
2. 75%
3. 50%
4. 25%

Depth of Knowledge Level 2

9. In certain rats, black fur is the dominant trait while white fur is the recessive trait.  If two heterozygous rats are mated, their offspring would be expected to have

1. three different genotypes and three different phenotypes.
2. four different genotypes and two different phenotypes.
3. two different genotypes and three different phenotypes.
4. three different genotypes and two different phenotypes.

10. In human polydactylism, having more than five fingers on one hand, is a dominant trait. A man who has five fingers on each hand marries a woman who is polydactyl.  The couple has 8 children, only 3 of the children are polydactyl.  What is the genotype of mother?

1. Homozygous dominant
2. Heterozygous
3. Homozygous recessive
4. Hemizygous

Content Benchmark L.12.A.5

Students know how to predict patterns of inheritance. E/S

Answers to Sample Test Questions

1. D, DOK Level 1
2. A, DOK Level 1
3. C, DOK Level 1
4. D, DOK Level 2
5. A, DOK Level 2
6. B, DOK Level 1
7. C, DOK Level 1
8. A, DOK Level 1
9. D, DOK Level 2
10. B, DOK Level 2

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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

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