CHAPTER 12: PATTERNS OF INHERITANCE
WHERE DOES IT ALL FIT IN?
Chapter 12 applies the information on meiosis covered in Chapter 11 to the principles of classical
inheritance. Students are likely to have many misconceptions about inheritance patterns and they
usually do not connect meiosis to inheritance. It is important to reinforce to students the goals and
outcomes of meiosis before starting this chapter. Chapter 12 will serve as an important reference for
Chapter 13.
SYNOPSIS
Early geneticists believed that genetic material from each parent blended in the offspring and that
variability was not introduced from outside the species. Blending and lack of variability, though,
should result in individuals that greatly resemble rather than differ from one another. This
paradox was partly solved by early plant breeders who found that hybrids differed greatly from
their parents and often from one another. They reported that certain physical traits disappeared
for a generation and reappeared in the next. Gregor Mendel cross-bred seven well-documented
varieties of a pea. Most importantly, he quantified his experiments, meticulously counted seeds
of hundreds of crosses and grouped them by apparent physical traits. Mendelian genetics is
derived from the mathematical ratios that describe the segregation and assortment of hereditary
material.
Mendel’s model states that each parent transmits a set of information about its traits in its
gametes. Therefore, each individual possesses two factors (genes) for each trait. Each factor
exhibits many possible forms (alleles) that do not influence one another; each remains discrete
within the cell. An individual may be homozygous and possess two identical alleles, or
heterozygous and have two different alleles. The presence of a factor does not ensure its
expression; dominant traits are expressed while recessive traits are generally not expressed. The
existence of the recessive allele in a heterozygote causes that factor to be masked for a
generation. Additionally, there is a difference between an individual’s phenotype, or overall
appearance, and its genotype, its precise genetic blueprint. Mendel’s First Law of Heredity
explains how alleles randomly segregate in the gametes, each gamete has an equal chance of
receiving either allele. His second law explains that different alleles assort into gametes
independently of one another, the presence of an allele of one trait does not preclude the
presence or absence of any other allele of any other trait.
LEARNING OUTCOMES
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Understand the historical background for Mendel’s pea experiments.
Know the key details in Mendel’s experiments that enabled him to postulate his laws of
inheritance where others had failed.
State Mendel’s model of heredity and Sutton’s theory of chromosomal inheritance.
Understand how gene segregation and independent assortment are different but yet related.
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Calculate expected phenotypic and genotypic ratios from various crosses using the Punnett
square method.
Explain the experimental rationale behind the classical testcross. Explain how Mendelian
inheritance changes with respect to continuous variation, pleiotropic genes, lack of complete
dominance, environmental modifications of genes, and epistasis
COMMON STUDENT MISCONCEPTIONS
There is ample evidence in the educational literature that student misconceptions of information
will inhibit the learning of concepts related to the misinformation. The following concepts
covered in Chapter 12 are commonly the subject of student misconceptions. This information on
“bioliteracy” was collected from faculty and the science education literature.
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Students believe that sexual reproduction is merely for increasing populations
Students have trouble connecting the events of meiosis with germ cell formation
Students have trouble connecting the events of meiosis with patterns of inheritance
Students think that traits skip generations
Students do understand that a Punnett square represents offspring probabilities
Students believe that gender in all organisms is determined by X and Y chromosomes
Students confuse the roles of autosomes and sex chromosomes
Students do not associate gene expression with inherited characteristics
Students believe sexual reproduction always involves mating
Students do not understand other mechanisms of sexual reproduction besides mammalian
reproduction
Students do not fully understand the role of genetics and environment on determining
observable variation in organisms
Students believe acquired characteristics can be inherited
Students think that all genetic disorders are homozygous recessive
Students believe that inbreeding causes genetic defects
Students do not take into account the role of crossing over in classical inheritance
variation
Students believe that chromosomes are segregated into gametes that contain either a pure
maternal or pure paternal homologous sets
INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE
Mendelian genetics is one of the classic discussions in introductory biology. Most students are
introduced to this in high school, but few really understand what is meant by segregation and
independent assortment. Segregation of alleles is hard to visualize without a good understanding
of meiosis. One can truly respect Mendel’s scientific ability when realizing that neither
chromosomes nor meiosis had been discovered yet.
The most frequent mistake a beginner makes in calculating a dihybrid cross is putting the two
alleles for the same trait in a single gamete. Make them separate (segregate) those letters! A
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normal gamete can have only one of each allele.
There is a lot of new terminology associated with genetics. Homozygous and heterozygous are
frequently confused as are phenotype and genotype and, for some strange reason, allele and
locus. Again, understanding the meanings of prefixes, suffixes, and root words helps
enormously. A background in a romance language has enormous benefits.
Many of your students are medical-school bound and relate to the topic of this chapter because
they see its direct application to their futures. Examining one’s own genetic background is to
some extent health-oriented fortune telling. By studying the ailments of one’s parents,
grandparents, and other relatives, a picture of one’s own future begins to develop. Most serious
diseases are not under strict genetic control, but research indicates a strong genetic component in
many, including cancer and heart disease.
HIGHER LEVEL ASSESSMENT
Higher level assessment measures a student’s ability to use terms and concepts learned from the
lecture and the textbook. A complete understanding of biology content provides students with the
tools to synthesize new hypotheses and knowledge using the facts they have learned. The
following table provides examples of assessing a student’s ability to apply, analyze, synthesize,
and evaluate information from Chapter 11.
Application
Analysis
Synthesis
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Have students predict a monohybrid cross Punnett square for a simple
trait in their family.
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Have students explain why a family who had four females children in a
role sit have an equal change of having a boy or a girl as the next child.
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Ask students explain the relationship between meiosis and the assignment
of alleles in a Punnett square.
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Ask students to hypothesize why inbreeding populations are likely to have
an abundance or a lack of genetic disorders.
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Ask students explain the effects of a 4N complement of DNA on the
expression of traits.
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Ask students to explain why certain characteristics appear very rarely in a
population of organisms.
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Ask students to come up with a reason why gender in alligators does not
follow the predicted Mendelian pattern of inheritance.
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Have students explain why a certain dominant characteristic only appears
in male offspring of an organism and does not show up in females.
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Evaluation
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Ask students to explain how child of a father with type AB blood and
mother with type O blood was born with type O blood.
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Ask students evaluate the benefits of drugs claiming to slow down the
genetic progression of aging.
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Ask students to discuss the pros and cons of inbreeding crops and
agricultural animals.
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Ask students to evaluate the impact of crossing over during meiosis on
polygenic traits.
VISUAL RESOURCES
Many different kinds of apparatus are available to illustrate Mendelian genetics, including
modeling clay and pop-it beads. Several very sophisticated bead kits are sold through biological
supply houses; unfortunately, most are too small to be useful in a class larger than twenty-five
students.
Much of the visual material is better handled in the lab, after initial exposure to the basics in the
lecture. Try to keep the genetics-oriented lab instructors from showing too many of their own
short-cuts. Have them stick to the old Punnett square. Students that derive short cuts on their own
may gain a better understanding of the material.
IN-CLASS CONCEPTUAL DEMONSTRATIONS
A. Animated Karyotype
Introduction
This fun and fast demonstration engages students in developing a human karyotype. The
click and drag animation allows the instructor to interact with students while selecting
chromosomes to build a karyotype diagram.
Materials
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Computer with live access to Internet
LCD projector attached to computer
Web browser with bookmark to Karyotype Animation at:
http://www.gla.ac.uk/medicalgenetics/nhs/karyotypemovie.htm
Procedure & Inquiry
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Introduce the concept of karyotyping
Pull up the Karyotype Animation website
Touch the cursor to a chromosome and watch where it ends up on the karyotype diagram
See if the students can match it with the homologous chromosome
Repeat several times until the diagram is complete
Ask the students to describe the features used to match up the pairs of homologous
chromosomes
B. Virtual Punnett Practice
Introduction
This demonstration uses an on-line animated Punnett square to review the calculation of
offspring probabilities. It immediately draws the Punnett squares for monohybrid and dihybrid
crosses. In addition, it gives the offspring probability ratios. The animation is useful for in-class
formative evaluation of Mendelian inheritance.
Materials
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Computer with live access to Internet
LCD projector attached to computer
Web browser with bookmark to Punnett Square Calculator at:
http://www.changbioscience.com/genetics/punnett.html
Sheets of writing paper for students
Procedure & Inquiry
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Introduce the topic of meiosis and how it related to Punnett squares.
Pull up the Punnett Square Calculator
Pick a simple monohybrid cross from the drop-down windows
Ask the students to write the Punnett square for cross
Then show the cross
Repeat this with several crosses while questioning and surveying students about their
ansswers
USEFUL INTERNET RESOURCES
1. Breast cancer is an excellent teaching model for evaluating the Mendelian inheritance of
genetic disorders. It also evaluates other factors that contribute to the expression of genes.
The Program on Breast Cancer and Environmental Risk Factors at Cornell University has
an informative website looking a breast cancer risk factors. This website can be found at
http://envirocancer.cornell.edu/.
2. Cold Spring Harbor Laboratory provides a valuable website for teaching the basic
genetics needed to understand classical patterns of inheritance. It provides many
animated discussions and videostreams of Mendelian genetics. The website can be found
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at http://www.dnaftb.org/dnaftb/.
3. Animations are a pedagogical tool known to reinforce the teaching of complex concepts
such as meiosis. This website hosted by PBS provides a useful narrated movie of
animated meiosis. This website is available at:
http://www.pbs.org/wgbh/nova/baby/divi_flash.html.
4. Case studies are a wonderful way to reinforce complex concepts related to classical
genetics. A case study provided by the University of Buffalo uses skin color as a model
for understanding the outcomes of polygenetic traits. The website can be found at
http://www.sciencecases.org/skin_color/skin_color_notes.pdf.
LABORATORY IDEAS
The mathematical calculation of Mendelian offspring probabilities is best reinforced
when students are able to see the outcomes of genetic crossing. This activity uses genetic corn as
a model for investigating the probabilities of various dihybrid crossing.
a. Provide a group of students with the following materials without tell them anything about
the genetic nature of the corn ears:
a. Ear of pure smooth yellow corn
b. Ear of pure yellow wrinkly corn
c. Ear of heterozygous X heterozygous corn cross - purple/yellow: smooth/wrinkled
d. 4 X 4 Punnett square diagram as shown below:
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e.
b. First ask the students to explain the differences between the three ears of corn.
c. Have students discuss which corn is typical of edible corn and whether that is the natural
characteristics that would be found in a large population of corn left to grow in the wild.
d. Instruct the students to use the Punnett square to calculate the offspring probabilities of
breeding two corn parents heterozygous for kernel color and shape. Provide students with
the following information:
a. Purple - P (dominant)
b. Yellow - p (recessive)
c. Smooth - S (dominant)
d. Wrinkly - s (recessive)
They should calculate a 9:3:3:1 ratio
e. Now tell the students if they believe the purple/yellow corn has a 9:3:3:1.
f. Ask them how they would determine this using the corn given to them.
g. Direct the students to count the different types of kernels on the purple/yellow corn. They
should be questioned to see if they recognize the four different types of kernels
h. Have the students determine how close they came to a 9:3:3:1 ratio from counting kernels
on the purple/yellow ear of corn. They should record their information on a table such as
provided below:
Phenotype:
_______ ______ _______ _______
Number:
_______ _______ _______ _______
Ratio:
______
:
______ : ______ : ______
i. Have the students hypothesize the genotypes of the yellow smooth and yellow wrinkly
corn. Ask them which corn will always produce pure lineages of offspring that resemble
the parents. Also ask the students how the corn sold in groceries stores is bred to have its
characteristics.
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LEARNING THROUGH SERVICE
Service learning is a strategy of teaching, learning and reflective assessment that merges the
academic curriculum with meaningful community service. As a teaching methodology, it falls
under the category of experiential education. It is a way students can carry out volunteer projects
in the community for public agencies, nonprofit agencies, civic groups, charitable organizations,
and governmental organizations. It encourages critical thinking and reinforces many of the
concepts learned in a course.
1. Have students present a forum on the benefits and risks of monoculture to a civic group.
2. Have students design an educational animated PowerPoint presentation about Mendelian
genetics for middle school teachers.
3. Have students tutor middle school or high school biology students studying classical
genetics.
4. Have students present a talk to elementary students about the inheritance of genetic
disorders.
This project is funded by a grant awarded under the President’s Community Based Job Training Grant as implemented by the U.S.
Department of Labor’s Employment and Training Administration (CB-15-162-06-60). NCC is an equal opportunity employer and
does not discriminate on the following basis:
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against any individual in the United States, on the basis of race, color, religion, sex, national origin, age disability, political
affiliation or belief; and
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against any beneficiary of programs financially assisted under Title I of the Workforce Investment Act of 1998 (WIA), on
the basis of the beneficiary’s citizenship/status as a lawfully admitted immigrant authorized to work in the United States, or
his or her participation in any WIA Title I-financially assisted program or activity.
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