Skip to main content
Biology LibreTexts

4.2: Genetics Teacher Preparation Notes

  • Page ID
  • Overview

    This activity helps students to understand the basic principles of genetics, including:

    • How genotype influences phenotype via the effects of DNA on the synthesis of proteins
    • How genes are transmitted from parents to offspring through the processes of meiosis and fertilization.

    The Student Handout includes three modules:

    • An introductory module with an analysis of albinism that teaches both the relationship of genotype to phenotype (including the concepts of dominant and recessive alleles) and how genes are inherited by meiosis and fertilization (including understanding Punnett squares); this module uses model chromosomes and is designed for use after "Meiosis and Fertilization – Understanding How Genes Are Inherited" (available at
    • A Coin Toss Genetics activity that helps students understand the probabilistic nature of inheritance and Punnett square predictions
    • An analysis of the inheritance of sickle cell anemia that introduces the important points that a single gene often has multiple phenotypic effects and alleles are often neither completely dominant nor completely recessive.

    The Genetics Supplement includes three modules:

    • An alternative version of the introductory module with an analysis of albinism that does not use model chromosomes
    • An analysis of student data on the sex makeup of sibships that helps students understand the probabilistic nature of inheritance and Punnett square predictions; this module can be used instead of the Coin Toss Genetics activity
    • Pedigree analyses for recessive and dominant alleles, including challenge questions that introduce the role of new mutations and engage students in evaluating the advantages and disadvantages of Punnett squares and pedigrees as models of inheritance.

    We recommend that you start with either version of the introductory module. After that, the other modules can be used in whatever combination and sequence best meets your learning goals for your students. Before beginning this activity, your students should have a basic understanding of meiosis and fertilization. For this purpose, we recommend the hands-on activity "Meiosis and Fertilization – Understanding How Genes Are Inherited" (available at These activities are part of an integrated sequence of learning activities for teaching genetics, presented in "Genetics – Major Concepts and Learning Activities" (available at


    Learning Goals

    How Genotype Influences Phenotype

    • A gene provides the instructions for making a protein. Different versions of a gene are called alleles; different alleles give the instructions for making different versions of a protein. The different versions of a protein can result in different phenotypic characteristics.
    • Each cell has two copies of each gene. If both copies of the gene have the same allele, the individual is homozygous for that gene. If a person is heterozygous (has two different alleles for a gene), often one (dominant) allele affects the phenotype and the other (recessive) allele does not. In other cases, neither allele is completely dominant or completely recessive.
    • Recessive alleles often code for non-functional proteins; the dominant allele codes for enough functional protein to ensure the normal phenotype.
    • A single gene may influence more than one phenotypic characteristic.

    Meiosis and Fertilization → Inheritance

    The behavior of chromosomes during meiosis and fertilization provides the basis for understanding the inheritance of genes.

    The combination of meiosis and fertilization results in each offspring having one copy of each gene from his or her mother and another copy of each gene from his or her father. Consequently, children tend to resemble their parents and their siblings.

    However, meiosis results in genetically diverse sperm and eggs which, together with random fertilization, result in genetic diversity of the zygotes and children produced by the same mother and father.

    Punnett Squares → Probabilistic Predictions of Inheritance

    • The processes of meiosis and fertilization can be summarized in Punnett squares to make predictions about the genotypes and phenotypes of offspring.
    • These predictions are accurate for large samples, but random variation in the genetic makeup of the sperm and egg that unite to form each zygote often results in substantial discrepancies between the Punnett square predictions and the outcomes observed in small samples such as individual families.
    • Each fertilization event is independent of other fertilization events, so the genetic makeup of each child is independent of the genetic makeup of any siblings.
    • Punnett squares are a useful model for understanding how meiosis and fertilization result in the various possible genotypes and phenotypes of offspring. Pedigrees are a useful model for analyzing inheritance in individual families. (See page 9 of these Teacher Preparation Notes for further discussion of the advantages and disadvantages of Punnett squares and pedigrees as models of inheritance.)

    Learning Goals Related to National Standards

    In accordance with the Next Generation Science Standards:

    • Students will gain an understanding of several Disciplinary Core Ideas:
    • LS1.A: Structure and Function – "All cells contain genetic information in the form of DNA molecules. Genes are regions in the DNA that contain the instructions that code for the formation of proteins."
    • LS3.A: Inheritance of Traits – "Each chromosome consists of a single very long DNA molecule, and each gene on the chromosome is a particular segment of that DNA. The instructions for forming species' characteristics are carried in DNA."
    • LS3.B: Variation of Traits – In sexual reproduction, meiosis can create new genetic combinations and thus more genetic variation.

    • Students will engage in several Scientific Practices:
    • developing and using models
    • analyzing and interpreting data
    • constructing explanations
    • engaging in argument from evidence.
    • This activity provides the opportunity to discuss the Crosscutting Concept, "Systems and system models".
    • This activity helps to prepare students for the Performance Expectations
    • HS-LS3-1, "Ask questions to clarify relationships about the role of DNA and chromosomes in coding the instructions for characteristic traits passed from parents to offspring."
    • HS-LS3-2, "Make and defend a claim based on evidence that inheritable genetic variations may result from: (1) new genetic combinations through meiosis..."
    • HS-LS3-3, "Apply concepts of statistics and probability to explain the variation and distribution of expressed traits in a population."

    The sickle cell anemia module will also help students meet Common Core English Language Arts Standards for Science and Technical Subjects, including "cite specific textual evidence to support analysis of science and technical texts" and "write arguments focused on discipline-specific content".



    For "Inheritance of Albinism" in the introductory module in "Genetics"

    • Model Chromosomes (This module is designed for use after "Meiosis and Fertilization – Understanding How Genes Are Inherited" (available at i_edu/waldron/#meiosis). Instructions for making the model chromosomes are provided on pages 3-4 in the Teacher Preparation Notes for that activity. You will need the modified model chromosomes described in the paragraph that begins on page 3 and ends on the top of page 4.)
    • Chalk or tape to outline rectangles (See page 2 of the Student Handout.)

    For the "Coin Toss Genetics" module in the Student Handout:

    • Pennies (or checkers) (1 per student)
    • Paper cup (optional, 1 per student; having each student shake a coin in a paper cup may result in more random tossing and less chance of coins on the floor)

    General Instructional Suggestions

    In the Student Handout, numbers in bold indicate questions for the students to answer.

    If you use the Word version of the Student Handout to make changes, please check the PDF version to make sure that all figures and formatting are displayed properly in the Word version on your computer.


    To maximize student learning, we recommend that you have your students, complete groups of related questions in the Student Handout individually or in pairs and then have a class discussion of these questions. In each discussion, you can probe student thinking and help them to develop a sound understanding of the concepts and information covered before moving on to the next part of the activity.

    If you would like to have a key with the answers to the questions in the Student Handout, please send a message to The following paragraphs provide additional background information.


    Instructional Suggestions and Background Biology for Genetics Student Handout

    A. The Introductory Module on Pages 1-3 of the Genetics Student Handout

    They introduce students to both basic principles of genetics:

    • How genotype influences phenotype via the effects of DNA on the synthesis of proteins
    • How genes are transmitted from parents to offspring through the processes of meiosis and fertilization.

    On page 1 of the Student Handout, you will probably want to emphasize how the table shows the effects of genotype on proteins which in turn influence phenotype.


    This section includes a definition of a gene as a segment of DNA that gives the instructions for making a protein. A more sophisticated contemporary definition of a gene is a segment of DNA that codes for an RNA molecule, which may be messenger RNA that codes for the sequence of amino acids in one or more proteins, ribosomal RNA, transfer RNA or regulatory RNA. There is no single universally agreed-upon definition of a gene at this time. As you probably know, the definition of a gene has changed as scientific understanding has progressed. Initially, a gene was conceived as a unit of heredity that determines a phenotypic characteristic. The changing definition of a gene illustrates the constantly evolving nature of science as scientists develop a progressively more sophisticated understanding of concepts such as the gene. For additional information about the challenges and complexities of defining a gene, see .


    The allele for albinism is recessive because it codes for a defective enzyme for producing melanin, while the normal allele codes for the functioning enzyme, and even when there is only one copy of the normal allele there is enough of this functioning enzyme to produce enough melanin to prevent albinism. Recessive alleles often code for a non-functional protein, while dominant alleles often code for a functional protein.

    On page 2 of the Student Handout, students are instructed to draw the rectangles from this chart on their lab table with chalk. Or you may prefer to provide them with tape instead of chalk. This will help students to carry out the fertilization part of the activity in a systematic manner.


    As students model meiosis and fertilization for two heterozygous parents, they should notice that a heterozygous zygote can arise in two different ways (dominant allele from mother or from father). This observation should help students understand why the heterozygous genotype is twice as likely as either homozygous genotype.


    The second page of the Student Handout is designed to foster student understanding of how meiosis and fertilization result in inheritance of genes (one copy of each gene from the mother and one copy of each gene from the father). In interpreting Punnett squares, it is important for students to realize that the person who develops from a zygote has the same genetic makeup as the zygote. The zygote undergoes many rounds of mitosis to produce the cells in the person's body, and mitosis produces daughter cells with the same genetic makeup as the original cell (see question 8 on the second page of the Genetics Student Handout).


    Questions 8-10 engage students in analyzing examples that illustrate how inheritance contributes to the tendency of children to resemble their parents.


    Question 11 should stimulate students to realize that parents with the phenotype of a recessive allele must be homozygous for the recessive allele and therefore won't have a child with the dominant allele (unless there is a new mutation). In contrast, two parents with the phenotype of the dominant allele may both be heterozygous so they could have a child who has inherited two copies of the recessive allele and has the associated phenotype. These insights are crucial for pedigree analysis.


    In the most common form of albinism, the lack of the pigment melanin affects not only skin and hair color, but also the appearance and function of the eyes. For additional information about the various forms of albinism see and OMIM = Online Mendelian Inheritance in Man (available at;search for 606952 (oculocutaneous albinism)).


    Students may ask questions concerning the distinction between inherited albinism and vitiligo. Albinism is the inability of the body's cells to produce melanin and affects the whole body. Vitiligo is a patterned loss of melanin pigment resulting from the destruction of melanocytes; the hypopigmented areas appear on the skin of a person with normal pigmentation. (Additional information is available at


    B. The Coin Toss Genetics Module

    This helps students understand the importance of random variation, particularly in small samples. Discussion of random variation will help your students to reconcile their experience of variation in outcomes in real-world families with the predictions of Punnett squares in the classroom. This module also introduces students to the independence of each fertilization event, so the genotype of each child is independent of the genotypes of any older siblings.

    Students will observe that results for an individual family of 4 "coin toss children" often deviate substantially from the results predicted by the Punnett square. The table below illustrates the high probability that the genotypes of 4 children born to two heterozygous parents will differ from the predictions of the Punnett square.

    Observed Outcome for 4 Coin Tosses Probability
    aa 32%
    aa 42%
    2 or more aa 26%
    AA + 2 Aa + 1 aa 19%


    When your students carry out the coin tosses to create 4 families of 4 children each, there is a 78% probability that they will get at least one family with no albino (aa) children and a 70% probability that they will get at least one family with 2 or more albino children.

    The results for larger samples are generally closer to the predicted distribution and less likely to show extreme deviations. For example, for two heterozygous parents a finding of no albino children is expected in 32% of families of 4 children, but in only 1% of samples of 16 children, and less than one in a million samples of 100 children (which should be roughly the size of your sample for the total class data for the "coin toss children").


    C. The Genetics of Sickle Cell Anemia Module:

    • Introduces a basic understanding of the biology of sickle cell anemia (additional information given below)
    • Reinforces some basic concepts of inheritance (question 2) and
    • Uses student analysis of the boxed reading on page 6 to extend their understanding of genetics by introducing two complexities that are common in genetics:
    • Multiple phenotypic effects of a single gene (question 3)
    • Many alleles are neither completely dominant nor completely recessive, as illustrated by the fact that the phenotype of an individual who is heterozygous for the sickle cell and normal hemoglobin alleles is not the same as the phenotype of an individual who is homozygous for either of these alleles (question 4).

    Sickle cell hemoglobin is less soluble in the watery cytosol of the red blood cells than normal hemoglobin, particularly when oxygen concentrations are low. Thus, sickle cell hemoglobin tends to clump into long stacks or rods of hemoglobin molecules; this results in the sickled and other abnormal shapes of some of the red blood cells in a person who is homozygous for the sickle-cell allele. The abnormally shaped red blood cells tend to clog the capillaries, thus blocking the circulation in various parts of the body. Also, these red blood cells do not survive as long as normal red blood cells, contributing to a tendency to anemia. Together, these effects result in the multiple symptoms of sickle cell anemia, including pain, physical weakness, impaired mental functioning, and damage to organs such as the heart and kidneys.


    Even in a person who has severe sickle cell anemia, most red blood cells are not sickled. The degree of clumping of sickle cell hemoglobin, sickling of red blood cells, and consequent symptoms are influenced by multiple factors, including oxygen levels in the blood, dehydration, and other genes. A sickling crisis with pain and organ damage can be triggered by an infection that induces vomiting and diarrhea, resulting in dehydration; dehydration increases the hemoglobin concentration in red blood cells and thus increases the tendency of sickle cell hemoglobin to clump into long rods and produce sickled red blood cells which block the circulation in the small blood vessels. These observations illustrate how the environment and genotype interact to influence phenotype. A useful summary of the medical aspects of sickle cell anemia, including symptoms, diagnosis, and treatment is available at For additional information, see OMIM (Online Mendelian Inheritance in Man (; search for 603903 (sickle cell anemia))


    In a person who is heterozygous for the sickle cell and normal hemoglobin alleles, each red blood cell has both sickle cell and normal hemoglobin. The amount of normal hemoglobin is sufficient to prevent the symptoms of sickle cell anemia in almost all cases. The sickle cell hemoglobin in each red blood cell decreases the severity of malaria in heterozygous individuals because the malaria parasite doesn't grow as well in red blood cells containing sickle cell hemoglobin. (The heterozygous individual is said to have sickle cell trait.)


    Malaria infections are common in many tropical countries where there are lots of the type of mosquitoes that transmit the malaria parasite. In areas where malaria is widespread, people who are heterozygous for the sickle cell allele are less likely to become seriously ill and die. Because the sickle cell allele contributes to increased survival of heterozygous individuals, this allele became relatively common in regions like West Africa where malaria is common. Since African-Americans are descended from populations in which the sickle cell allele was relatively common, African-Americans have relatively high rates of the sickle cell allele (approximately 8% are heterozygous for this allele and 0.16% are homozygous).


    Instructional Suggestions and Background Biology for the Genetics Supplement

    A. The First Four Pages of the Genetics Supplement

    They can provide an alternative version of the introductory module in the Genetics Student Handout (see background information and instructional suggestions on pages 4-5 of these Teacher Preparation Notes). The Genetics Supplement version will be appropriate if your students have not completed "Meiosis and Fertilization – Understanding How Genes Are Inherited" (available at and/or you do not want to use model chromosomes.


    As students complete question 5 in this module, they should realize that a heterozygous zygote can arise in two different ways (dominant allele from mother or from father). This should help your students understand why they heterozygous genotype is twice as likely as either homozygous genotype.


    B. The Genetics of Sex Determination Module

    This can be used in place of the Coin Toss Genetics module in the Genetics Student Handout or together with the Coin Toss Genetics module to reinforce student understanding of the probabilistic nature of inheritance and Punnett square predictions.


    If your class is sex-biased, you should modify the instructions to prevent biased results due to whatever factors have resulted in a preponderance of males or females in your class. Specifically, the students should exclude themselves when answering questions 4-5 and just count all of their siblings and step-siblings born to their mother, each of whom represents an independent fertilization event and thus should be unaffected by whatever bias has affected enrollment in your class.


    The table below shows the expected ranges of results for different sample sizes. Even with relatively large samples, a rather substantial variation from one class to the next will be relatively common.

    Number of Children For All the Mothers in a Class

    If data were collected for a large number of classes,

    68% of the results are expected to be in this range:

    95% of the results are expected to be in this range:

    20 39%-61% males 28%-72% males
    40 42%-58% males 34%-66% males
    60 43.5%-56.5% males 37%-63% males
    80 44.4%-55.6% males 39%-61% males


    It should be mentioned that these ranges have been calculated based on several simplifications. Specifically, we have not taken into account the fact that slightly more males than females are born (51% males in the US, slightly lower for African-Americans and slightly higher for Asian-Americans). Also, there appears to be some biological tendency for some couples to produce more female or more male offspring, and this would increase expected variation in results.


    The Y chromosome contains the SRY gene which codes for a protein that binds to regulatory DNA and activates multiple genes that stimulate the gonads to develop into testes instead of ovaries. The testes secrete testosterone and other chemical messengers that stimulate the genitalia to develop into penis, scrotum, vas deferens, etc. In the absence of the SRY gene, the gonads develop into ovaries, and in the absence of testosterone, the genitalia develops into the clitoris, labia, uterus, etc. Additional genes on multiple chromosomes contribute to the normal development of male and female reproductive organs.


    Students often ask questions concerning the various anomalies in sexual development. Some of these are due to too many or too few copies of the sex chromosomes in each cell, e.g. Kleinfelter and Turner Syndromes (see,, and Teacher Notes for "How Mistakes in Cell Division Can Result in Down Syndrome and Miscarriages", available at It should be noted that a zygote must have at least one X chromosome to survive and develop into an embryo.


    Some other anomalies in the development of male or female reproductive organs result from defective hormone receptors or defective enzymes to produce hormones:

    • Androgen Insensitivity Syndrome results from a lack of functional molecular receptors for testosterone and dihydrotestosterone. Due to the lack of these molecular receptors, testosterone and dihydrotestosterone do not affect the cells in the fetal genitalia of an XY fetus with Androgen Insensitivity Syndrome, so female external genitalia develop. These individuals are raised and live as females, but they have testes instead of ovaries. They are infertile. This syndrome is typically detected when a teenage female fails to menstruate.
    • Congenital Adrenal Hyperplasia (also called Adrenogenital Syndrome) develops when an enzyme needed to produce cortisol is defective or missing, resulting in abnormal hormonal feedback which leads to excessive production of androgens by the adrenal cortex. The elevated androgen levels in a XX fetus result in varying degrees of masculinization of the external genitalia. As a result, the baby's sex may appear ambiguous or even be mistaken for males.

    C. The Pedigree Analysis Module:

    • Introduces analysis of pedigrees
    • Reinforces understanding of basic principles of inheritance
    • Helps students to understand that some dominant alleles are rare
    • Introduces the concept that some genetic conditions are not inherited, but rather are due to new mutations
    • Engages students in thinking about the advantages and limitations of Punnett squares and pedigrees as models of inheritance.

    This module can be used as an extension activity after completing the Genetics Student Handout or this module could be carried out immediately after the Inheritance of Albinism.


    The analysis of the first pedigree assumes that students know that the allele for albinism is recessive. The conclusion that the allele for albinism is recessive is also indicated by the pedigree since two unaffected parents have an affected offspring. (This could be the result of a new mutation for a dominant allele, but this is unlikely since an affected offspring of unaffected parents occurs twice within three generations of this family.) This pedigree also indicates that the allele for albinism is autosomal recessive and not X-linked recessive since the affected son (6) inherited an allele for albinism from his father (3), but he did not inherit an X chromosome from his father.


    Other conditions with this mode of inheritance are mentioned, including phenylketonuria (PKU). PKU is due to recessive alleles that code for defective versions of the enzyme that converts phenylalanine to tyrosine (an important step in disposing of excess phenylalanine). As noted in the Genetics Supplement, PKU "results in mental retardation unless phenylketonuria is detected at birth and treated with a special diet". This provides the opportunity to discuss the important point that phenotype is determined by the effects of both genes and environment. To enhance student understanding of this important point, you can:

    • Discuss how mental retardation can be prevented in an individual with phenylketonuria by minimizing phenylalanine in the diet by avoiding the artificial sweetener aspartame and high-protein foods (e.g. meat, fish, milk, cheese, eggs, nuts, beans, tofu, and even foods with flour) and substituting special low-phenylalanine foods; minimum intake of phenylalanine is especially important for babies and young children when the brain is developing rapidly; for additional information, see and
    • Mention the influence of environmental factors on symptoms of sickle cell anemia (see page 6 in these Teacher Preparation Notes)
    • Mention the effect of sun exposure/tanning on skin color (see "II. Were the babies switched?" in "Soap Opera Genetics", SoapOperaGenetics).

    The second pedigree in this module indicates that the allele for achondroplasia is dominant since two affected parents have normal children. (Furthermore, this allele must be autosomal dominant and not X-linked dominant, since an affected father (1) has an unaffected daughter.) The allele for achondroplasia is considered dominant because an individual who is heterozygous for this allele and the normal allele has the dwarf phenotype. However, there are important differences between a heterozygous individual (~7% risk of infant death) and an individual who is homozygous for the achondroplasia allele (~100% early mortality, due to difficulty breathing as a result of a small rib cage plus brain problems resulting from abnormalities of the skull). The specific mutation responsible for achondroplasia results in a protein that is overactive in inhibiting bone growth.


    Additional information about achondroplasia is available at


    Question 3 stimulates students to notice that achondroplasia is an example of a condition caused by an allele which is dominant, but rare in the population. 99.99% of the population is homozygous for the normal recessive allele for this gene. Achondroplasia is rare because there is substantial selection against inheritance of the achondroplasia allele and the mutation rate is low (estimated ~1/10,000).


    Question 4 raises the important point that achondroplasia is an example of a condition which is genetic, but usually not inherited. In more than 80% of cases, neither parent has the allele for achondroplasia and the person has achondroplasia due to a new mutation which occurred during production of one of the gametes (see "This Genetic Condition Was Not Inherited", available at


    Question 5 stimulates students to think about and evaluate Punnett squares and pedigrees as models of inheritance. One advantage of Punnett squares as a model of inheritance is that a Punnett square summarizes how the processes of meiosis and fertilization contribute to inheritance of different alleles of a gene. For parents with specified genotypes, Punnett squares can identify the possible combinations of alleles in offspring and the resulting possible phenotypes, and Punnett squares can make quantitative predictions concerning the frequency of these genotypes and phenotypes in large samples of the children of this type of couple. Limitations of Punnett squares as models of inheritance include the lack of information about likely variation in small samples such as individual families and the lack of information about population prevalence of parental genotypes (so no predictions can be made about population prevalence of offspring genotypes and phenotypes). Also, the predictions of a Punnett square model may be inaccurate if important complexities are omitted (e.g. the effects of multiple genes or the possibility of mutation). The failure to take account of all the complexities is, of course, a general limitation of models, which are simplified representations of complex processes.


    Pedigrees can be useful for figuring out the mode of inheritance for a phenotypic condition observed in multiple members of a family, and pedigrees can provide a useful basis for genetic counseling. Pedigrees can be quite complex to interpret, e.g. if a mutation has occurred, if the environment influences the phenotype, and/or if more than one gene influences the phenotype. Also, pedigrees do not directly represent the underlying biological processes of meiosis and fertilization.


    Follow-up Activity

    Soap Opera Genetics – Genetics to Resolve Family Arguments


    This analysis and discussion activity contains three "soap opera" episodes that contribute to student understanding of the principles of inheritance and the relevance of genetics to everyday life. In the first episode, students explain the biology summarized in a Punnett square to answer the probing questions of a skeptical father who wants to know how his baby could be albino when neither he nor his wife is albino. Concepts covered in the second episode include co- dominance, incomplete dominance, polygenic inheritance, and the combined effects of genes and the environment on phenotypic characteristics. In the third episode, students analyze sex-linked inheritance. Each episode can be used separately or with other episodes, depending on your teaching goals. This activity is aligned with the Next Generation Science Standards.


    We recommend that you use the first episode of the "Soap Opera Genetics" activity to engage your students in active recall of key concepts presented in this "Genetics" activity. You can enhance student learning and retention of important concepts and vocabulary by having your students complete the first episode of "Soap Opera Genetics" without referring to their notes, and then providing prompt feedback to clarify any points of confusion and correct any misconceptions (e.g. by having a class discussion of student answers).


    Additional Activities for Teaching/Learning Genetics

    Genetics – Major Concepts and Learning Activities


    This overview summarizes important genetic concepts and proposes an integrated sequence of learning activities to develop student understanding of these key concepts. Part 1 provides an outline of key concepts needed to understand how genes influence phenotypic characteristics and how genes are transmitted from parents to offspring. Part II recommends an integrated sequence of learning activities that are aligned with the Next Generation Science Standards and provides links for additional resources for helping students to understand genetics.