The Student Handout includes three modules:
The Genetics Supplement includes three modules:
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 http://serendip.brynmawr.edu/sci_edu/waldron/#meiosis). These activities are part of an integrated sequence of learning activities for teaching genetics, presented in "Genetics – Major Concepts and Learning Activities" (available at http://serendip.brynmawr.edu/exchang. neticsConcepts).
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.
In accordance with the Next Generation Science Standards:
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"
For the "Coin Toss Genetics" module in the Student Handout:
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 iwaldron@sas.upenn.edu. The following paragraphs provide additional background information.
They introduce students to both basic principles of genetics:
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 http://www.biologyreference.com/Fo-Gr/Gene.html .
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 http://www.nlm.nih.gov/medlineplus/e. cle/001479.htm and OMIM = Online Mendelian Inheritance in Man (available at www.ncbi.nlm.nih.gov/omim/;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 http://www.mynvfi.org/about_vitiligo.)
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 |
| 0 aa | 32% |
| 1 aa | 42% |
| 2 or more aa | 26% |
| 1 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").
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 http://www.mayoclinic.com/health/sic. anemia/DS00324. For additional information, see OMIM (Online Mendelian Inheritance in Man (http://www.ncbi.nlm.nih.gov/omim/; 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).
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 http://serendip.brynmawr.edu/sci_edu/waldron/#meiosis) 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.
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
68% of the results are expected to be in this range:
95% of the results are expected to be in this range:
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 http://ghr.nlm.nih.gov/condition/turner-syndrome, http://ghr.nlm.nih.gov/condition/klinefelter-syndrome, and Teacher Notes for "How Mistakes in Cell Division Can Result in Down Syndrome and Miscarriages", available at http://serendip.brynmawr.edu/exchang. es/mmfmistakes). 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:
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:
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 http://ghr.nlm.nih.gov/condition/achondroplasia.
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 http://serendip.brynmawr.edu/exchang. eticsInherited).
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.
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).
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.
4.2: Genetics Teacher Preparation Notes is shared under a CC BY-NC license and was authored, remixed, and/or curated by LibreTexts.
