Genetics Supplement (These supplementary modules, a Genetics Student Handout, and Teacher Preparation Notes with suggestions for implementation are available at http://serendip.brynmawr.edu/sci_edu/waldron/#genetics. By Drs. Scott Poethig, Ingrid Waldron, and Jennifer Doherty, Dept. Biology, Univ. Pennsylvania, 2013.) We all know that children tend to resemble their parents in appearance. Parents and children tend to have similar appearance because children inherit genes that influence characteristics such as skin and hair color from their parents. What is a gene? A gene is a segment of a DNA molecule that gives the instructions for making a protein. The protein can influence our characteristics. For example, one gene gives the instructions for making a protein enzyme which helps to make melanin, the pigment which contributes to the color of skin and hair. Different versions of this gene (called alleles) code for different versions of the protein which result in different skin and hair color, as explained in the table below. Genes in DNA Protein Characteristic Two copies of A allele provide instructions to make normal protein enzyme Two copies of a allele provide instructions to make defective protein enzyme Normal enzyme makes melanin (dark pigment in skin and hair) Defective enzyme does not make melanin Normal skin and hair color Albino How does a baby inherit genes from his or her mother and father? Genes are parts of DNA molecules that are contained Mother Father in chromosomes in the nucleus of each cell in our Meiosis Meiosis bodies. When we talk about genes being inherited from one generation to the next, we are really talking about how these gene-carrying chromosomes move egg sperm Fertilization during meiosis and fertilization. As a result of meiosis and fertilization, a baby inherits two copies of each zygote gene, one from the mother and one from the father. Mitosis baby Inheritance of Albinism To learn more about how genetic traits are inherited, we will start with a specific question: If each parent has one A allele and one a allele (i.e. both parents are Aa), what different combinations of A and/or a alleles would you expect to observe in the children of these parents? To answer this question your group will use model chromosomes to show how meiosis separates the chromosomes with the two alleles during the formation of eggs or sperm and then fertilization produces different combinations of alleles in different zygotes. The pair of homologous chromosomes for each parent will include one model chromosome with an A allele and another with an a allele. 1a. One of you should be the mother and use your model chromosomes to demonstrate how meiosis produces different types of eggs, and another should be the father and demonstrate how meiosis produces different types of sperm. In the chart on the next page, write in the genetic makeup of the two types of eggs and the two types of sperm produced by meiosis.
1b. Next, model fertilization, using the model chromosome for each type of sperm to fertilize each type of egg. Write the genetic makeup of the resulting zygotes in the chart. Biologists use a similar chart to analyze inheritance However, biologists omit much of the detail shown above and use a simplified version called a Punnett Square: A a A AA Aa a Aa aa 2. For this couple, what fraction of the mother's eggs have an a allele? What fraction of the father's sperm have an a allele? What fraction of this couple's children would you expect to be aa? Explain your reasoning. (To answer this question, remember that each zygote undergoes repeated mitosis to become a child, so the child will have the same genetic makeup as the zygote.) As explained on the previous page, children who have aa alleles will have albinism, and children who have AA alleles will have normal skin and hair color. These children are homozygous for the a allele or the A allele. Homozygous means that both copies of the gene have the same allele. The next question is: Will the parents and children who have Aa alleles have normal skin and hair color or be albino? This type of combination of two different alleles is called heterozygous. Often, one allele in a heterozygous pair of alleles is dominant and the other allele is recessive; the dominant allele determines the observable characteristic of the heterozygous individual. The A allele is dominant because it codes for normal, functional enzyme and, even in a heterozygous individual, there is enough of this normal, functional enzyme to produce enough melanin to result in normal skin and hair color. The a allele is recessive because it codes for a non-functional enzyme which does not affect skin or hair color in a heterozygous individual. 3. Do the Aa parents have normal skin and hair color or albinism? 4. What fraction of this couple's children would you expect to have normal skin and hair color?
The genotype refers to the genetic makeup of an individual. The phenotype refers to the observable physical and physiological characteristics of an individual. 5. Give an example of two individuals who have different genotypes for the albinism gene, but the same phenotype. Explain how two individuals with different genotypes can have the same phenotype. 6. To evaluate whether albino parents are more likely to have an albino child, complete the following chart. For each parent in the first two columns, indicate which genotype or genotypes (AA, Aa, and/or aa) could produce this phenotype. Then, draw the Punnett squares for these genotypes. Circle each albino offspring in these Punnett squares. Phenotype and Punnett Square or Squares Genotype or Genotypes for a Couple or Couples with these Genotypes Mother Father Albino Albino Albino Normal skin and hair color Normal skin and hair color Normal skin and hair color Which couple would be the most likely to have albino children? Which couple would be the least likely to have albino children? 7. Explain why two albino parents will not have any children with normal skin and hair color, but two parents with normal skin and hair color could have an albino child.
Genetics of Sex Determination As you probably know, human males have an X and a Y chromosome (XY), whereas females have two X chromosomes (XX). The gene that results in the development of male anatomy is located on the Y chromosome. This gene is called SRY, which stands for sex-determining region of the Y chromosome. If a zygote has a Y chromosome with the SRY gene, the embryo will develop testes and male anatomy. If a zygote does not have a Y chromosome with the SRY gene, the embryo will develop ovaries and female anatomy. 1. In the figure below, use X and Y to show the separation of X and Y chromosomes in meiosis (mother on the left and father on the right). Then show the formation of a zygote that will develop into a female (on the left) or a male (on the right). Meiosis Fertilization 2. Draw a Punnett Square which shows the inheritance of the sex chromosomes. Use X to indicate an egg or sperm with an X chromosome and Y to indicate a sperm with a Y chromosome. 3. Based on this Punnett Square, what percent of children would you expect to be male? 4. To test this prediction, begin by writing down the initials of all the children your mother has had. Arrange these initials in order from the youngest to the oldest, indicating whether each was male or female. Use this information and the information from the other students in your group to complete the upper rows of the table on the next page. Add your group's data to the class data your teacher is collecting.
5. Complete the following table. Your mother's children Sex of each child 1 st 2 nd 3 rd 4 th 5 th + Total number of children Number of males % males Children of the mother of another student in your group Children of the mother of another student in your group Children of the mother of another student in your group Children of the mothers of all the students in your class Predicted percent from Punnett square 6. Use your group's data and data from nearby groups to answer the following questions. If a mother's first child is a son, is the next child necessarily a daughter? If a mother's first child is a daughter, is the next child necessarily a son? If a mother's first two children are the same sex, is the next child necessarily the opposite sex? These observations illustrate that you cannot predict the sex of the next child based on the sex of a previous child or children. Each time a sperm fertilizes an egg, this fertilization event is independent of any previous fertilizations that resulted in older brothers or sisters. 7. Compare the predicted percent male with the observed percent male for your mother s children and for the children of the mothers of each of the other students in your group. How similar to the prediction are the observed results for each of these families? How similar to the predictions are the results for the children in all of the families combined? How do you interpret these results?
Pedigree Analysis Geneticists illustrate the inheritance of a gene within a family by using a pedigree chart. In a pedigree chart, males are symbolized by a square ( ) and females are symbolized by a circle ( ). People who are affected by a condition or disease are symbolized by a dark square or circle. This pedigree chart shows the inheritance of albinism in three generations of a family. 1 and 2 represent a couple who had five children, including a son who is labeled 3 and a daughter who is labeled 5. Only one of their children, 5, was an albino. One of their sons (3) and his wife (4) had four children, including a son (6) who was an albino. 1 2 3 4 5 6 7 1. Write the genotypes of each individual who is labeled with a number in the pedigree. Use A to represent the dominant allele and a to represent the recessive allele. Begin by writing in the genotypes of 5 and 6. How do you know their genotypes? Explain how you can determine the genotypes of 1 and 2. Show the Punnett Square for these parents, and write their genotypes in the pedigree. Write the genotypes of 3 and 4 in the pedigree. Explain how you can figure out the genotype of 7 and write her genotype in the pedigree. Many other conditions are the result of homozygous recessive alleles, so these conditions are inherited in the same manner as albinism. These include: sickle cell anemia cystic fibrosis (a genetic disease that results in difficulty in breathing and serious illness) phenylketonuria (a genetic disease that results in mental retardation unless phenylketonuria is detected at birth and treated with a special diet).
This pedigree shows the inheritance of a different condition called achondroplasia (ay-kon-druh-playzhuh), a form of dwarfism. Dark circles or squares indicate individuals with achondroplasia. 1 2 3 4 5 6 7 2. Is the allele that causes achondroplasia recessive or dominant? How do you know? To answer these questions, you will need to think about 5 and 6 and their children. Include the Punnett Square for 5 and 6 in your answer; use D to represent the dominant allele and d to represent the recessive allele. In the pedigree, write the genotypes of 5 and 6. Write the genotypes of 2, 3 and 7. How do you know their genotypes? 3. Determine the genotypes of 1 and 4. Explain your reasoning. 4. Based on the frequency of dwarfs among the people you have seen in your lifetime, do you think that the allele for achondroplasia is common or rare in the population? Explain your reasoning.
Challenge Questions 5. Most people who have the achondroplasia allele did not inherit this allele from their parents. For people with the achondroplasia allele who did not inherit it from their parents, what biological process is the most likely explanation for their achondroplasia allele? We have analyzed two types of models of inheritance: a Punnett Square and a pedigree. A model is a simplified representation of a biological process that demonstrates important aspects of the process. Models can make it easier to understand important features of a complex biological process. 6. What are some advantages of a Punnett Square as a model of inheritance? What is one limitation of a Punnett Square as a model of inheritance? What is one advantage of a pedigree as a model of inheritance? What is one limitation of a pedigree as a model of inheritance?