Name: Dr. Gonzalez BSC2005 In-Class Worksheet MENDELIAN GENETICS Introduction In sexually reproducing animals, genetic information is passed from the parents to offspring by means of haploid gametes (egg and sperm) which, in most organisms, unite to form a diploid zygote. The zygote receives half of its chromosomes from one parent and half from the other. The alleles that the parents pass on determine not only the genotype of the offspring, but also its observable traits -- its phenotype. For example, a child could have the blood type A, B, AB, or O depending on what alleles were inherited from the parents. Most complex organisms have separate sex chromosomes as distinguished from the others which are called autosomes. In mammals, the females have two full-sized X sex chromosomes. Males, in contrast, have one X and one Y chromosome. The Y chromosome is not full sized, and lacks most of the loci on the X chromosome. It follows that, if a gene is located on the X chromosome, a female will get two alleles for that gene while the male gets only one. This creates some potential problems for males related any gene that is sex-linked. We will study the inheritance of sex-linked genes that result in some diseases and disabilities that occur mainly in males. Although we will not be covering the subject in this worksheet, you should know that not all phenotypic characters and traits exhibit simple patterns of Mendelian inheritance. They can result from complex interplays among many genes and their interactions with the environment. Human height and intelligence are among these complex polygenic characters. Objectives: 1. Demonstrate the following skills necessary for genetic problem solving: representing genotypes using the proper symbols, determining the gametes produced by parents in a cross, setting up and completing a Punnett square, and finding the genotypic and phenotypic ratios of offspring from a cross. 2. Relate the genetic makeup of gametes to the process of meiosis. 3. Explain the concept of dominance and solve problems involving monohybrid crosses with simple dominance. 4. Explain the concepts of incomplete and codominance (multiple alleles) and solve problems involving monohybrid crosses with genes showing these interactions. 5. Demonstrate using a Punnett square how sex is determined in mammals. 6. Compare the structures of X and Y chromosomes and explain implications for sexlinked genes. 7. Solve problems involving sex-linked genes. Activity 1: Vocabulary Define the following terms heterozygous homozygous genotype phenotype genome diploid haploid allele gene homologous chromosomes dominant allele recessive allele test cross trait 1
Activity 2: The Genotype of a Cell 1. It is often important to summarize what alleles (gene variants) are present in a cell by noting its genotype. This notation will also be the genotype for the multicellular organism to which the cell belongs because all the body (somatic) cells of one individual have the same alleles on their chromosomes. Using Figure 1 as an example, we represent the genotype by: Figure 1 a) including a letter for each allele that is present on the chromosomes. What alleles are present? b) placing the upper case (capital) letter first when two different alleles for a gene are present (ex., Aa make sure to use letters that can be easily distinguishable. For example, Y, P, W are not good choices). 2. Write the correct genotype for the cell in Figure 1: 3. In the hypothetical case that the alleles present were G, a, A, t, T, and G, what would be the correct genotype?. Activity 3: Skills for Genetic Problem Solving These exercises will help you learn the skills needed to determine the patterns of inheritance from one generation to the next. These same skills are used by geneticists, livestock breeders, and health counselors in their work. The general question to be answered is: "What are the genotypes and phenotypes of the offspring likely to be if individual X mates with individual Z?" We will use a monohybrid cross with dominance to learn the skills. 1. What is a cross? What symbol represents a cross? State in words what is meant by Tall plant x short plant. 2. How many genes are involved in a monohybrid cross? 3. In one type of cross between parents that have different traits (phenotypes) for a character, only one of the traits is expressed in all of the offspring. The relationship revealed from this cross is called. 4. For example, when a tall plant is crossed with a short plant, all the offspring (progeny) are tall. Tallness is the trait. Shortness is the trait. 2
5. The tall plant and short plant are called the parents, or P generation, in the cross. Usually the parents are represented, not by words (like tall plant ), but by their genotypes. If T is the allele for tallness and t is the allele for shortness, what would be the genotypes for the following plants? a. a short plant b. a tall homozygous plant c. a heterozygous plant 6. The offspring of a cross, called progeny, are represented the same way, with their genotypes. If the offspring from a cross were all tall, what might their genotype(s) be? 7. The offspring of the parents in a cross are members of the first filial (F 1 ) generation. Determining the genotypes and phenotypes of the F 1 generation involves three important steps: Step 1: Write out the cross using the genotypes of the parents. Step 2: Determine the possible gametes that can be formed by the parents. Step 3: Use a Punnett square to identify the genotypes of all expected offspring from the cross. 8. Determining gametes is an essential step that often gives students trouble. You will remember that meiosis I sorts the two chromosomes from each homologous pair into different daughter cells. The final result is that each gamete has one of the two homologous chromosomes of the parent cell, and not the other. a. For example, if the parent cell had T and t, what will the gametes have? b. About how many gametes will have each allele?. 9. Step #3: In setting up a Punnett square, the gametes of 1 parent are represented by symbols for the alleles (like T and t) placed at the top of each column in the matrix. The gametes of the second parent are represented by allele symbols on the left-hand side of the square next to each row. Study Fig 2 to make sure you understand how to do it. The gametes from which parent are shown at the top of the table? On the left side? 3
Figure 2 10. In many genetics problems, the final tasks are to determine the genotypic ratio and phenotypic ratio of the offspring. Ratios represent the relative number of each kind that is present. For example, if a workshop has 10 men and 5 women, the ratio is written 10:5 or 2:1 a. To obtain the genotypic ratio, you must first determine the number of different genotypes that are represented among the progeny. For example BB, Bb, and bb are three different genotypes. b. Next count up the number of individuals with each genotype. c. Finally express them as a ratio. For example: 1 BB: 2 Bb: 1 bb (see Fig. 2) d. If there is only one genotype represented among the progeny, no ratio is possible. In this case you would simply summarize the results by saying, for example, "All are genotype Tt." 4
11. The phenotypic ratio is determined by the number of observable traits for the character studied in the cross. For instance, if the character is flower color, as in Fig. 2, there are two different phenotypes (traits) purple and white. a. First, determine how many different phenotypes are represented among the progeny. (Note that some different genotypes may have the same phenotypes.) b. Second, count the number of offspring of each phenotype c. Third, express the numbers as a ratio. For example 3 talls : 1 short d. Again, if there is only 1 phenotype represented, no ratio is possible. For example, you might simply report, "All are tall." Activity 4: Skills in Genetic Problem Solving Use the information provided in table 1 to set up the crosses given below. DO NOT solve the crosses, unless instructed to do so by your professor. Include all possible genotypes. The first is done as an example. Table 1. Genotypes for garden pea plants and people. Organism Genotypes Phenotypes (Traits) Garden Pea Plants AA or Aa Purple flowers aa White flowers GG or Gg Yellow seeds gg Green seeds RR or Rr Smooth seed coat rr Wrinkled seed coat TT or Tt Tall plants tt Short plants Humans EE or Ee Free earlobes ee Attached earlobes RR or Rr Rh positive blood rr Rh negative blood FF or Ff Freckles ff Uniform pigment distribution a. Tall homozygous plant with short (dwarf) plant. Answer: TT x tt b. Pea plants with purple flowers (heterozygous) and with white flowers c. humans with free earlobes (heterozygous) and with attached earlobes d. humans with Rh+ blood (homozygous) and with Rh- blood e. Person with freckles (whose mother had no freckles) and person with no freckles 5
Activity 5: Solving Complete Dominance Genetics Problems If you need more space, solve problems in a separate sheet of paper. 1. In a given plant, the yellow seed color allele (G) is dominant to green seed color (g). Give the genotypic and phenotypic ratios for the following crosses: a. homozygous yellow x green b. heterozygous yellow x green c. heterozygous yellow x heterozygous yellow 2. In tigers, there are rare white individuals that lack the normal orange pigment in the fur but still have black stripes. This condition is caused by the expression of a recessive allele, t. The dominant allele is T. A pair of normally colored parents produces a white cub and a normally pigmented cub. a. What are the possible genotypes of the cubs? b. What are possible genotypes of the parents that produced the two cubs? c. Explain your reasoning by showing a cross that could give the observed results. 3. A guinea pig with black fur was bred to one with brown fur and all offspring had black fur. Breeding two pigs from the F1 generation produced a combination of black and brown individuals in the F2 generation. a. Give symbols to the alleles for coat color in guinea pigs. What is the dominant allele for coat color? How do you know? b. Show the genotypes of the parental pair, the alleles in the gametes from each one, and the genotypes of the F1 offspring. Use a Punnett Square to summarize the crosses. c. Determine the genotypic and phenotypic ratio for the F1 offspring. d. Show the cross between F1 individuals that produced the F2 generation e. Suggest what the genotypic and phenotypic ratio would be among the F2 offspring. 4. By simply looking at an organism (the phenotype), how can you distinguish between a homozygous individual (TT) and a heterozygous one (Tt)? Explain the concept of a test cross. 5. Phenylketonuria is a genetically based disease in which the individual is unable to breakdown an amino acid called phenylalanine which, with its by-product phenylpyruvate, accumulates in the blood stream causing mental retardation and other side effects. The disease is the result of the expression of a recessive allele. A normal man and woman produce a child with phenylketonuria. a. Make up symbols for the alleles in this cross. b. Suggest the genotypes of the man, woman and baby and show how normal parents can have a baby with the disease. c. If the woman gets pregnant again, what is the probability that she will give birth to a child with the disease? Give the expected ratio of normal offspring to those with the disease. d. Can you think of any way these parents can avoid giving birth to a child with this disease? Activity 6: Codominance and Multiple Alleles 1. Some traits are determined by genes with more than two alleles. Blood type in humans is an example. Three alleles are involved: I A is for type A blood, I B is for type B blood, and i is for type O blood. What does each allele code for in the chemistry of blood cells? 2. There are dominance relationships among the three alleles. Based upon the symbols alone what would you predict about the relationships? Explain. 6
3. Using I A and I B explain the concept of codominance. What is the relationship of these alleles to i? 4. Fill out table 2 to establish relationship between genotypes and phenotypes. The first one is filled out as an example. Table 2. Blood Types and Genotypes in Humans Blood type (phenotype) Genotype Can donate blood to A I A I A or I A i B AB O Can receive blood from Activity 7: Sex Chromosomes and Sex-Linked Genes 1. In human cells there are 46 chromosomes making up 23 homologous pairs. How many of these pairs are autosomes? How many pairs are sex chromosomes? 2. The two chromosomes in an autosomal pair are the same size and shape. What about in the sex chromosome pair for women? For men?. Give the symbols for male sex chromosome and female sex chromosome. 3. In Fig. 4 complete the Punnett square by: a. entering the gametes with sex chromosomes identified (X or Y) for the two parents b. determining the phenotypic ratio for the cross Fig. 3: Sex chromosomes Fig. 4: Sex determination 4. In random assortment of sex chromosomes into gametes what is the expected ratio of girl and boy babies?. 5. On the Y sex chromosome there are some genes related to male traits. On the X chromosome there are hundreds of genes, most of which are absent from the Y chromosome. For a sex-linked gene (x-linked gene; i.e., present on X and absent on Y), how many alleles are present in female cells? How many alleles in male cells? 7
6. Sex-linked genes control the expression of sex-linked traits. Examples are genes for blood clotting and color vision. People with defective genes may suffer from hemophilia or red-green color blindness. a. Draw a pair of sex chromosomes for a female. Near the top place a locus for the color vision gene B/b. Make the sex chromosomes heterozygous for the alleles. b. Draw a pair of sex chromosomes for a male. Place the color vision gene on the X chromosome, but not the Y. Place a recessive allele on the X chromosome. 7. We represent sex-linked genes as superscripts over the X symbol. For example, in the case of the gene for color vision (B/b), X B represents the allele for normal vision, while X b is the recessive allele which is expressed in colorblindness. The Y chromosome is represented with no superscript since it lacks either allele. A colorblind male would be. 8. If the B allele is expressed in the phenotype, a person will have normal color vision. If the b allele is expressed, the person will be colorblind. What are the phenotypes of the man and woman you represented in your drawings in #6? man: woman: 9. Make the following cross using the steps for solving a genetics problem: A woman who is heterozygous for the color vision allele and a colorblind man. a. show the genotypes of the parents b. show the gametes each can produce c. set up the Punnett square and fill it in d. determine the genotypic and phenotypic ratio (be sure to include gender as well as vision in the phenotypes you identify). 10. Explain why colorblindness is more common in males. 11. With what combination of alleles could you get a colorblind female? Activity 8: Nondisjunction The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II is termed disjunction. When the separation is not normal, it is called nondisjunction. This results in the production of gametes which have either more or less of the usual amount of genetic material, and it is a common mechanism for trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis II phases of gametogenesis, or even during mitosis. Nondisjunction during either oogenesis or spermatogenesis is a cause of several medical conditions in humans, including: o Down Syndrome - trisomy of chromosome 21 o Patau Syndrome - trisomy of chromosome 13 o Edward Syndrome - trisomy of chromosome 18 o Klinefelter Syndrome - an extra X chromosome in males 8
o o o Turner Syndrome - only one X chromosome present in females Jacob Syndrome - an extra Y chromosome in males Triple X Syndrome - an extra X chromosome in females 1. A biologist marked a cell that she knew was about to undergo meiosis. A short time later she observed the four cells produced by the original marked cell; their chromosome numbers were: 17, 17, 18, 16. She knew these numbers indicated that something abnormal had occurred during meiosis. Using this information, answer the following three questions: 1) What kind of mistake occurred? 2) What would be the normal diploid number for this cell? (2n =?) 3) At which specific stage of meiosis did the mistake take place? Activity 9: Practice Genetics Problems For each of the problems below: a. state the category of which it is an example (e.g., monohybrid cross with dominance, codominance, incomplete dominance, dominance with multiple alleles, sex-linked). b. state what clues led to your decision. c. solve the problem showing your work, step by step. 1. A spotted rabbit and solid-colored rabbit were crossed. They produced all spotted offspring. When these F 1 rabbits were crossed the F 2 consisted of 32 spotted and 10 solid colored rabbits. Which characteristic is determined by a dominant gene? Give a summary of the crosses using appropriate symbols for the two alleles involved. 2. What proportion of the F 2 spotted rabbits in the prior problem would be heterozygous? Homozygous? How many of the F 2 solid-colored rabbits are expected to be homozygous? 3. What breeding experiment could be used to tell which of the spotted rabbits in the above problem were heterozygous and which, homozygous? Explain using a summary of the crosses. 4. Red-green color blindness is a sex-linked recessive trait. a. A color-blind man marries a woman who is heterozygous for the color vision gene. What are the expected genotypic ratios of their sons? of their daughters? Give the phenotypic ratios. b. A man with normal color vision marries a heterozygous woman. What are the expected genotypic ratios of their sons? Of their daughters? Give the phenotypic ratios. 5. A married couple living together has a child with B blood type. The man has blood type AB and the woman has type O blood. a. What is the genotype of the child? b. A question of paternity arises when payments for child support are sought by the mother after the couple splits up. The father claims that the woman had been unfaithful and the child was fathered by an acquaintance whose blood type is A. Is this possible? Explain. c. Can the mother donate blood to the father? Can the father donate blood to the mother? Which parent can donate blood to the child? 6. What are the possible genotypes and phenotypes for children of a couple that have blood types A and B if both are heterozygous for the blood type gene? If both are homozygous for the blood type gene? 9
7. A couple has three girls and one boy. How does this compare with the expected ratio of boys to girls among four offspring? Are their chances of a boy greater now if they have a fifth child than when they had their first child? Explain why or why not. 8. Queen Victoria of England was married to Prince Albert. Neither had hemophilia (an expression of a sex-linked recessive gene) but one son, Leopold, was a hemophiliac and two of their normal daughters had grandsons with hemophilia. a. Which parent, king or queen, carried the recessive hemophilia gene? Explain. b. What is the expected ratio in their offspring of normal sons : hemophiliac sons? normal daughters : hemophiliac daughters? c. In the descendants of Victoria and Albert there were 10 male hemophiliacs and no female hemophiliacs. Explain why this was so. 9. A color-blind man marries a woman with normal vision whose father was color-blind. What are the chances that their male children will be color-blind? Their female children? Give the genotypic ratios and phenotypic ratios by sex as well as color-blindness. 10. A woman is heterozygous for free-earlobes and her husband has attached earlobes. They have four children. a. What is the genotype for each parent? b. What are the possible genotypes and phenotypes of their four children? Rewritten from: Progressions: Peer-Led Team Learning The Workshop Project Newsletter. Fall 2005, vol.7, issue 1. By: Joseph G. Griswold and David L. Wilson. David Gosser, Consulting Editor, A.E. Dreyfuss, Editor, Michael Gaines, Co-Editor, Biology Modules, Joseph G. Griswold, Co-Editor, Biology Modules. Website: www.pltl.org Reproduction of material appearing in Progressions is encouraged with complete citation. Progressions (ISSN 1539-1752 print; ISSN 1539-7483 online) is published by the PLTL Workshop Project. 10