Genetic Traits Topic The traits of offspring are determined by genetic instructions received from the mother and the father. Introduction Traits are passed down from parent to offspring through genetic material in cells called deoxyribonucleic acid (DNA). Therefore, the DNA that an individual inherits determines his or her characteristics. Within cells, molecules of DNA form structures called chromosomes. The instructions for specific traits are contained on sections of chromosomes called genes. Offspring receive genes from both their mother and their father for each genetic trait. Many traits are determined by multiple genes, but some traits are regulated by only two genes. Often, one of these genes is dominant over the other. A dominant gene is one that masks the appearance of a recessive gene. For this reason, a dominant trait will appear in an individual even if only one of the genes is present. For a trait carried on a recessive gene to show up, both genes in the pair must carry the trait. The combination of genes for a specific trait is referred to as an allele. Gene combinations may be heterozygous or homozygous. A homozygous allele for a trait can be homozygous dominant, carrying two dominant genes, or homozygous recessive, with two recessive genes. An allele with a heterozygous combination of genes carries one dominant and one recessive gene. Most genetic traits are carried on body chromosomes, or autosomes, but a few are carried on sex chromosomes. Each individual possesses only one pair of sex chromosomes: XX for a female and XY for a male. Of the two types, the X chromosome carries many more traits than the Y chromosome. Consider an imaginary scenario in which a new species of insect has recently been discovered. Because it can be found buzzing around chocolate, the new species has been nicknamed the chocolate fly. Genetically, chocolate flies are simple animals. Each has only four autosomes in addition to its two sex chromosomes.
GENETIC TRAITS 2 In chocolate flies, only a few genes have been located and researched. The gene for eye color is found on the X chromosome. Since the Y chromosome lacks genes, X chromosomes determine eye color in chocolate flies. The rest of the genes that have been studied in chocolate flies are located on the four autosomes. Each of these genes carries traits for color. For example, a set of genes on chromosome 1 codes for wing color, and pink wings are dominant to gray wings. A set of genes on chromosome 2 carries the traits for body color. The gene for white body color is dominant to the gene for blue body color. Chromosome 3 carries genes for head color, and a yellow head is dominant to an orange one. The genes on chromosome 4 are responsible for the color of the insects legs. The trait for brown legs is dominant to the trait for black legs. In today s lab, craft sticks represent the chromosomes of chocolate flies, and dots represent traits (see Data Table 1). Red craft sticks correspond to X chromosomes, and green craft sticks stand for Y chromosomes. White dots drawn or taped on the craft sticks indicate genes for white eyes, and purples dots indicate the genes for purple eyes. Wood-tone craft sticks are numbered from 1 to 4 to represent the flies autosomes. The traits, and their representative craft sticks, are detailed in Data Table 1. Remember that a male fly has 1 X chromosome, 1 Y chromosome, and four sets of autosomes. A female fly has two X chromosomes and four sets of autosomes. Data Table 1: Craft sticks and dots represent traits on chocolate flies Chromosome Traits Combination of craft sticks pair X chromosome eye color 1 red craft stick with white dot Y chromosome eye color 1 green craft stick Autosomes 1 wing color 2 craft sticks labeled 1, one with a pink dot, the other with a gray dot Autosomes 2 body color 2 craft sticks labeled 2, one with a white dot, the other with a blue dot Autosomes 3 head color 2 craft sticks labeled 3, one with a yellow dot, the other with an orange dot Autosomes 4 leg color 2 craft sticks labeled 4, one with a brown dot, the other with a black dot
GENETIC TRAITS 3 Time Required 60 to 75 minutes Materials pencil paper 4 empty brown paper bags (such as lunch bags) 1 small paper bag that contains craft sticks representing chromosomes of a male chocolate fly (prepared earlier by the teacher) 1 small paper bag that contains craft sticks representing chromosomes of a female chocolate fly (prepared earlier by the teacher) Safety Note Please review and follow the safety guidelines. Procedure 1. In each lab group, assign one lab partner the role of male chocolate fly and the other the role of female chocolate fly. 2. From the teacher, obtain a paper bag that contains craft sticks representing chromosomes for either a male or female chocolate fly. For the purposes of this lab, the chromosomes of both the male and female flies are heterozygous for each autosomal trait. In addition, both the male and female flies have white eyes. The combinations of chromosomes for each gender of fly are detailed in Data Table 2.
GENETIC TRAITS 4 Data Table 2: Combinations of chromosomes for male and female flies Sex of Sex Eye color on X Wing color on Body color on Head color on Leg color on chocolate chromosomes chromosome autosome 1 autosome 2 autosome 3 autosome 4 fly (E = white; (W = pink; (B = white; (H = yellow; (L = brown e = purple) w = gray) b = blue) h = orange) l = black) Male XY white pink white yellow brown (X E Y) (Ww) (Bb) (Hh) (Ll) Female XX white pink white yellow brown (X E X e ) (Ww) (Bb) (Hh) (Ll) 3. The male chocolate fly empties the chromosomes in his paper bag on the lab table. Each male should match his autosomal alleles together by number. The two sex chromosomes should also be paired (Figure 1). paper sack of "male chromosomes" male chocolate fly 1 1 red craft stick with dot 2 2 3 3 4 4 two number 4 craft sticks with dots two number 1 craft sticks with dots two number 2 craft sticks with dots green craft stick two number 3 craft sticks with dots Figure 1 4. The male transfers one craft stick (chromosome) from each of the matching pairs into one of the empty paper bags. The remaining crafts sticks should be placed in the other empty paper bag, then the tops of both bags rolled down. This step in the procedure represents meiosis of the male chromosomes. Meiosis is the type of cell division in which the number of chromosomes is reduced by half.
GENETIC TRAITS 5 5. The male places the two bags behind his back, then mixes them up by switching hands several times. The female partner then chooses a hand to select a bag. The selected bag should be set aside on the desk top. The other bag can be stored out of the way under the desk or lab table. 6. The female lab partner repeats steps 3 through 5, letting the male select a bag. This step in the procedures represents meiosis of the female chromosomes. 7. Each partner pours the contents of the two chosen bags onto the lab table to represent the combination of male and female chromosomes, or fertilization. These contents represent the genes of their offspring. Partners match up the colored craft sticks to find the sex of the baby fly, then match the autosomal sticks to find out the baby fly s body characteristics. Partners record these results on Data Table 3 in the row labeled Fly offspring 1. 8. Replace the chromosomes in the male and female paper sacks, then repeat steps 3 through 7 to find the genes for the second offspring. Record these data on Data Table 3 in the row labeled Fly offspring 2. 9. Repeat this entire process six more times until you have produced a total of eight offsprings and completed Data Table 3. Data Table 3: Chromosomes of male, female, and offspring Fly Sex Eye color Wing color Body color Head color Leg color offspring chromosomes (E) (W) (B) (H) (L) 1 2 3 4 5 6 7 8
GENETIC TRAITS 6 Analysis 1. Did any of the female offspring have purple eyes? Did any of the male offspring have purple eyes? Based on your knowledge of genetics, explain which of these two combinations would have been impossible to achieve. 2. Some inheritable diseases are transmitted on sex chromosomes. Explain why such diseases are more often transmitted through the mother s family than through the father s. 3. Assume that a pair of chocolate flies has eight offspring. Use a Punnett square to determine the expected number of offspring that would have each of the following traits: pink wings, white body, yellow head, black legs. A Punnett square is a simple chart that shows the possible combinations of parental genes. The chart is shaped like a window with four panes. The genes of one parent are written across the top of the Punnett square, and the genes of the other parent are written along the side (Figure 2). 4. Use a Punnett square to answer the following: Of four offspring, how many would you expect to have orange heads if one parent had a yellow head (Hh) and the other parent had an orange head (hh)? genes of one parent A a Punnett square genes of another parent A AA Aa a Aa aa Figure 2
GENETIC TRAITS 7 What s Going On? In this lab the parents are heterozygous (one dominant and one recessive gene) for the four autosomal traits. That means that 75 percent of their offspring express the dominant traits. The female parent has white eyes, but she also carries a gene for purple eyes on one of her X chromosomes. White eyes are expressed because the gene for white is dominant over purple. The male has white eyes because he has a gene for white eyes on his X chromosomes, but his Y chromosome does not carry a gene for eye color. All of the female offspring of this couple will have white eyes, although some of them will also possess the recessive gene for purple eyes. Male offspring may have white or purple eyes. Want to Know More? See Our Findings.
OUR FINDINGS GENETIC TRAITS Notes to teacher: Before the experiment, prepare paper bags of chromosomes for each student. On the day of the experiment, pair each student with a lab partner. One student will receive a paper bag containing craft sticks that represent the chromosomes of a male chocolate fly, and the other student will receive a bag containing craft sticks that represent the chromosomes of a female chocolate fly. To prepare the sex chromosomes, use markers or paint to color a supply of the craft sticks red and a smaller supply of craft sticks green. Each mating pair (set of two students) will need three red sticks and one green stick. On two of the red craft sticks, draw a purple dot, or tape a purple dot punched out from construction paper. This indicates the presence of the gene for purple eye color. Use Wite-Out to paint a white dot on the other red craft stick indicating the presence of the gene for white eyes. White eye color is dominant to purple. No genes will be present on the green craft sticks. Each lab group should receive: four craft sticks labeled 1 four craft sticks labeled 2 four craft sticks labeled 3 four craft sticks labeled 4 For each student, create a bag of craft sticks to represent his or her genome. Each male bag should contain the following (see the table of male chromosomes) : A. One set of sex chromosomes, made from a red craft stick (X chromosome) with a white dot (gene for white eyes) and 1 green craft stick (Y chromosome) B. Four sets of autosomes Set 1: Two #1 craft sticks, one with a pink dot and the other with a gray dot Set 2: Two #2 craft sticks, one with a white dot and one with a blue dot Set 3: Two #3 craft sticks. Place a yellow dot on one and an orange dot on the other Set 4: Two #4 craft sticks Place a brown dot on one and a black dot on the other
2 Data Table: Male Chromosomes Chromosome pair Traits Combination of craft sticks Sex chromosomes Eye color one red craft stick with white dot and one green craft stick Autosomes 1 Wing color two #1 craft sticks, one with a pink dot, the other with a gray dot Autosomes 2 Body color two #2 craft sticks, one with a white dot, the other with a blue dot Autosomes 3 Head color two #3 craft sticks, one with a yellow dot, the other with an orange dot Autosomes 4 Leg color two #4 craft sticks, one with a brown dot, the other with a black dot Each female bag should contain the following (see the Table of female chromosomes): A. One set of sex chromosomes, made from two red craft sticks, one with a white dot on it and the other with a purple dot B. Four sets of autosomes Set 1: Two craft sticks labeled 1 that represent wing color, one with a pink dot and the other with a gray dot Set 2: Two craft sticks labeled 2 that represent body color, one with a white dot on one of the craft sticks and a blue dot on the other Set 3: Two craft sticks labeled 3 representing head color, one with a yellow dot and the other with an orange dot Set 4: Two craft sticks labeled 4 representing leg color. Place a brown dot on one and a black dot on the other
3 Data Table: Female Chromosomes Chromosome pair Traits Combination of craft sticks Sex chromosomes Eye color one red craft stick with white dot and one red stick with a purple dot Autosomes 1 Wing color two #1 craft sticks, one with a pink dot, the other with a gray dot Autosomes 2 Body color two #2 craft sticks, one with a white dot, the other with a blue dot Autosomes 3 Head color two #3 craft sticks, one with a yellow dot, the other with an orange dot Autosomes 4 Leg color two #4 craft sticks, one with a brown dot, the other with a black dot Analysis question 3 asks students to prepare a Punnett square. If students are not familiar with this technique, go over it before the experiment. Analysis 1. No, yes. A female offspring cannot inherit purple eyes because she inherits a gene for white eyes from the father s X chromosome. 2. The males would either inherit the X chromosome that carries the normal gene from the mother or the one for the diseased trait from the mother. That X chromosome would be paired with the father s Y chromosome (which would not carry a gene) dictating whether the male offspring inherited the disease or not. 3. pink wings 6 white body 6 yellow head 6 black legs 6 4. 50 percent
4 Data Table Subject Sex Eye color (E) Wing color (W) Body color (B) Head color (H) Leg color (L) chromosomes Father XY White Pink White Yellow Brown (X E Y) (Ww) (Bb) (Hh) (Ll) Mother XX White Pink White Yellow Brown (X E X e ) (Ww) (Bb) (Hh) (Ll) Fly offspring 1 Answers will vary on the rest of the data table
SAFETY PRECAUTIONS Review Before Starting Any Experiment Each experiment includes special safety precautions that are relevant to that particular project. These do not include all the basic safety precautions that are necessary whenever you are working on a scientific experiment. For this reason, it is necessary that you read and remain mindful of the General Safety Precautions that follow. Experimental science can be dangerous, and good laboratory procedure always includes carefully following basic safety rules. Things can happen very quickly while you are performing an experiment. Materials can spill, break, or even catch fire. There will be no time after the fact to protect yourself. Always prepare for unexpected dangers by following the basic safety guidelines during the entire experiment, whether or not something seems dangerous to you at a given moment. We have been quite sparing in prescribing safety precautions for the individual experiments. For one reason, we want you to take very seriously every safety precaution that is printed in this book. If you see it written here, you can be sure that it is here because it is absolutely critical. Read the safety precautions here and at the beginning of each experiment before performing each activity. It is difficult to remember a long set of general rules. By rereading these general precautions every time you set up an experiment, you will be reminding yourself that lab safety is critically important. In addition, use your good judgment and pay close attention when performing potentially dangerous procedures. Just because the text does not say be careful with hot liquids or don t cut yourself with a knife does not mean that you can be careless when boiling water or punching holes in plastic bottles. Notes in the text are special precautions to which you must pay special attention. GENERAL SAFETY PRECAUTIONS Accidents caused by carelessness, haste, insufficient knowledge, or taking an unnecessary risk can be avoided by practicing safety procedures and being alert while conducting experiments. Be sure to check the individual experiments in this book for additional safety regulations and adult supervision requirements. If you will be working in a lab, do not work alone. When you are working off site, keep in
SAFETY PRECAUTIONS 2 groups with a minimum of three students per group, and follow school rules and state legal requirements for the number of supervisors required. Ask an adult supervisor with basic training in first aid to carry a small first-aid kit. Make sure everyone knows where this person will be during the experiment. PREPARING Clear all surfaces before beginning experiments. Read the instructions before you start. Know the hazards of the experiments and anticipate dangers. PROTECTING YOURSELF Do not horseplay. Tie back long hair. Follow the directions step-by-step. Do only one experiment at a time. Locate exits, fire blanket and extinguisher, master gas and electricity shut-offs, eyewash, and first-aid kit. Make sure there is adequate ventilation. Keep floor and workspace neat, clean, and dry. Clean up spills immediately. If glassware breaks, do not clean it up; ask for teacher assistance. Never eat, drink, or smoke in the laboratory or workspace. Do not eat or drink any substances tested unless expressly permitted to do so by a knowledgeable adult. USING EQUIPMENT WITH CARE Set up apparatus far from the edge of the desk. Use knives or other sharp-pointed instruments with care. Pull plugs, not cords, when removing electrical plugs. Clean glassware before and after use. Check glassware for scratches, cracks, and sharp edges.
SAFETY PRECAUTIONS 3 Clean up broken glassware immediately. Do not touch metal conductors. Do not use reflected sunlight to illuminate your microscope. Use alcohol-filled thermometers, not mercury-filled thermometers. USING CHEMICALS Never taste or inhale chemicals Label all bottles and apparatus containing chemicals Read labels carefully. Avoid chemical contact with skin and eyes (wear safety glasses, lab apron, and gloves). Do not touch chemical solutions. Wash hands before and after using solutions. Wipe up spills thoroughly. HEATING SUBSTANCES Wear safety glasses, apron, and gloves when boiling water. Keep your face away from test tubes and beakers. Use test tubes, beakers, and other glassware made of Pyrex glass. Never leave apparatus unattended. Use safety tongs and heat-resistant gloves. If your laboratory does not have heat-proof workbenches, put your Bunsen burner on a heat-proof mat before lighting it. Take care when lighting your Bunsen burner; light it with the airhole closed, and use a Bunsen burner lighter in preference to wooden matches. Turn off hot plates, Bunsen burners, and gas when you are done. Have a fire extinguisher on hand. Keep flammable substances away from flames and other sources of heat.
SAFETY PRECAUTIONS 4 FINISHING UP Thoroughly clean your work area and any glassware used. Wash your hands. Be careful not to return chemicals or contaminated reagents to the wrong containers. Do not dispose of materials in the sink unless instructed to do so. Clean up all residues and put them in proper containers for disposal. Dispose of all chemicals according to all local, state, and federal laws. BE SAFETY CONSCIOUS AT ALL TIMES!