Week 4 Day 1 Lab: MENDELIAN TRAITS and INHERITANCE Part 1: Mendelian Traits Alleles are alternative versions of one gene. Alleles are found at the same locus on homologous chromosomes, but may code for different phenotypes. The genotype of a trait is the combination of two inherited alleles at one locus (either of these alleles can be dominant or recessive). The genotype determines phenotype, or how the trait is expressed. Mendelian traits are those that are inherited simply and are expressed based on the presence or absence of dominant and recessive alleles. If one dominant allele is present in the genotype, the dominant phenotype will be expressed. In order for the recessive allele to be expressed, both alleles must be present. If both alleles are the same this is called homozygous (both the same). If one of the alleles is dominant and the other recessive, this is called heterozygous EXAMPLE: Pretend that purple hair is inherited as a Mendelian trait. Having purple hair is a recessive trait, with the allele represented by the p. Having white hair is the dominant trait, with the allele represented by the letter P. An individual who has the homozygous dominant genotype is written PP and has the phenotype of white hair. An individual who has the homozygous recessive genotype is written pp and has the phenotype of purple hair. An individual who has the heterozygous genotype is written Pp and has the phenotype of white hair. A. Below is a table and list of traits that are inherited in this way. Record which phenotype you display for each trait by marking the appropriate column with an X (for these traits this will be presence or absence of the trait). The photographs supplied will help you determine whether you do or do not exhibit the trait. After presence/absence data are recorded for each student, total the number of students displaying each phenotype in the class. TRAIT PRESENT ABSENT Dominant Class Total State (Present): 1. Widow s peak X Present 2. Attached earlobe X Absent 3. Hitchhiker s thumb X Absent 4. Mid-digital hair X Present 5. Short hallux X Present 6. Left thumb hand clasping X Present 1
Trait Descriptions: 1. Widow s peak: A distinct downward point of the hairline at the center of the forehead is called a widow s peak (if you are a lady and have this trait you are supposed to outlive your husband). A receding hairline due to baldness will mask this trait, so try to remember what you may have looked like in your younger days. The dominant phenotype for this trait is to have a widow s peak, while a straight hairline is recessive. 2. Attached earlobes: The inheritance of a dominant allele results in earlobes that hang free rather than attaching directly to the side of the head. Those with attached earlobes are homozygous recessive for the trait. 3. Hitchhiker s thumb: Hold your hand up like you re trying to hitch a ride. If the end of your thumb bends backwards more than 45 degrees, you have hitchhiker s thumb. If your thumb is straight, start thinking of another way to get a ride. Straight thumbs are dominant, while expression of the hitchhiker s thumb is recessive. 4. Mid-digital hair: The presence of hair on the middle segment of the fingers is due to the presence of a dominant allele. Make fists with both hands. Examine the surface of the middle finger segments for the presence of any hair. Those with no hair are homozygous recessive. 5. Short hallux: Compare the length of your big toe (hallux) to your second toe. If your big toe is shorter than your second toe, you have inherited a dominant allele. If your big toe is equal to or longer than your second toe, you are homozygous recessive for this trait. 6. Left thumb hand clasping: Fold your hands together by interlacing your fingers. Which thumb is on top? Try interlacing your fingers the opposite way. It will feel awkward and may require some thought to accomplish. Those who place the left thumb over the right have inherited a dominant allele, while those who place the right thumb over the left are homozygous recessive. B. Imagine your lab section is a random sample of the population, and use what you have learned from this exercise to answer the following questions: 1. Were the dominant phenotypes of the traits the ones that were most often expressed? Were any of the traits particularly rare in your sample? After using online populations samples, it was found that about 63% of those sampled conformed to the dominant state of the phenome type. In other words, about 63% of the time, if the dominant state was absent or present, the sample population were correspondingly absent or present. 2
2. What does the variation found in your sample mean for the entire population? Given the sample sizes used and that it was from the U.S., the variation is a good representation of the U.S. population. However, if this were a limited classroom situation, I do not feel it would have been a good representation of the facts as it is narrow in scope diversity. 3. Why is variation in traits, phenotypes and genotypes important for natural selection? Variation is important because it enables us to adapt and allows those better equipped to make better use of their resources, eventually either out-surviving inter-breeding these environmentally-adapted variences into a better organism or species. Part 2: Mendelian Inheritance Mendel came to some amazing conclusions about heredity through observation and some very simple cross-breeding experiments with pea plants. For instance, he was able to identify that genes are passed down from both parents, can be expressed in different ways, and act as discrete units. Two principles were illustrated: The Law of Segregation simply states that the two alleles a parent has for a given gene will separate during meiosis so that each gamete has only one of the two alleles. The Law of Independent Assortment tells us that genes coding for different traits and found on different chromosomes, will separate independently of one another in the formation of gametes. (i.e. chromosomes do not influence the segregation of other chromosomes) Punnett Squares To help understand Mendel s laws, we can use a Punnett Square. Punnett Squares are designed to illustrate how alleles for a gene can be passed on from parents and combined and inherited in offspring. A. Punnett Squares: Monohybrid Cross (crossing one gene) In the example below, the letters H and h represent the two alleles found for the gene expressing mid-digital hair. Remember from earlier that having hair on your knuckles is the dominant state, and is represented here by H. A Punnett Square is organized with the potential gametes produced by parents (the F 0 generation) above and to the side of the square, here indicated by the sperm and eggs. Only one allele from each parent will be 3
passed on in each gamete, as described in Mendel s Law of Segregation. The interior of the Punnett Square (shaded gray) is where you find all possible combinations of alleles that could occur in the offspring (the F 1 generation) once the gametes from both parents find combine. Each potential offspring genotype is equally likely to occur. QUESTIONS: 1. What is the genotype of the mother? Hh 2. What is the phenotype of the mother? Observable as Hh - trait will be visible. 3. List all the genotypes possible in the offspring from this mating? The three groups are: HH, Hh, hh 4. Out of the four zygotes produced how many are homozygous dominant? One is homozygous dominant 5. Out of the four zygotes produced how many are heterozygous? Two are heterozygous 6. Out of the four zygotes produced how many are homozygous recessive? One is homozygous recessive. *****This results in a genotypic ratio of 1:2:1***** 7. What are the possible phenotypes of the offspring? Phenotypes are observable. The only observable possibilites are Hh and hh. 8. What would the phenotypic ratio be? 3:1 is the phenotypic ratio in this situation. Read the explanations and answer the questions below based on what you have learned from the Punnett Square above. Make sure you understand the basics, because more complex squares are next. QUESTION 9: (Part 1) Mendel discovered that his pea plants could have either green pea pods or yellow pea pods. G represents the dominant green color, and g represents the recessive yellow color. Mendel found that one of his favorite plants was heterozygous for pod color. a. What is his plant s genotype for pod color? His plant's genotype is Gg. b. What is this plant s phenotype for pod color? The phenotype is observable as Gg. c. In order for the plant to have the recessive pod color, what would the genotype have to be? The recessive gene the plant would need to have is gg. 4
QUESTION 9: (Part 2) You decide you want to have pea pods with both yellow and green coloration in your garden. You begin your experiment with the seeds from a homozygous green plant and a homozygous yellow plant. These parents are the F 0 generation of your experiment. (***Remember that green is dominant and yellow is recessive***) a. What are the genotypes for each plant? Yellow: The genes are bb. b. Fill in the Punnett Square below to illustrate a cross between the two plants. Green: The genes are BB. B B b Bb Bb arent 2 ( GG ) b Bb Bb c. What is/are the possible genotype(s) for the offspring produced by crossing the plants? This group of offspring is the F 1 generation. The only type for this generation is Bb. d. What is/are the possible phenotype(s) for the F 1 generation? All would be observable as Bb, the dominant trait will be visible. 5
QUESTION 9: (Part 3) You discover that the F 1 generation does not provide you with both color pea pods to compare in your garden. You decide you may as well not let all these pea pods go to waste, so you make a cross between two of the F 1 generation to discover what might happen next. The product of this second cross is called the F 2 generation. B b a. Fill in the Punnett Square below to illustrate a cross between two members of the F 1 generation B BB Bb b Bb bb b. What is/are the possible genotype(s) for the F 2 generation? The possible genotypes consist of: BB, Bb, bb. c. What genotypic ratio does this result in? One : : Two One d. Does this second generation give you the variety that you were originally looking for in your garden? Yes, it should eventually provide me with the color I am looking for. e. In your own words, explain how the Law of Segregation works when gametes are created: By splitting the pair of alleles, the Law of Segregation allows genes to cross during the reproductive sequece and obtain a differing allel from the other reproductive donor. 6