Answers Lab Mendelian Genetics

Mendelian Genetics Part 1: Terminology Beginning students of biology always learn about Mendelian genetics. Inevitably, the study of inheritance always leads to additional questions. In fact, Mendelian inheritance patterns are exceedingly rare, especially in humans. We now know that inheritance is much more complex, usually involving many genes that interact in varied ways. Nonetheless, a clear understanding of basic inheritance patterns that follow Mendel’s original observations will provide a springboard for understanding current scientific exploration. Inheritance patterns that follow Mendelian rules are as follows: • Traits are governed by single genes • There are two alternate forms of a gene, know as alleles • Alleles are expressed as dominant and recessive It just so happened that the traits Gregor Mendel observed in his pea plants did indeed conform to these rules. After collecting and analyzing his data, Gregor Mendel developed 2 laws of inheritance: The Law of Segregation and the Law of Independent Assortment. 1. Describe these laws below: A. The Law of Segregation Alternative alleles of a characteristic segregate independently during meiosis B. The Law of Independent Assortment Genes located on different chromosomes assort independently from one another. 2. Before you can work with problems involving Mendelian inheritance, you need to be comfortable with the following terms: Diploid 2 matched sets of chromosomes Haploid Allele Dominant 1set of chromosomes The specific variation of a gene A dominant trait will be inherited unchanged in a hybridization. The dominant allele is the variety of the gene that will produce the dominant trait. A recessive trait will disappear in the offspring in a hybridization. The recessive allele is the variety of the gene that will produce the recessive trait. Recessive Genotype The alleles present for a characteristic. Homozygous Heterozygous Phenotype The presence of one type of allele for a characteristic in the genotype. The presence of 2 different alleles for one characteristic in the genotype. The physical appearance. Part 2: Mendel’s First Law- Law of Segregation The Law of Segregation states that alternative alleles of a trait segregate independently during meiosis. Using a technique known as Punnett Square analysis, we will see how Mendel analyzed his monohybrid crosses to come up with the Law of Segregation. Procedure Carefully follow each step to create a Punnett square analysis. You can use these SAME general procedures to analyze EVERY Punnett Square you do! Problem: In pea plants, height is coded for by the “T” gene. The dominant allele (T) codes for the tall phenotype while the recessive allele (t) codes for the short phenotype. Make a cross between a true breeding tall pea plant and a true breeding short pea plant. 1. What are the phenotypes of the parent plants? The parents are considered the P generation. tall short 2. Determine the genotypes of each parent plant. TT t 3. Imagine each parent goes through MEIOSIS to produce gametes. List the genotype(s) of the possible gametes that each parent would produce. T t 4. Create a Punnett square that displays the genotypes of the possible offspring. Also label the PHENOTYPES of the possible offspring. These offspring are considered the F1 (first filial) generation. t T Tt 5. Now allow the F1 generation to self-pollinate. What are the possible gametes that each F1 parent can produce? T t 6. Create a Punnett square that displays the genotypes of the possible offspring. Also label the PHENOTYPES of the possible offspring. These offspring are considered the F2 (second filial) generation. T T t ______________ TT Tt t Tt tt Note: Always reduce the phenotypic and genotypic ratios to their lowest terms. 1. What is the phenotypic ratio of the F1 generation? Tall : short 4:0 2. What is the genotypic ratio of the F1 generation? TT : Tt : tt 0 :4:0 3. What is the phenotypic ratio of the F2 generation? Tall : short 3:1 4. What is the genotypic ratio of the F2 generation? TT : Tt : tt 1 :2:1 Part 3: Probability Do the expected and observed phenotypic and genotypic ratios always match up in real life? In the case of flipping coins, we would expect to see heads 50% of the time and tails 50% of the time. But, does this always occur? Let’s explore! Materials 2 coins Procedure 1. Working with a partner, take two coins and assume that heads represent the dominant allele (A) and tails represents the recessive allele (a). The genotype for each coin is heterozygous (Aa). 2. Assume that each coin represents one parent. When a single coin is flipped, one gamete is formed (through the process of meiosis). If the flipped coin is on heads, then the gamete has the dominant allele (A). When both coins are flipped simultaneously, there will be two possible gametes which can combine through fertilization to form a zygote. Each time you flip both coins, you will record the “genotype” of the offspring. 3. Flip the coins 100 times and record your results in the chart below. Expected r esults (After 100 flips) Genotype Expected count Ratio Class results Your r esults (# of flips with each outcome) Tally Observed count (4*count) total flips Ratio Observed count Ratio AA 25 1 127 1.02 Aa 50 2 253 2.02 aa 25 1 120 0.96 Total flips 100 -- Total flips 100 Total 500 flips Calculate the ratios using this formula: Genotypic Ratio = # of possible combinations (4) x # of flips of a given genotype (from tally) total number of flips counted (100) Note: If calculating class totals, the demoninator in this equation is equal to the total of all flips counted by all students in the class. 4. Record your results on the classroom computer and your instructor will report the numbers for the “Class Totals” column. 5. What is the expected genotypic ratio for a cross between two Aa coins? TT : Tt : tt 1 :2:1 1. Did the observed and expected genotypic ratios match? Why or why not? They did not match perfectly. The class ratio matched closer than our group ratio. The class ratio had a larger sample size. Part 4: The Law of Independent Assortment The Law of Independent Assortment states that genes located on different chromosomes assort independently from one another. To see the effects of this law, you must examine two different genes that are carried on two different chromosomes. We can investigate this phenomenon by looking at baby. For this experiment, each group will examine a special ear of corn. These ears were created when a mama and papa corn plant, both heterozygous for SEED COLOR (P = purple, p= yellow) and SEED SHAPE (S = smooth, s = wrinkled), made baby corns. Corn is cool, because an EAR of corn is just a WHOLE BUNCH OF BABIES held in one place! By counting the corn babies (each kernel is a baby), you can investigate the principle of Independent Assortment. Materials 1 ear of corn/group Procedure 1. What were the phenotypes of the mama and papa corn plants that gave rise to your cob of babes? (Read the previous paragraph to answer this question!) Purple smooth 2. What were the genotypes of the mama and papa corn plants that gave rise to your cob of babes? PpSs PpSs 3. What were all the possible gametes each parent corn could produce? Mama corn: Papa corn: PS pS Ps ps PS pS Ps ps 4. Make a dihybrid cross illustrating ALL the POSSIBLE baby corns produced by these parents. Then calculate the EXPECTED phenotypic ratios of the babies. 9 Purple Smooth: 3 Purple Wrinkled: 3 Yellow Smooth: 1 Yellow Wrinkled PS pS Ps ps PS PSPS PSpS PSPs PSps pS pS pSpS pSPs pSps Ps PsPS PspS PsPs Psps ps psPS pspS psPs psps 5. Count and record the phenotypes of 100 kernels on your cob. Record your results below. Smooth Purple Tally Total Expected ratio Phenotypes of possible offspring Smooth Yellow Wrinkled Purple Tally Total Tally Total Wrinkled Yellow Tally Total 56 9 19 3 19 3 6 1 201 8.12 99 4 54 2.18 42 1.7 Total # counted Observed Ratio (16*count) total flips Observed Ratio (Class average) Calculate the ratios using this formula: Phenotypic Ratio = # of possible combinations (16) x # of kernals of a given phenotype total number of kernals counted (100) Note: If calculating class totals, the demoninator in this equation is equal to the total of all kernals counted by all students in the class. 1. Do the observed phenotypes agree with the expected phenotypes? Why or why not? They did not match perfectly. The class ratio matched closer than the group ratio. The class ratio had a larger sample size. 2. Can you determine the genotypes of the purple kernels or the smooth kernels in this lab exercise? Why or why not? No, the genotype of the purple kernels could be PP or Pp. 3. Can you determine the genotypes of the yellow kernels or the wrinkled kernels in this lab exercise? Why or why not? Yes, the genotype of the yellow kernels is pp.