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Understanding Sex-Linked Genes and Linked Genes in Eukaryotic Genetics, Lab Reports of Biology

Morehouse CollegeBiology

The concepts of linked genes and sex-linked genes through the example of pea plants and human genetics. Students will learn about the difference between sex chromosomes, the location of genes on chromosomes, and the independent assortment of genes during meiosis. Exercises and punnett squares to help students understand these concepts.

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Morehouse College
BIO 111 General Biology
1
Peer-Lead Team-Learning (PLTL) Workshop 11
Eukaryotic Genetics 2
Introduction
In this second workshop on eukaryotic genetics, we will explore the phenomena of linked
genes and sex linkage. This workshop builds on the concepts and skills developed in the
previous workshop, Eukaryotic Genetics 1, so you should do these two workshops in
sequence. Linked genes are genes that are physically located on the same chromosome.
Thus, linked genes are different loci on the same DNA molecule. Sex linked genes are
genes that are physically located on the X-chromosome. There are some genes located
on the Y-chromosome but these are called Y-linked rather than sex linked.
Pre-Workshop Assignment
Prior to your laboratory class meeting you should complete the questions given below
and bring your completed work to your laboratory class meeting.
Activity A. Dihybrid crosses. In this activity you review the skills for genetic problem
solving as they apply to setting up and solving a dihybrid cross. Write out all the answers
and be ready to share them at the workshop.
1. In garden peas, purple flower color is dominant over white flower color, and smooth
seed coat is dominant over wrinkled seed coat. The genes for seed coat and flower color
are located on separate chromosomes. Shown below in Fig 11.1 is a cross between a
parent plant that produces smooth seeds and purple flowers and one with wrinkled seeds
and white flowers. Each is homozygous for both genes. Using the rules for writing
genotypes, assign a genotype to each plant:
2. Label the alleles in Fig 11.1 for the parents, gametes and offspring. What is the
genotype of the F1 offspring? ___________ Are any other combinations possible?
Explain. _____________
3. What kinds of seed coats and flowers will be produced by the F1 offspring?
______________
4. Set up and complete a Punnett square for this cross. What are the genotypic and
phenotypic ratios? ___________________________________________________
5. The F1 offspring are called dihybrids, referring to the fact that each of two gene pairs
is heterozygous.
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BIO 111 General Biology Peer-Lead Team-Learning (PLTL) Workshop 11 Eukaryotic Genetics 2 Introduction In this second workshop on eukaryotic genetics, we will explore the phenomena of linked genes and sex linkage. This workshop builds on the concepts and skills developed in the previous workshop, Eukaryotic Genetics 1, so you should do these two workshops in sequence. Linked genes are genes that are physically located on the same chromosome. Thus, linked genes are different loci on the same DNA molecule. Sex linked genes are genes that are physically located on the X-chromosome. There are some genes located on the Y-chromosome but these are called Y-linked rather than sex linked. Pre-Workshop Assignment Prior to your laboratory class meeting you should complete the questions given below and bring your completed work to your laboratory class meeting. Activity A. Dihybrid crosses. In this activity you review the skills for genetic problem solving as they apply to setting up and solving a dihybrid cross. Write out all the answers and be ready to share them at the workshop.

  1. In garden peas, purple flower color is dominant over white flower color, and smooth seed coat is dominant over wrinkled seed coat. The genes for seed coat and flower color are located on separate chromosomes. Shown below in Fig 11.1 is a cross between a parent plant that produces smooth seeds and purple flowers and one with wrinkled seeds and white flowers. Each is homozygous for both genes. Using the rules for writing genotypes, assign a genotype to each plant:
  2. Label the alleles in Fig 11.1 for the parents, gametes and offspring. What is the genotype of the F 1 offspring? ___________ Are any other combinations possible? Explain. _____________
  3. What kinds of seed coats and flowers will be produced by the F 1 offspring?
  1. Set up and complete a Punnett square for this cross. What are the genotypic and phenotypic ratios? ___________________________________________________
  2. The F 1 offspring are called dihybrids , referring to the fact that each of two gene pairs is heterozygous.

BIO 111 General Biology A cross of the F 1 dihybrids (PpRr x PpRr) produces a somewhat more complex situation, although the steps are largely the same as in a monohybrid cross of heterozygous individuals. Fig 11.2 shows how to do the most difficult step, forming the gametes. a. The first pair of alleles (P and p) assorts into 2 different gametes. b. The second pair of alleles (R and r) assorts independently of the first pair so that R may be in the same gamete as either P or p forming PR or pR. Likewise, r ends up about half the time with P and half the time with p making Pr or pr.

  1. Label alleles in the F1 parent and the gametes in Fig 11.2. How many different types of gametes will be produced by a dihybrid heterozygous parent? _____________What is their ratio? __________________________.
  2. A Punnett square is set up to determine the genotypes of the offspring produced by the cross. This Punnett square (Table 11.1) has 4 rows and 4 columns. Why?
  3. Complete the Punnett square for this dihybrid cross by: a. entering the gametes for each parent across the top and down the left side. b. fill in the genotypes of the offspring in the 16 cells using the genotype rules
  4. The final step is to determine the phenotypic ratio of offspring. (The genotypic ratio is complex and will not be used in our study.) a. determine the phenotype of each individual in the 16 cells of the Punnett square. b. list each of the different phenotypes expressed (ex: purple-round) in the space below. c. count the number of each phenotype and enter it below the phenotype. d. write out the phenotypic ratio: ___________________________ purple-round : _____________ : _____________ : ____________ count _______ _______ _______ ______

BIO 111 General Biology

  1. State in your own words the Principle of Independent Assortment. Use the cross you just completed to illustrate your definition.
  2. Summarize the steps in solving a dihybrid cross as a flow chart in the box below. __9. Do genetics problems #4 and #10 at the end of this workshop.
  3. Write the correct genotype for the cell in Figure 11.1. __________________________
  4. If the alleles present were G, a, A, t, T, and G , what would be the correct genotype? ____________________________.
  5. On Fig. 1.2, add 2 R alleles at the top end of the smaller chromosomes. Now write out the genotype. _____________________________________________ Activity B. Linkage.
  6. In Figure 11.3, three genes are shown. Two have loci on the larger chromosome, and one on the smaller chromosomes. Based on physical appearances alone, which gene pairs (ex. E/e & F/f) are linked and which are not linked? Indicate all possible pairs.
  7. Draw all possible gametes that could be formed from the meiotic divisions of this cell and label all the alleles. Is it possible for a gamete could contain E, D, and F? If so, how does this happen?
  8. How many different combinations did you get for the genes D/d and F/f? ___________ How many for the genes D/d and E/e? ________________. Why is the number of combinations different? Explain using a cell diagram.

BIO 111 General Biology

  1. Genes that are linked on the same chromosome assort independently into gametes during meiosis. True or false? Explain.
  2. How does the Mendel’s law of independent assortment apply to: a. different genes with loci on different chromosomes? b. different genes with loci on the same chromosome?
  3. Two plants with the same genotype as the one in Fig. 11.3 were crossed. Provide the following information about the cross. a. genotypes of the parents b. the gametes each parent can produce c. a Punnett square showing the cross d. a genotypic ratio e. a phenotypic ratio. Figure 11.3. Linked and Non linked Genes

BIO 111 General Biology

  1. We represent sex-linked genes as superscripts over the “ X ” symbol. For example, in the case of the gene for color vision (C/c), X C represents the allele for normal vision, while Xc^ 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 ___________.
  2. If the C allele is expressed in the phenotype, a person will have normal color vision. If the c allele is expressed, the person will be colorblind. What are the phenotypes of the man and woman you represented in your drawings in Question #7?
  3. Make the following cross using the steps for solving a genetics problem: a woman who is heterozygous for the color vision allele and a normal 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).
  4. Color blindness is more common in males. Explain.
  5. With what combination of alleles could you get a colorblind female? Figure 11.4. Sex chromosomes Figure 11.5. Sex determination

BIO 111 General Biology

PLTL Workshop Problems

Activity 1. Skills in genetic problem solving. Pair-problem solving. Each pair gets one of the following problems and has two minutes to solve it. Each pair presents its solution to the rest of the workshop. Use Table 11.2 for genotypes of the parent cell. Table 11.2. Genotypes for garden pea plants and people. Organism Genotypes Phenotypes (Traits) PP or Pp Purple flowers pp White flowers YY or Yy Yellow seeds yy Green seeds RR or Rr Smooth seed coat rr Wrinkled seed coat TT or Tt Tall plants Garden Pea Plants tt Short plants EE or Ee Free earlobes ee Attached earlobes RR or Rr Rh positive blood rr Rh negative blood FF or Ff Freckles Humans ff Uniform pigment distribution

  1. Represent the parent cell and different gametes that come from that cell by the alleles they carry. (In some problems there may be more than one possibility. List all possibilities.) a. A dihybrid heterozygous tall pea plant with purple flowers b. A short pea plant that is heterozygous for seed coat c. A pea plant that produces yellow seeds with a smooth coat d. A human who is heterozygous for earlobes and freckles

BIO 111 General Biology

  1. Draw Punnett squares for the six crosses you summarized in #2a-f and fill in the F 1

genotypes.

  1. Determine the genotypic ratios for six crosses in #2 and express them with the genotypes written below the numbers. 5.. Determine the phenotypic ratios for the crosses in #2. Indicate the phenotypes next to the numbers. Activity 2. Concepts in patterns of inheritance. Pair problem solving.
  2. a. Explain briefly Mendel’s Law of Independent assortment. b. Give an illustration that helps in your explanation. c. Give an example of situations in which it applies and doesn’t apply to inheritance patterns.
  3. a. Using a diagram, illustrate the physical basis for linkage or non-linkage between two alleles.

BIO 111 General Biology b. Explain a linkage pattern between genes can be changed. Use a diagram. c. Using two genes, compare the ratio of offspring with different genotypes expected when the genes are linked and non-linked. Explain with a diagram.

  1. What type of test cross might you conduct in order to tell if two genes are linked or non-linked? Set it up and show the expected outcomes.
  2. Using a diagram, explain the difference between a sex-linked and autosomal gene.
  3. a. Using a diagram, explain why sex-linked recessives genes are more often expressed in male than female mammals. b. In what situation can the recessive alleles be expressed in females?

BIO 111 General Biology

  1. 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.
  2. 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.
  3. A woman with AB blood is heterozygous for free-earlobes and her husband has type O blood and 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?
  4. In fruit flies, body color is normally gray, the expression of a dominant allele (b+), while black color is the expression of a recessive allele (b). Normal long wings result from an allele (vg+) and short vestigial wings are the expression of a recessive allele (vg). A male that is heterozygous for both genes is mated with a black, vestigial winged female. The cross produced 1000 offspring of which 470 had gray bodies and long wings and 480 had black bodies and vestigial wings. 24 had gray bodies and vestigial wings and 26 had black bodies and long wings. Determine if the two genes are linked or not by showing the predicted outcomes with and without linkage. What explains the gray-vestigial and black-long winged flies?
  5. In the following cross: AaBB x aaBb where A , red eye color is dominant over a , green eye color and B , bald is dominant over b , lots of hair, a. What are the resulting genotypes and what are their ratios (assuming no linkage between the two gene loci)? b. What are the resulting phenotypes and what are their ratios? c. What fraction of the offspring is heterozygous for eye color? d. What fraction of offspring is homozygous for baldness?
  6. Consider the following cross: AabbCcDd x aaBbccDd a. What are the odds of the F1 generation being homozygous recessive at gene locus B? b. What are the odds of the F1 generation being homozygous dominant at gene locus D?

BIO 111 General Biology c. What are the odds of the F1 generation being both homozygous recessive at gene locus B and homozygous dominant at gene locus D? [Hint: just combine the two results above. Can you determine how the combination is to be done?] d. Using the way you answered the above three questions, can you think of a general way to approach such multiple-gene-locus probabilities?

  1. Consider the following pedigree. squares = males; circles = females. A filled square or circle indicates that the individual has a disorder that causes premature aging. Procedure: Break the group into three teams. Have each team decide on the answer to questions a and b, below, and then have team one determine the genotype of individual 1, above; team two determine the genotype of individual 2, above; and team three determine the genotype of individual 3, above. Reassemble to discuss the answers and reasons for them. Figure 11.6 Pedigree 1 (Question 12) To answer the questions, use A = dominant allele, a = recessive allele. a. Is the trait dominant or recessive? How do you know? b. Is there any indication that the trait is sex-linked? Give your reasoning. c. What is the genotype of the three individuals (1, 2, 3) indicated above?

BIO 111 General Biology c. Between which pairs is the linkage pattern going to be changed most frequently? Least frequently? Explain why. d. If the percentage of offspring in which the pattern is changed is 20% between A/a and C/c , predict the approximate percentages between A/a and B/b. Between B/b and C/c. e. Share your results with the group before going on.

  1. Imagine that you have performed a cross and seen that there are 499 parental types and 502 recombinants, for a total of 1,001 offspring. a. What is the recombination frequency? b. Are these two genes linked? c. Could they still be located on the same chromosome?
  2. In a cross there are 500 parental types and 100 recombinants. a. What is the recombination frequency? b. Are these two genes linked?
  3. If the recombination frequency between gene A and gene B is 4% and that between gene A and gene C is 8%, is gene B or gene C closer to gene A? Procedure: For question #5, pairs of workshop participants should come forward to a blackboard and add/subtract options on a growing map of the four genes whose recombination frequencies are listed below. They should discuss and justify their reasons (The recombination frequencies below are given in percents, which are the same as map units).
  4. a. Use the following recombination frequencies to map four genes, A-D. Workshop pair Genes Recombination frequency One A, B 8% Two A, C 4% Three A, D 4% Four B, C 4% Five/One B, D 11%

BIO 111 General Biology Procedure: For the whole workshop group to discuss:

  1. b. Consider that there is a fifth gene E that also is linked. If you know that the A-to-E recombination frequency is 4%, can you locate E on the map you constructed above?
  2. c. Why isn’t the recombination frequency between B and D = 12%? Activity 5. Complex Patterns of Inheritance: Challenge Section. If time permits in the workshop, divide the group into two teams and have each solve one of the two problems below. At the end of a prescribed period, the teams should present their work, even in a partially complete state to the other team. Together with guidance from the peer leader, the problems can be solved.
  3. Consider the following cross: AabbCcDd x aaBbccDd a. What are the odds of the F1 generation being homozygous recessive at gene locus B? b. What are the odds of the F1 generation being homozygous dominant at gene locus D? c. What are the odds of the F1 generation being both homozygous recessive at gene locus B and homozygous dominant at gene locus D? [Hint: just combine the two results above. Can you determine how the combination is to be done?] d. Using the way you answered the above three questions, can you think of a general way to approach such multiple-gene-locus probabilities?
  4. The product of one gene can influence the phenotypic expression of another gene. Consider the Black/Brown gene locus in mice and the influence of the alleles at a second gene locus, designated C. BB and Bb animals are black, and bb animals are brown UNLESS the animals also are homozygous recessive at the C locus, in which case they have white fur color. a. Consider the cross: BBCc x BbCc i What color are each of these parents? ii. What percent of the F1 offspring will be black, what % brown, and what % white? b. Consider the cross: BBCc x Bbcc i. What color are each of these parents. ii. What percent of the F1 offspring will be black, what % brown, and what % white?

BIO 111 General Biology Activity 2. Determining the genetic makeup of gametes. For each of the diploid cells in Fig. 11.8, show the genetic makeup of gametes that could result from meiotic divisions of each cell. Note that the chromosomes can be distributed in different ways, giving rise to different genetic combinations. Show all the possible combinations. Activity 3. Genetics Problems. For each of the problems below: a. state the category of which it is an example (e.g., dihybrid cross with dominance, sex-linked, linkage, etc.). b. state what clues led to your decision. c. solve the problem showing your work, step by step.

  1. In guinea pigs, black fur is expressed by a dominant gene B while brown fur is the expression of the recessive allele b. Short hair S is dominant to long hair s. In a cross of individuals with dominant phenotypes for both characters, the total F1 offspring of several matings were as follows: 18 black, short-haired, 7 black long-haired. a. What were the genotypes of the parents? b. Show how the cross would result in the observed outcome of offspring c. What is the relationship between predicted numbers of offspring and actually observed numbers? d. How is cross similar to and different from a monohybrid cross of two heterozygous individuals?
  2. If a woman who is homozygous for normal color vision marries a man who is color blind (a sex-linked, recessive trait), what proportion of their offspring (separately indicate results for males and females among their offspring) would: a. be carriers of the colorblindness trait? b. be colorblind?
  3. If a woman is heterozygous for colorblindness (sex-linked, recessive) and marries a male with normal color vision, among their offspring (separately consider males and females), what proportion would: a. be carriers? b. be colorblind?
  4. In sweet pea plants, two forms of the flower color gene are P and p , the first expressed as purple flowers and the second as red flowers. Also pollen grains (sperm containing reproductive cells) can be with long ( L ) or round ( l ). The two genes are found on the same autosome. Two plants that are heterozygous for both genes are crossed. What are the predicted genotypes and phenotypes of their offspring? Give the genotypic and phenotypic ratios. Set up the problem and show your work.

BIO 111 General Biology

  1. A person who did not have the information about linkage in the previous problem decided to determine if the two genes were linked or not. a. Show the expected outcome of the cross above if the two genes were not linked. b. Compare it to the results in the previous question. c. Suggest some things to look for in distinguishing linked and non-linked genes.
  2. Tomato plants generally have round fruit which is an expression of a dominant allele R while the recessive allele r is expressed in elongated fruit shape. Likewise smooth skin is an expression of S while the recessive s produces tiny hairs (fuzz) on the skin. a. A dihybrid homozygous dominant plant was crossed with a homozygous recessive plant. Show the genotypes of the parents, the possible gametes and the genotypes of the F1 offspring. What are the genotypic and phenotypic ratios? b. Show the cross of two F1 offspring, including the genotypic and phenotypic ratios of the F2 generation. c. Among the F2 are a large number that show both dominant phenotypes. What are the potential genotypes of such individuals? d. Show how you would determine if a particular individual was homozygous for heterozygous for the two genes. This workshop was based on Module 12: Mendelian Genetics 2 by Joseph G. Griswold, PhD, City College of New York, CUNY and David L. Wilson, PhD, University of Miami, FL in PLTL Introductory Biology Modules CD-ROM. 2005. Joseph G. Griswold and Michael Gaines eds. Edited and revised 9/2006 by L. Blumer.