Download Dragon Genetics: Exploring Independent Assortment and Linkage and more Lecture notes Genetics in PDF only on Docsity!
Dragon Genetics
-- Independent Assortment and Genetic Linkage
This activity, by Dr. Ingrid Waldron and Jennifer Doherty, Department of Biology, University of Pennsylvania, © 200 8 , incorporates ideas from Dragon Genetics Lab, 2002, Bob Farber, Central High School, Philadelphia, PA and Dragon Genetics by Dr. Pamela Esprivalo Harrell, in the January 1997 issue of Science Scope, 20:4, 33- 37.^1 In this activity you will study the patterns of inheritance of multiple genes in (imaginary) dragons. These dragons have two pairs of homologous chromosomes in each cell. You will see that, since genes are carried on chromosomes, the patterns of inheritance are determined by the behavior of chromosomes during meiosis and fertilization.
The Law of Independent Assortment
-- Inheritance of Genes on Different Chromosomes
For this activity, we will only consider one gene on each chromosome. These genes are described in the following table. Dominant Alleles Recessive Alleles Chromosome 1 W = has wings w = no wings Chromosome 2 H = big horns h = small horns The mother dragon is heterozygous for the wing gene ( Ww ) and the horn gene ( Hh ). The father is homozygous recessive for the wing gene ( ww ) and the horn gene ( hh ). What phenotypic traits will each parent have? (Phenotypic traits are the observable bodily characteristics.) Draw the appropriate characteristics for each parent. Mother Father Review of Inheritance of Single Genes Draw a Punnett Square to show the inheritance of the horn alleles for a mating between this mother and father. On average, what fraction of the baby dragons will have big horns? (^1) Teachers are encouraged to copy this student handout for classroom use. A Word file (which can be used to prepare a modified version if desired), Teacher Preparation Notes, comments, and the complete list of our hands-on activities are available at http://serendipstudio.org/sci_edu/waldron/.
Predictions of Inheritance of Two Genes on Different Chromosomes To predict the inheritance of the wing and horn genes, you first need to determine the genotypes of the eggs produced by the heterozygous ( WwHh ) mother dragon and the sperm produced by the homozygous ( wwhh ) father dragon. Considering both the wing and horn genes, what different genotypes of eggs could the heterozygous mother dragon produce? Use the figure below to answer this question. Notice that, in a cell that is prepared for meiosis 1, the homologous chromosomes are always paired with each other, but the specific arrangement of the chromosomes can differ. Describe this difference and the effect that this has on the genotypes of the eggs produced. What genotypes or genotype of sperm can the homozygous ( wwhh ) father dragon produce? Draw a diagram to show how meiosis would occur in the father, starting with a diploid cell ready to undergo meiosis 1 and ending with four haploid sperm. Two equally probable chromosome arrangements in Meiosis I:
or
Meiosis II:
or
Gametes
or
with Genotypes:
or
Procedure to Test Inheritance of Two Genes on Different Chromosomes To test whether baby dragons with wings and baby dragons without wings will be equally likely to have big horns, you will carry out a simulation of the simultaneous inheritance of the genes for wings and horns. Since the father is homozygous ( wwhh ), you know that all of the father's sperm will be wh. Therefore, to determine the genetic makeup of each baby dragon produced in your simulation, you will only need to determine the genetic makeup of the egg which is fertilized to become the zygote that develops into the baby dragon. During meiosis, each egg randomly receives one from each pair of homologous chromosomes. Your simulation will mimic this process. For this simulation, each of the mother's pairs of homologous chromosomes will be represented by a popsicle stick with the genes of one chromosome shown on one side and the genes of the other homologous chromosome shown on the other side.^2 Since the mother dragon is heterozygous for both genes ( WwHh ), you will have one Popsicle stick representing a pair of homologous chromosomes which are heterozygous for the wing gene ( Ww ) and another Popsicle stick representing a pair of homologous chromosomes which are heterozygous for the horn gene ( Hh ).
- Hold one Popsicle stick in each hand about 6 inches above the desk. Hold each Popsicle stick horizontally with one side facing toward you and the other facing away (with one edge of the Popsicle stick on the bottom and the other edge on the top). The two Popsicle sticks should be lined up end-to-end, simulating the way pairs of homologous chromosomes line up in the center of the cell during the first meiotic division. Simultaneously drop both Popsicle sticks on the desk. The side of each Popsicle stick that is up represents the chromosome that is contained in the egg. This indicates which alleles are passed on to the baby dragon. Put a I in the appropriate box in the chart below to record the genotype of the resulting baby dragon. Mother ( WwHh ) wh wH Wh WH Father ( wwhh
)^
wh Genotype of baby = wwhh Number of babies with this genotype =____ Genotype of baby = wwHh Number of babies with this genotype =____ Genotype of baby = Wwhh Number of babies with this genotype =____ Genotype of baby = WwHh Number of babies with this genotype =____
- Repeat step 1 three times to make and record three more baby dragons. (^2) For the purposes of this activity, we will ignore the sister chromatids of each chromosome, since they are not relevant for understanding the genetics discussed in this activity. Also, we assume that both of the chromosomes under investigation are autosomes.
Summary and Interpretation of Data
- Compile the data for all the baby dragons produced by all the students in the following chart. Mother (WwHh) wh wH Wh WH Father ( wwhh
wh Genotype of baby =________ Number of babies with this genotype =___ Phenotype: Wings __ or no wings __ Horns big __ or small __ Genotype of baby =________ Number of babies with this genotype =___ Phenotype: Wings __ or no wings __ Horns big __ or small __ Genotype of baby =________ Number of babies with this genotype =___ Phenotype: Wings __ or no wings __ Horns big __ or small __ Genotype of baby =________ Number of babies with this genotype =___ Phenotype: Wings __ or no wings __ Horns big __ or small __
- Do any of the baby dragons with wings have small horns? Does either parent have the combination of wings and small horns? (See page 1.) How did this new combination of characteristics (wings and small horns) arise in some of the baby dragons? (Your answer will include events during meiosis and fertilization, so you may find it helpful to review the diagram of meiosis on page 2 and the chart of fertilization on page 3.)
- On page 3 you used your understanding of meiosis and fertilization to predict whether baby dragons with wings and without wings would be equally likely to have big horns. What was your prediction? Use the class results shown above to complete the following table. Fraction that have big horns Baby dragons with wings Baby dragons without wings Results Do your results match your prediction?
Genetic Linkage
-- Inheritance of Genes which are
Close Together on the Same Chromosome
Obviously, real chromosomes have more than one gene each. In this activity, you will analyze the inheritance of multiple genes which are close together on the same chromosome. We will consider three genes on Chromosome 1 and one gene on Chromosome 2, as indicated in the following table. Dominant Alleles Recessive Alleles Chromosome 1 W = has wings w = no wings F = fire-breathing f = no fire-breathing N = long fangs n = short fangs Chromosome 2 H = big horns h = small horns For this activity, the father is heterozygous for each of these genes ( WwFfNnHh ), and the mother is homozygous for the recessive alleles ( wwffnnhh ). For the three genes on chromosome 1 for the father, the dominant alleles are all located on one chromosome and the recessive alleles are all located on the other homologous chromosome. The Chromosome 1 pairs for the father and mother look like this. Father Mother
( W F N ) (w f n )
(w f n ) (w f n ) Draw the genes for the Chromosome 2 pairs for the father and mother. Father Mother
( ) ( )
( ) ( )
- In the figures below, draw the phenotypes of the heterozygous father and the homozygous mother. Father Mother
- What different combinations of alleles can be found in the different types of sperm that the heterozygous ( WwFfNnHh ) dragon father can produce?^3 To answer this question, complete the genotypes in the figure below. (It may help to note that the alleles on the large white chromosomes are w, f, and n.) How many different combinations of alleles can be found in the eggs produced by the homozygous ( wwffnnhh ) mother dragon? Show the genotype(s) of the mother's eggs.
- Draw a Punnett square to show the genotypes of the baby dragons that could be produced by this father and mother. You may want to use the popsicle sticks representing the father's and mother's chromosomes to help you visualize the different combinations of alleles. Based on this Punnett square, will any of the baby dragons have the dominant allele W for wings, but not the dominant allele F for fire-breathing? Will any of the baby dragons have the dominant allele W for wings, but not the dominant allele H for big horns? Explain the difference in results for the inheritance of the wing and fire-breathing genes vs. the inheritance of the wing and horn genes. (^3) Assume that the wing, fire-breathing and fang genes are so close together on Chromosome 1 that there is no crossing over within this region of the chromosome. Two equally probable chromosome arrangements in Meiosis I:
or
Meiosis II:
or
Gametes
or
with Genotypes:
or
Discussion
The principles of inheritance in these dragons also apply to inheritance in humans, other animals, and plants. However, inheritance in humans, other animals, and plants is much more complex than inheritance in these dragons. First, list the two principles of inheritance illustrated by this Dragon Genetics activity. What are some additional complexities of inheritance in humans, other animals, and plants? Discuss the following questions.
- Genetic linkage is very strong for genes which are located close to each other on the same chromosome. What happens in the case of two genes which are far apart on the same chromosome?
- Are all alleles either completely dominant or completely recessive?
- Do any genes have more than two alleles?
- Does each gene influence only one phenotypic trait?
- Is each phenotypic trait influenced by only one gene (i.e. one pair of alleles on a pair of homologous chromosomes)?
- For the genes that are on the X chromosome in humans and other mammals, what are the differences in inheritance for males vs. females?
