X-chromosome Genetics (The X-files)

GENA workshop 2009 Berry and Holmes Lesson Plan Overview In mammals the behavior of genes on the X-chromosome does not follow the canonical patterns of inheritance and expression exhibited by genes on the autosomes. In this unit students engage in hypothesis building activities to discern the mechanism of sex determination and the nature of sex-linked traits. For more advanced students we also present a hypothesis-building exercise that examines the nature and consequences of X-chromosome inactivation. In each stage students will be asked to use their existing knowledge and new data to formulate hypotheses to learn a new concept. Pre-tests will test prior knowledge of the subjects, while bell work questions will be used as formative assessments to measure mastery of material. The goal will be to learn new material while consolidating previously covered concepts. This lesson is part of a unit on Mendelian and non-Mendelian genetics. The unit culminates in student research into, and presentation of, information about specific human genetic disorders. Student misconceptions addressed in the unit

• Genes on the X-chromosome are responsible for female-specific traits • X-linked traits will be more common in females • The X-chromosome is only involved in sex-determination • Genes on the X-chromosome obey simple Mendelian rules • Changes in gene dosage will have neutral effects

Connecticut State Standards Addressed in the Unit

Heredity and Evolution 10.4 In sexually reproducing organisms, each offspring contains a mix of characteristics inherited from both parents D38 Deduce probable mode of inheritance from pedigree diagrams (e.g., sex linked traits)

Major Science Concepts Addressed

• Genetics of sex determination in mammals • Genetics of sex-linked traits • Dosage compensation in mammals

Objectives of the Unit

• Students will be able to analyze a pedigree to identify the mode of inheritance • Students will be able to predict the inheritance of X-linked traits using Punnett squares • Students will be able to recognize a trait affected by gene dosage • Students will be able to form hypotheses given a set of observations

Prerequisite skills that students will bring to the unit

• Students should be familiar with basic patterns of chromosome transmission and karyotypes.

• Students should be familiar with meiosis and crossing over.

• Students should be familiar with the patterns of inheritance of autosomal genes (Mendelian patterns)

• Students should be familiar with basic pedigrees (dominant and recessive traits) • Students should be familiar with the use of Punnett squares

Procedure

Pretest:

Before the lesson a pre-test is given to determine prior knowledge of sex determination and sex-linked traits. Engagement:

Students are asked “What determines sex in mammals?” Students are then shown pictures of male and female mammals exhibiting sexual dimorphism. Explore and Explain:

Day 1

1. Sex determination: Students are shown human karyotypes in which the sex of each individual is identified.

2. Students work in small groups to formulate hypotheses for how sex is determined. Groups present their possible hypotheses.

Expectation: At least two hypotheses should emerge.

A. The presence of two X chromosomes causes the development of females.

B. The presence of a Y chromosome (or one X and one Y) determines the development of males.

Also possible: C. The presence of only one X chromosome causes the development of males.

3. Students are then given additional data in the form of more karyotypes. These karyotypes will include XXY and XO individuals. Students will again break into small groups and test/refine their hypotheses.

4. Additional karyotypes are shown (XXX, XYY). Further discussion should result in a unifying hypothesis for sex determination.

5. Class discussion may then include the introduction of additional data: SRY gene on the Y chromosome, and individuals whose SRY gene has translocated from the Y chromosome to the X chromosome (i.e., are female by the criteria of karyotype, but male by most phenotypic measures).

Extend/Explain: Emphasis is given that the X chromosome contains many genes unrelated to sex, but crucial to body development and functioning. The concept of the pseudoautosomal region allowing for crossover between the X and Y chromosomes during meiosis is also developed.

6. Extend: Students are shown that other animals have different means of sex determination.

Day 2

1. Bell work: “True or false- Genes on the X chromosome are responsible for female specific traits. Explain.”

2. Sex linked traits/X-linked inheritance: Students are presented with a description of T.H. Morgan’s discovery of white-eye color in a male fruit fly.

3. Students will formulate strategies to determine the inheritance pattern of white eyes.

4. Students will be reminded of Mendel’s procedures if they do not come up with a strategy.

5. Students will learn that Morgan crossed the white-eyed male to a red-eyed female and the resulting F1 generation was all red eyed.

Students will be asked what pattern of inheritance the results seem to support (recessive), and what the next step would be (F2). The resulting 3:1 ratio in the F2 generation will presented, showing a recessive pattern, but with the information included that all the white-eyed flies were male. Explain: After ideas for explanations for the pattern are elicited, corresponding Punnett squares will be used to demonstrate the X-linked inheritance.

Explore/Extend:

6. To further reinforce the concept of X-linked transmission of traits students will examine the pattern of inheritance of color blindness and hemophilia.

7. Students will practice Punnett square word problems as class work or homework.

Day 3

1. Bell work: “Are X–linked recessive traits more common in males or females? Explain.”

2. X-inactivation: Students are presented with phenotypic information about calico cats.

3. Students are challenged to form hypotheses in small groups about why there is a female-specific coat-color pattern.

4. Following discussion of these hypotheses students are shown the corresponding genotypes for the different coat colors.

5. Student groups will refine their hypotheses.

6. Students will be presented with an explanation for calico coat patterns. During follow up discussion the importance of gene dosage is emphasized (e.g., by reviewing the causes of Down Syndrome).

Assess:

Day 4

1. Bell work and question during lessons provide information as formative assessment.

2. The pre-test is given again as a post-test.

Results and feedback: This lesson was implemented in two advanced level freshman and sophomore biology classes. One class had 23 students, and the other had just 13 students. In the future, this lesson will be modified for use in general biology also. Students increased scores from 42% correct on the pre-assessment (note that answering at random is expected to yield average scores of 25%) to 78% correct on the post-assessment. Many students reported finding the hypothesis building exercises helpful in retaining the concepts. Some students came up with a very good hypothesis for the calico cat color pattern. They presented X-linked co-dominance as a hypothesis. The hypothesis showed very good reasoning, and provided a good starting point for explaining X-inactivation. However, some students retained this misconception until the summative assessment at the end of the unit. The knowledge gained during this lesson provided students with a deeper understanding of the genetic disorders they researched toward the end of the unit. Students were able to use the concept of gene dosage to infer the effects of gene dosage of the SHOX (short stature homeobox) on height. SHOX is located on the pseudoautosomal region of X and Y. Individuals presented Klinefelter’s, Turner’s, and XYY reported height differences as symptoms. Putting that information together allowed the class to hypothesize that gene dosage of the SHOX gene was responsible for differences in height. This informal and unplanned part of the unit was most gratifying. Reflections: The experience of working with other professionals who were interested in genetics and committed to quality education through the GENA project was very enjoyable. My experience collaborating with Scott was nothing but positive. I appreciate his depth of knowledge and ideas for hypothesis driven learning. One sophomore student is now working with Scott at the University. I hope Scott and I will be able to continue the collaboration and cooperation in the future. The ability to foster student’s interest in biology at a higher level is very rewarding.

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Sex Determination and Sex Linkage

http://www.physorg.com/news177315628.html

http://www.wwd.com/lifestyle‐news/eye/joe‐dimaggio‐and‐marilyn‐monroe‐continued‐1963733//?full=true

KaryotypesFemale Male

Other KaryotpesFemale  Male

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Even more KaryotypesFemale Male

A closer look at the sex chromosomes:Pseudoautosomal regions allow for synapsis and 

crossing over during meiosis

Sometimes there are errors in crossing over (illegitimate crossing over)

SRY

Phenotypically Female Phenotypically Male

– Other systems of sex determination exist in other animals and plants

22+XX

22+X

76+ZW

76+ZZ

32 16

Figure 9.22 D

Figure 9.22 C

Figure 9.22 B

•9.23 Sex‐linked genes exhibit a unique pattern of inheritance

– All genes on the sex chromosomes are said to be sex‐linked

– In many organisms the X chromosome carries many genes unrelated to sex

– In Drosophilawhite eye color is a sex‐linked trait.  T.H. Morgan found a few white eyed fruit flies in his population

Figure 9.23 A

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• How do we determine the inheritance pattern of white eyes?

• Plan:

What Morgan did:1. Bred pure strain of white eyed fruit flies 2. Cross red eyed and white eyed

F1 Result: All red eyed offspring, so white is    

Recessive

3. Cross F1 Offspring:F2 Result: 3:1 ratio (3 red eyed : 1 white eyed), 

so confirms recessive, BUT …

• All white eyed offspring were male

• What happened?

Female Male

Sperm 

Xr Y 

XR

Xr Y ×XR XR

XR Xr XR YEggs 

R = red‐eye alleler = white‐eye allele

Female Male

Sperm 

XR Y 

XR

XR Y ×XR Xr

XR XR XR Y

Eggs 

Xr Xr XR Xr Y

Female

Sperm 

Xr Y 

XR

Xr Y ×XR Xr

XR Xr XR Y

Eggs 

Male

Xr Xr Xr Xr Y

Figure 9.23 B Figure 9.23 C Figure 9.23 D

Figure from http://www.accessexcellence.org/RC/VL/GG/sex.php

CONNECTION•Recessive Sex‐linked disorders affect mostly males

– Most sex‐linked human disorders • Are due to recessive alleles• Are mostly seen in males

Figure 9.24 A

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Figure from http://www.sciencecases.org/hemo/hemo.aspFigure from http://www.sciencecases.org/hemo/hemo.asp

Calico Cats Color patterns when breeding calico cats

Why no Male calico cats?

Calico Cat Genotypes X‐Inactivation

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• Because females have twice as many X chromosomes as males, Dosage compensation must take place to even out gene expression.

• Dosage compensation is achieved by one X chromosome in each female cell coiling up into a Barr Body.

During embryonic development (a few hundred cells or fewer into development), one copy of each X chromosome coils up into a Barr body.  Which copy coils up is a random occurrence.  However, each cell produce by mitosis after that point has the same copy coiled up. 

Advanced Biology- Sex Determination and Sex Linkage Name:____________ Date:___________ 1. Color blindness is a recessive X-linked trait. Are males or females more likely

to be colorblind? 2. If a woman is colorblind, what is her genotype? Was her father colorblind?

Explain. 3. If a boy is colorblind, do you know if his father was colorblind? Why or why

not? 4. A woman carrier for the colorblind gene marries a man who can see color.

What is the genotype of the man? What will the phenotypic and genotypic ratio of their children be? Include the sex of the children.

5. A colorblind man marries a carrier woman (heterozygous). What ratio of their

children will be colorblind? Show work. Include the gender of the children. 6. Why are Y-linked traits less common? Explain.

GENAreflections� Pete Berry The experience of working with other professionals who were interested in genetics and committed to quality education through the GENA project was very enjoyable. My experience collaborating with Scott was nothing but positive. I appreciate his depth of knowledge and ideas for hypothesis driven learning. One sophomore student is now working with Scott at the University. I hope Scott and I will be able to continue the collaboration and cooperation in the future. The ability to foster student’s interest in biology at a higher level is very rewarding. �ScottHolmesFrom this geneticist’s standpoint, the GENA experience was extremely positive. I appreciated the opportunity to interact with Pete and the other high school teachers. As was noted several times, many or most college faculty aren’t explicitly trained to be educators. I appreciated the opportunity that the workshop gave me to think more deeply about learning strategies, teaching plans, assessment, etc. This was timely, as I began teaching the freshman introductory biology course at Wesleyan for the first time this past fall. Pete was very effective in communicating the challenges and opportunities of teaching high school biology, particularly the constraints of the relatively short class time and the need to cover state-mandated standards. It was clear that the ideas I came to the workshop with were overly ambitious. I thought we were successful in formulating a plan that delivered valuable content in the context of a hypothesis-building exercise. An additional positive outcome of the relationship that the GENA project fostered was that Pete put a Middletown High School sophomore in touch with me, and she has been working in my lab at Wesleyan since December. �