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Reebop Reproduction Why aren’t all baby Reebops the same?

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Presentation on theme: "Reebop Reproduction Why aren’t all baby Reebops the same?"— Presentation transcript:

1 Reebop Reproduction Why aren’t all baby Reebops the same?
This presentation provides a link from the hands-on experiences with the Reebops to understanding the sources for and importance of variation in populations of organisms. It provides a brief introduction to the terms mitosis and meiosis. This PowerPoint presentation provides enough information for students to connect the Reebop activity to these important ideas. Without going into further detail about mitosis and meiosis, they are prepared to go on to the later Reebop activities on Mendelian and population genetics.

2 You just conducted a Reebop breeding program.
What were some of the observable differences between the parent Reebops and their babies? What were some of the observable differences between the babies? In other words, how did the phenotypes of the Reebops differ? Give students an opportunity to share some of their observations from the breeding activity. Instructor note: While phenotype may sometimes be defined as the outward appearance of an organism, phenotype is actually the complete expression of the genotype, be it at the morphological, anatomical, physiological, behavioral, or molecular level. For the purposes of this activity, morphological traits are the only part of the phenotype being considered.

3 How did the phenotypes of the Reebops differ?
Antennae Tails Body segments Humps Eyes Legs Noses Note that the concepts of genotype and dominant and recessive traits will be discussed in the Reebop Ratios activity and PowerPoint. 1, 2, or no antennae Curly or straight tail 2 or 3 body segments 1, 2, or 3 green humps 2 or 3 eyes Blue or red legs Red, orange, or yellow nose

4 Quick-Think Time Having Baby Reebops with identical phenotypes would be better for the Reebops. Do you agree or disagree? Explain your thinking. Give students a few moments to write down their thoughts. Ask for some to share. Continue with a discussion of the benefits of variation. For example, ask students to consider the effect of a new predator that only preyed on Reebops WITH antennae. Would all the Reebops be eaten? Why or why not? Also discuss the idea that being identical could be beneficial if the Reebops are well adapted to an environment that does NOT change. Discuss whether such a constant environment is likely or not, and what could happen to an identical population when something in the environment does change. Note: The concept of natural selection and its effects on gene pools will be more fully developed in the activity Reebop Populations.

5 Benefits of Variation Natural selection acts on random variation that exists in a population of organisms. Variations provide the potential to be adapted to changes in the environment when they happen. Heritable variations already exist in the population and do not arise because of a change in the environment. EMPHASIZE that natural selection acts on pre-existing variation in the gene pool. A common misconception is that individual organisms change IN RESPONSE to changes in the environment and that these phenotypic changes are passed on to their offspring. This erroneous concept is called Lamarckian inheritance after Lamarck, the 18th century scientist who first proposed it as a mechanism for evolutionary change. Be sure that students know the meaning of the word “random.” According to Merriam-Webster OnLine, random means “without definate aim, direction, or rule.” It is important to point out that traits (phenotypes) are not directly transmitted to offspring. Only genes are transmitted. Developmental changes in phenotype that may result from interactions with the environment do not change an individual’s genotype. Discuss how changes in the environment (such as a new predator) can cause a previously existing trait to become an advantage. For example, the Reebop population already included some individuals with no antennae, but perhaps it was neither an advantage nor a disadvantage. The appearance of a new predator resulted in the already existing “no antennae” trait becoming an advantage. Note: The concepts of natural selection and its effects on gene pools will be more fully developed in the activity Reebop Populations.

6 Quick-Think Time Think–Pair–Share
How did the Reebop babies end up being different from their parents and from each other? Provide a minute for thinking and jotting down ideas. Ask students to share with a partner for another minute. Call on a few students to share their ideas with the class. You can vary the procedure by asking students to share their partner’s ideas rather than their own. After giving students a chance to share, continue with the next slides, connecting to various student ideas as you go.

7 Sources of Variation: Each Reebop parent has two copies of each chromosome in all of its body cells. A cell with two sets of chromosomes is diploid. Traits are determined by genes carried on chromosomes. Different forms of a gene for the same trait are called alleles. Both copies of a chromosome might carry the same allele or they might carry different alleles. Instructor Note: For the purposes of this discussion, we are not distinguishing between germline cells and somatic cells. The cells destined to produce sperm and eggs (germline) are set aside very early in embryonic life. There can be different forms of a gene as a result of mutations. Different forms of the gene are called alleles. A gene can consist of a variety of different forms, but only two forms are ever present in a diploid cell. Emphasize that because each parent has two alleles for each trait, and these alleles may differ from one another, much more genetic diversity is possible than if each parent had only one allele.

8 Quick-Think Time Look at the students holding the signs.
The “t” on each sign represents an allele for antennae. If the Reebop parents have the alleles shown on these signs, what kind of antennae do you expect their babies to have? As an attention-getter, use four signs made from pieces of heavy paper with alleles written on them with marker. It is important to note that each sign should have a small “t” written on it. Have four student volunteers come up and hold them. Students should realize that all of the Reebop Babies will have the trait for “no antennae.” Discuss how this result is different than what they observed in the activity. Ask students to imagine that both Reebop parents had two of the SAME ALLELE for EVERY trait. Lead students to understand that if the parents had the same alleles for every trait, all of their babies would be identical. Emphasize that the variation in parent’s alleles is an important source of variation in the population.

9 Sources of Variation: Reproductive Cells (Gametes)
You randomly selected one copy of each Reebop Mom chromosome for an egg cell (female gamete).

10 7 pairs of Reebop Mom chromosomes 14 chromosomes in all
Q q E e D d M m T t L l 7 pairs of Reebop Mom chromosomes 14 chromosomes in all

11 Randomly choose one of each…
q e D m T L a Q E d M t l

12 7 single Reebop Mom chromosomes 7 chromosomes in all
Reebop egg a Q E d M t l 7 single Reebop Mom chromosomes 7 chromosomes in all

13 Sources of Variation: Reproductive Cells (Gametes)
You randomly selected one copy of each Reebop Mom chromosome for an egg cell (female gamete). Chances of selecting the exact same set of chromosomes a second time are extremely small. Each egg contains one set of chromosomes. A cell with one set of chromosomes is haploid. The egg contains one allele for each trait. Everything just said is the same for Reebop sperm (male gametes). In Reebops, there are 128 possible combinations of 7 chromosomes in a gamete due to random assortment. This value is calculated by 2 to the 7th power. For simplicity, all Reebop traits are single-gene traits. So for our purposes, a trait is equivalent to a gene. Later the concept of polygenic traits, such as height and eye color in humans, can be introduced. (See activity Meiosis to Mendel.)

14 Why do you think that gametes are haploid?
Quick-Think Time Why do you think that gametes are haploid? Tip: What would happen every generation if gametes were diploid? Give students a minute to write down their ideas. Through discussion, bring out the idea that if the gametes were diploid, the fertilized egg would have four sets of chromosomes. Explain that this actually can happen. The next slide introduces the concept of polyploidy.

15 Why are watermelons big?
Cells are polyploid if they contain more than two (2n) sets of chromosomes (and genes). For example, triploid (3n) and tetraploid cell (4n) cells are polyploid. Polyploidy is very common in plants, especially in angiosperms. Polyploidy is much rarer in animals. It is found in some insects, fishes, amphibians, and reptiles. From 30% to 70% of today’s angiosperms are thought to be polyploid. Species of coffee plant with 22, 44, 66, and 88 chromosomes are known. Polyploid plants not only have larger cells but the plants themselves are often larger. This has led to the deliberate creation of polyploid varieties of such plants as watermelons, marigolds, and snapdragons. Polyploidy is an important evolutionary process in the plant kingdom, actually enabling new species to be formed in one generation due to the incompatibility of gametes having a different chromosome number resulting in reproductive isolation. Until recently, no polyploid mammal was known. However, the 23 September 1999 issue of Nature reports that a polyploid (tetraploid; 4n = 102) rat has been found in Argentina. Polyploid cells are larger than diploid ones; not surprising in view of the increased amount of DNA in their nucleus. The liver cells of the Argentinian rat are larger than those of its diploid relatives, and its sperm are huge in comparison. Normal mammalian sperm heads contain some 3.3 picograms of DNA; the sperm of the rat contains 9.2 pg. (Source:

16 Extra or Missing Chromosomes
Normal Human Karyotype How is this Karyotype different? Give students a chance to point out differences in the Karyotype. Explain that instead of en entire extra set of chromosomes (as in polyploidy), only one chromosome may be extra. (You may want to introduce nondisjunction as the general term for errors in chromosome division.) Note that in asking students to look for differences, some of them will notice that one has two x chromosomes and the other an x and a y. After discussion, click on the slide to point out these chromosomes. Very few viable forms of aneuploidy (extra or missing chromosomes) exist in humans. A conservative estimate calculates that at least sixty percent of all miscarriages that occur before the twelfth week of gestation are due to an incorrect chromosome number in the developing fetus. One example is Down’s Syndrome. Most adolescents are familiar with a developmental disorder called Down’s Syndrome. A child with Down’s Syndrome has an extra number 21 chromosome. The extra chromosome can come from either the father or the mother, depending on if the nondisjunction occurred during the production of either the sperm or egg cell. The result of nondisjunction is shown on the current slide: The karyotype on the right has an extra #21 chromosome. Students may be interested in finding out how Down’s syndrome is detected in a fetus: If an expectant mother chooses to have an amniocentesis, she will learn some information about her baby’s chromosomes, but not about the baby's genes. Chromosomes are large enough to be seen with a microscope, genes are not. Specialized tests are required to look for a particular gene that can cause a genetic disorder. Typically, an amniocentesis is used to see if the baby has the correct number of chromosomes. The chromosomes of fetal cells taken from the amniotic fluid are examined in the procedure. The baby’s chromosomes are photographed through a microscope. Each pair of chromosomes differs in length. The chromosomes are cut out of the photo and arranged by length into 23 pairs. The pairs are numbered longest to shortest, with the longest pair labeled as number one. This chromosomal picture is called a karyotype. If a mistake occurs when cells are dividing to produce egg or sperm cells, the baby may end up with an incorrect number of chromosomes. This error would show up in the karyotype. Nondisjunction is the general term for errors in chromosome division. For example, a pre-sperm cell with 46 chromosomes could divide into one sperm cell with 22 chromosomes and another with 24. If the sperm cell with 24 chromosomes fertilizes an egg with 23 chromosomes, the baby will now have 47 chromosomes.

17 Extra or Missing Chromosomes
Image used with permission of Clinical Tools, Inc.,

18 Sources of Variation: Fertilization
You joined the egg and sperm to form a fertilized egg. The chances of any particular egg being fertilized by any particular sperm are very small. Any organism that results from sexual reproduction normally has an even number of chromosomes, because half of the chromosomes come from the male and the other half from the female. For example, in sexually reproducing animals such as humans, a sperm, the male gamete, and an ovum, female gamete, each normally having 23 chromosomes unite during fertilization to form a zygote containing 46 chromosomes. In the Reebops breeding exercise, each team created only one egg and one sperm from two sets of chromosomes randomly selected from each parent. How many different gene combinations in the gametes are possible for the Reebops? The answer is that the two sets of chromosomes in a diploid germ cell can be randomly assorted in 2 to the 7th power or 128 different ways. Students should be encouraged to verify that there are numerous possibilities by sorting their Reebop chromosomes from one parent into many different combinations in a systematic way. This may be done as a teacher led exercise using a transparent set of Reebop chromosomes on an overhead projector to get the systematic sorting initiated. After students have generated enough variants gametes to see that there are LOTS of possibilities, you may show them the short-cut to determining the total without having to sort out each one. Another question that should then be posed is: “How many possible different offspring can two parent Reebops produce with these 128 possible gametes?” Give students some time to try to calculate this and canvass the class for their ideas. The total possible different kinds of male gametes times the total possible different kinds of female gametes equals the total of different kinds of offspring (128 different Reebop sperm X 128 different Reebop ova = 22,384 different Reebop offspring). This is a good time to point out that math is a valuable tool for scientists which saves a lot of time and allows patterns to emerge that might otherwise be overlooked.

19 Quick-Think Time Fill in the blanks.
The fertilized egg has __________ set(s) _____of chromosomes (and genes). The fertilized egg is _____ploid. The fertilized egg contains _______ allele(s) for each trait. Students should now be able to provide the answers in the blanks. The fertilized egg has two sets of seven chromosomes. The fertilized egg is diploid. The fertilized egg contains two alleles for each trait.

20 Quick-Think Time Fill in the blanks.
The fertilized egg has two set(s) of seven chromosomes (and genes). The fertilized egg is diploid. The fertilized egg contains two allele(s) for each trait. Students should now be able to provide the answers in the blanks. The fertilized egg has two sets of seven chromosomes. The fertilized egg is diploid. The fertilized egg contains two alleles for each trait.

21 Image used with permission of Clinical Tools, Inc. , http://www

22 Sources of Variation: Summary
Parents have two alleles for each gene. These alleles can differ from one another, adding potential for variation in offspring. Parents produce haploid gametes with a random assortment of their chromosomes. Each gamete is different. As gametes are formed, another process called crossing over adds a great deal of variation to the gametes. The many possible combinations of sperm and egg produce even more variation. Bullet 1: This simple summary represents a sexually reproducing diploid organism. Bullet 2: In humans, there are over 8 million possible combinations of 23 chromosomes in a gamete due to random assortment. This value is calculated by 2 to the 23rd power. Bullet 3: This is not meant to be a complete explanation. Crossing over will be part of the discussion of meiosis in Part 2. Bullet 4: As previously mentioned, in humans, 8 million possible combinations of 23 chromosomes result from random assortment. That variation is multiplied by the next level of possible combinations that occur in fertilization, resulting in approximately 8 million x 8 million possible combinations. This value does not take into consideration the additional variation due to crossing over and mutations. The topic of mutations is covered in the activity and PowerPoint called DNA Code Cracking.

23 A process called MEIOSIS
Meiosis Basics You made a haploid gamete by picking strips of paper that represented chromosomes. In real organisms, how do we get from the diploid somatic cell to the haploid gamete? A process called MEIOSIS Bullet 1: We are now ready to connect the process for gamete formation that students followed in the Reebops activity to MEIOSIS. Bullet 2: In animals, the cells that give rise to gametes are called germline cells. Plants, in contrast to animals, form germ cells (sperm and eggs) from somatic tissues. Meiosis consists of two cell divisions. In the first cell division, shown with a yellow background, one diploid cell divides to form two diploid daughter cells. These daughter cells are not identical because of a process called crossing over in which duplicated chromosomes (called sister chromatids) swap genes. This genetic recombination is an important source of variation. In the second cell division, diploid cells divide (without duplicating their DNA) to become haploid gametes, or sex cells. 1 diploid cell 2 diploid cells 4 haploid cells

24 A process called MITOSIS
Mitosis Basics You built a Reebop baby by reading the chromosomes in the fertilized egg. In real organisms, how does a single cell (the fertilized egg) become a multicellular baby? A process called MITOSIS

25 2 identical diploid cells
Mitosis is the process of creating two genetically identical daughter cells from one pre-existing parent cell. These daughter cells are for growth or for the replacement of dead cells. The circular diagram on the lower left shows the life cycle of the cell. In the G1, S, and G2 phases, the cell prepares for mitosis by duplicating chromosomes and other cell parts. The phases of mitosis occur in M, ending in cytokinesis, the final division into two genetically identical cells. 2 identical diploid cells

26 Mitosis vs Meiosis
While the purpose of this lesson is not to provide a detailed discussion of the phases of mitosis and meiosis, this animation, which which provides a step-by-step comparison of meiosis and mitosis, allows you to point out the highlights. To view the animation, go to and choose: Go to "Mitosis vs. Meiosis” Your computer will need to have a Flash plugin. It's probably already there with newer browsers. You can also view a non-flash version at

27 Reebops Science Summary
Reebop populations have genetic diversity. Any reebop might have two different alleles for a trait. The process of producing gametes (meiosis) adds variation through random assortment and crossing over of chromosomes. Fertilization adds more variation. Natural selection by the environment acts on this variation by determining which individuals survive to reproduce viable offspring. In conclusion, we have observed that hereditary information, DNA, is passed to successive generations through mitosis and that genetically different gametes are produced from parents, through meiosis, and combine to form new individuals. Now, one can begin to understand how changes in the genetic composition of a population during successive generations may begin to occur, as a result of natural selection acting on the genetic variation among individuals, and possibly resulting in the development of new species. We will develop and explore these ideas further in the lessons Reebop Ratios and Reebop Populations. [Note that we are not yet considering mutation as a source of variation. The topic of mutations is covered in the activity and PowerPoint called DNA Code Cracking.]

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