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19 Reproduction, Life Cycles, and Heredity Eggs Chicks come from eggs. What other things come from eggs? Put an X next to each thing you think comes from an egg. _____ whale _____ butterfly _____ human _____ worm _____ soil _____ bacteria _____ rock _____ mouse _____ beetle _____ alligator _____ robin _____ bean plant _____ clam _____ catfish _____ single-celled organism _____ frog _____ spider _____ cow _____ cat _____ oak tree _____ turtle Explain your thinking. What rule or reasoning did you use to decide if something comes from an egg? _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ Uncovering Student Ideas in Life Science Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions. 117 19 Reproduction, Life Cycles, and Heredity Eggs Teacher Notes Purpose The purpose of this assessment probe is to elicit students’ ideas about sexual reproduction. The probe is designed to reveal whether students recognize that multicellular organisms develop from a fertilized egg. Related Concepts Egg, sexual reproduction, life cycle Explanation The best answer is: Everything on the list comes from an egg except for four things—soil, bacteria, rock, and single-celled organism. The rest are multicellular plants and animals that reproduce sexually. During sexual reproduction, an egg cell is fertilized by a sperm cell. This fertilized egg then divides and develops into an organism. Soil and rock are not living things, so they do not come from eggs (although soil does contain living organisms). Although we often refer to eggs as animal in origin, biologically plants also produce eggs. At the base of a 118 flower is the ovary. The ovary contains one or more ovules. Each ovule contains an egg that is fertilized by a sperm cell that develops in a pollen grain. The fertilized egg within the ovule develops into a seed, which may then germinate and develop into the plant. Although this is a simplified explanation of a more complex process, the big idea is that both plants and animals develop from fertilized eggs. Only multicellular organisms can produce eggs. Bacteria do not produce eggs. Bacteria and other single-celled organisms generally reproduce asexually by binary fission or other means of cell division. Bacteria are single-celled, as are egg cells; therefore, a bacterium does not contain eggs within its cell. Viruses are not considered cells and are generally considered not to be living things. They can only reproduce by replicating within a living cell, using the cell’s DNA. Rocks and soil are not living organisms and thus do not produce eggs, although tiny eggs of worms and insects can be found in soil. N a t i o n a l S c i e n c e Te a c h e r s A s s o c i a t i o n Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions. Reproduction, Life Cycles, and Heredity Curricular and Instructional Considerations Elementary Students In the elementary grades, students learn about the life cycles of plants and animals. They observe the life cycles of various animals such as butterflies, frogs, and chicks, and learn that their life cycles begin with an egg. They learn that some animals lay their eggs and that these eggs develop outside the animal’s body. They also learn that some animals develop inside the mother’s body. They learn about the life cycles of plants and recognize that the life cycle of a plant begins with a seed. Elementary students would not be expected to understand that a seed developed from an egg within the ovule of a flower. Later in middle school, students learn about sexual reproduction in plants and recognize that seeds develop from a fertilized egg. See “Administering the Probe” for modifications appropriate for elementary students. Middle School Students At the middle school level, students learn about the difference between sexual and asexual reproduction. They learn that multicellular plants and animals develop from an egg fertilized by a sperm cell. They learn about the structure and function of different parts of a plant and animal, including the structures where eggs are produced. As they learn about the parts of a flower and the process of pollination, they recognize where the egg and sperm come from and what happens when the egg is fertilized. High School Students At the high school level, students build on their basic understanding of sexual reproduction in plants and animals and examine life cycles in more detail, including reproduction in flowering and nonflowering plants. Students use terms such as gametophytes to describe 19 specialized cells involved in reproduction. The process of meiosis to produce eggs and sperm is emphasized at this level. However, some high school students will still revert to the common misconception that eggs come only from animals. Administering the Probe This probe is best used with students in grades 3–8. With younger children who are not yet ready to learn about plant reproduction, consider modifying the probe to include only animals. Younger students may only choose animals that lay their eggs outside of their bodies as egg producers, even though they learn that life cycles of animals begin with a fertilized egg. This probe can be administered as a card sort (Keeley 2008). Instead of a paper-pencil task, print each word from the list (p. 117 ) on cards. Give each small group a set of cards and have them sort them into two piles: things that produce eggs and things that do not produce eggs. Students discuss their justifications for each choice while they are doing the card sort. As you circulate among groups, listen carefully to students’ reasoning and probe further if needed. For older elementary students, you might replace bacteria with germ or leave it out altogether. Related Ideas in National Science Education Standards (NRC 1996) K–4 Life Cycles of Organisms • Plants and animals have life cycles that include being born, developing into adults, reproducing, and eventually dying. The details of this life cycle are different for different organisms. 5–8 Reproduction and Heredity In many species, including humans, females produce eggs and males produce Indicates a strong match between the ideas elicited by the probe and a national standard’s learning goal. Uncovering Student Ideas in Life Science Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions. 119 19 Reproduction, Life Cycles, and Heredity sperm. Plants also reproduce sexually— the egg and sperm are produced in the flowers of flowering plants. An egg and sperm unite to begin development of a new individual. That new individual receives genetic information from its mother (via the egg) and its father (via the sperm). Sexually produced offspring never are identical to either of their parents. 9–12 Molecular Basis of Heredity • Transmission of genetic information to offspring occurs through egg and sperm cells that contain only one representative from each chromosome pair. An egg and a sperm unite to form a new individual. • • Related Ideas in Benchmarks for Science Literacy (AAAS 2009) K–2 Human Development • All kinds of animals have offspring, usually with two parents involved. 6–8 Heredity In sexual reproduction, a single specialized cell from a female merges with a specialized cell from a male. The fertilized egg cell, carrying genetic information from each parent, multiplies to form the complete organism. Suggestions for Instruction and Assessment • • 6–8 Human Development • Human fertilization occurs when sperm cells from a male’s testes are deposited near an egg cell from the female ovary, and one of the sperm cells enters the egg cell. Related Research • Some children hold an “agricultural model” of reproduction, believing that eggs are “laid.” Some student use this model to think of human eggs as similar to hens’ eggs, incubated inside a mother’s body (Driver et al. 1994). A transition in understanding reproduction happens after a child tries to make sense of the relationship between a mother and father, acquiring information about sexual intercourse and ideas about sperm and eggs. By age 11, most children understand the role of the parents’ sperm and egg (Driver et al. 1994). In a study by Gott et al. (1985), 800 15-year-old students more often correctly identified sexual reproduction in animals than in plants. Many of the students did not believe that plants can reproduce sexually. Furthermore, studies confirmed by teachers showed that this view is very resistant to change, even after instruction. Biology instruction appears to have little effect in changing “folklore” concepts of reproduction (Driver et al. 1994). • Combine this probe with “Does It Have a Life Cycle?” in Uncovering Student Ideas in Science, Vol. 3: Another 25 Formative Assessment Probes (Keeley, Eberle, and Dorsey 2008). When younger students learn about life cycles, starting with the egg, you should explicitly develop the idea that this stage in the life cycle is similar for all animals. All animals start out as an egg, even those that develop inside their mothers, such as mammals. Later, when students are ready to learn about the reproductive parts of a flower, you can extend this generalization to plants. Once students have learned about cells, they can understand that a life cycle starts with a single-celled egg. Probe deeper to find out what students’ conceptions of an egg is inside animals that give live birth. Some students may Indicates a strong match between the ideas elicited by the probe and a national standard’s learning goal. 120 N a t i o n a l S c i e n c e Te a c h e r s A s s o c i a t i o n Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions. Reproduction, Life Cycles, and Heredity • still hold the “agricultural model” of an egg with a developing fetus inside it, rather than a model of cell division starting with a fertilized egg. Have students compare and contrast the everyday common use of the word egg with the scientific meaning of the word. For example, in everyday usage, we think of an egg as something that is laid or has a shell. Add this to students’ growing list of examples of the ways we use words in our everyday language that are not always the same in meaning when the word is used in a scientific context. Related NSTA Science Store Publications, NSTA Journal Articles, NSTA SciGuides, NSTA SciPacks, and NSTA Science Objects Koba, S., with A. Tweed. 2009. Hard-to-teach biology concepts: A framework to deepen student understanding. Arlington, VA: NSTA Press. Konicek-Moran, R. 2009. Flowers are more than just pretty. In More everyday science mysteries: Stories for inquiry-based science teaching, 121– 134. Arlington, VA: NSTA Press. Science Object: Cell Structure and Function: Cells— The Basis of Life 19 References American Association for the Advancement of Science (AAAS). 2009. Benchmarks for science literacy online. www.project2061.org/publications/ bsl/online Driver, R., A. Squires, P. Rushworth, and V. WoodRobinson. 1994. Making sense of secondary science: Research into children’s ideas. London: RoutledgeFalmer. Gott, R., A. Davey, R. Gamble, J. Head, N. Khaligh, P. Murphy, T. Orgee, B. Schofield, and G. Welford. 1985. Science in schools: Ages 13 and 15. Report No. 3. London, UK: Department of Education and Science, Assessment and Performance Unit. Keeley, P. 2005. Science curriculum topic study: Bridging the gap between standards and practice. Thousand Oaks, CA: Corwin Press and Arlington, VA: NSTA Press. Keeley, P. 2008. Science formative assessment: 75 practical strategies for linking assessment, instruction, and learning. Thousand Oaks, CA: Corwin Press and Arlington, VA: NSTA Press. Keeley, P., F. Eberle, and C. Dorsey. 2008. Uncovering student ideas in science, vol. 3: Another 25 formative assessment probes. Arlington, VA: NSTA Press. National Research Council (NRC). 1996. National science education standards. Washington, DC: National Academies Press. Related Curriculum Topic Study Guide (in Keeley 2005) “Reproduction, Growth, and Development (Life Cycles)” Uncovering Student Ideas in Life Science Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions. 121