188.8.131.52 Variation within Species
Use concepts from Mendel's Laws of Segregation and Independent Assortment to explain how sorting and recombination (crossing over) of genes during sexual reproduction (meiosis) increases the occurrence of variation in a species.
Use the processes of mitosis and meiosis to explain the advantages and disadvantages of asexual and sexual reproduction.
Explain how mutations like deletions, insertions, rearrangements or substitutions of DNA segments in gametes may have no effect, may harm, or rarely may be beneficial, and can result in genetic variation within a species.
Minnesota Standard in Lay Terms
- When homologs separate during the process of meiosis, the homologs with genes inherited from either the mother or the father separate and become part of separate gametes. When these homologs separate, approximately eight million different combinations are possible.
- These many different combinations may account for a new genotype in an organism which may give it an advantage over other organisms. However, it is also possible that the new organism has a combination of genes which is not as fit as the original organism.
- Chromosomes and genes can break and rejoin in such a way that sections of them switch parts or are lost completely and occasionally are duplicated. Some of these result in combinations that are lethal; others have no effect and only very rarely may increase the ability of the organism to survive an environmental situation.
Meiosis (sex cell division) is the basis of recombination of genes in organisms. This leads to new combinations and contributes to genetic diversity which can be either beneficial to the species or not. Chromosome rearrangements, although often detrimental, contribute to this diversity.
MN Standard Benchmarks
184.108.40.206.1 Use concepts from Mendel's laws of segregation and independent assortment to explain how sorting and recombination (crossing over) of genes during sexual reproduction (meiosis) increases the occurrence of variation in a species.
220.127.116.11.2 Use the processes of mitosis and meiosis to explain the advantages and disadvantages of asexual and sexual reproduction.
18.104.22.168.3 Explain how mutations like deletions, insertions, rearrangements, or substitutions of DNA segments in gametes may have no effect, may harm, or rarely may be beneficial, and can result in genetic variation within a species.
Meiosis Square Dance - YouTube
- NSES Standardsp. 185
- 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. The fact that the human body is formed from cells that contain two copies of each chromosome - and therefore two copies of each gene - explains many features of human heredity, such as how variations that are hidden in one generation can be expressed in the next.
- AAAS Atlas: Vol. 1, pp. 69 and 71 (see benchmarks below).
- Benchmarks of Science Literacypg 108. The information passed from parents to offspring is coded in DNA molecules.
- The sorting and recombination of genes in sexual reproduction results in a great variety of possible gene combinations from the offspring of any two parents. 5b/2
- Some new gene combinations make little difference, some can produce organisms with new and perhaps enhanced capabilities, and some can be deleterious.
- Genes are segments of DNA molecules. Inserting, deleting, or substituting DNA segments can alter genes. An altered gene may be passed on to every cell that develops from it. The resulting features may help, harm, or have little or no effect on the offspring's success in its environment. 5b/4
- Gene mutations can be caused by such things as radiation and chemicals. When they occur in sex cells, the mutations can be passed on to offspring; if they occur in other cells, they can be passed on to descendant cells only. The experiences an organism has during its lifetime can affect its offspring only if the genes in its own sex cells are changed by the experience. 5b/5
Framework for K-12 Science Education
The information passed from parents to offspring is coded in the DNA molecules that form the chromosomes. In sexual reproduction, chromosomes can sometimes swap sections during the process of meiosis (cell division), thereby creating new genetic combinations and thus more genetic variation. Although DNA replication is tightly regulated and remarkably accurate, errors do occur and result in mutations, which are also a source of genetic variation. Environmental factors can also cause mutations in genes, and viable mutations are inherited. Environmental factors also affect expression of traits, and hence affect the probability of occurrences of traits in a population. Thus the variation and distribution of traits observed depend on both genetic and environmental factors. 12LS3.B
In multicellular organisms individual cells grow and then divide via a process called mitosis, thereby allowing the organism to grow. The organism begins as a single cell (fertilized egg) that divides successively to produce many cells, with each parent cell passing identical genetic material (two variants of each chromosome pair) to both daughter cells. As successive subdivisions of an embryo’s cells occur, programmed genetic instructions and small differences in their immediate environments activate or inactivate different genes, which cause the cells to develop differently—a process called differentiation. Cellular division and differentiation produce and maintain a complex organism, composed of systems of tissues and organs that work together to meet the needs of the whole organism. In sexual reproduction, a specialized type of cell division called meiosis occurs that results in the production of sex cells, such as gametes in animals (sperm and eggs), which contain only one member from each chromosome pair in the parent cell. 12LS1.B
Common Core Standards
Math standards can be easily incorporated into this topic.
Math 22.214.171.124 Make reasonable estimates and judgments about the accuracy of values resulting from calculations involving measurements.
Students may make predictions as to the outcome of the diversity resulting from independent assortment and segregation.
Math 126.96.36.199 Design simple experiments and explain the impact of sampling methods, bias, and the phrasing of questions asked during data collection.
This standard can be addressed with all data collection methods. Statistical analysis and graphing can be done.
Common Core Language Arts: Students can write a laboratory report in the proper form, using their knowledge of technical writing skills. Common core standards addressed:
RST.9-10.1. Cite specific textual evidence to support analysis of science and technical texts, attending to the precise details of explanations or descriptions.
RST.9-10.2. Determine the central ideas or conclusions of a text; trace the text's explanation or depiction of a complex process, phenomenon, or concept; provide an accurate summary of the text.
RST.9-10.3. Follow precisely a complex multistep procedure when carrying out experiments, taking measurements, or performing technical tasks, attending to special cases or exceptions defined in the text.
- Meiosis is mitosis by two.
- Meiosis occurs in all cells of the body.
- Students often believe there is a blending of characteristics from the mother and father, with various ideas about either the mother or father having a stronger contribution and that girls inherit traits from mothers and boys from father. (AAAS 1993; Driver et al. 1994 - Hard to Teach Biology Concepts, pg 113)
- Students do not understand the role of chance in producing new heritable characteristics by formation of new combinations of existing genes or by mutations of genes. (AAAS 1993, Driver et al. 1994 - Hard to Teach Biology Concepts pg 114)
As the students enter the biology classroom they suddenly realize that there are place cards on the tables. There is a white paper wedding bell on the front counter. As they find their seats they realize that with a few exceptions, they are paired boy and girl. The teacher enters the room in a robe and announces that today we are having "mail order brides" and an arranged wedding. They cannot choose their spouse today as the assortment is random, but no worries as it is only for an hour.
The ceremony begins "Dearly beloved, we are gathered here today" and the students are properly "married." Now it's time to get down to business and "make babies." (Lots of snickering.) Students are given a booklet filled with traits from sex to eye color to face shape. The book contains instructions as to which kind of genes (dominant, recessive, multiple alleles) are responsible for which traits. The students are informed that some "creative license" has been taken with the genetics of the traits and that they have been simplified, but that real traits operate in a similar manner.
Students then flip coins (the girl flips one and the boy flips one). If it is heads, they donate a dominant allele. If it is tails, they donate a recessive allele. At the end of the coin flipping, the students make a birth announcement for their baby with a picture of the baby on the cover. The birth announcements are hung on the wall. This is not only a lively and fun experience but it becomes very obvious that all students began with the same possible alleles and that their babies (due to segregation and random assortment of genes) display a wide diversity of phenotypic traits. (Refer to The Genetics of Parenthood)
Suggested Labs and Activities
- 188.8.131.52.1 Sordaria Lab
- Sordaria is a laboratory that uses a fungus named Sordaria. When the sordaria are crossed the asci show segregation of gametes through the color of the spores in the asci. They are arranged by meiotic division. The lab itself can be complicated and expensive and is usually reserved for AP classes. However, the outcome of the laboratories (pictures of the asci) can be found online on AP biology websites. A lively discussion takes place as students analyze the outcome of the crosses. AP Central Web site
- 184.108.40.206.2 Why Sex?
- This is an outstanding video which talks about the advantages and disadvantages of sexual verses asexual reproduction. Recommended for high school but would be a little too mature for younger students. This is Part 5 (Why Sex?) of the PBS 'Evolution Project." This video is particularly effective in promoting outstanding student discussions about the evolution of sexual reproduction.
- 220.127.116.11.3 Karyotyping
- Old fashioned karyotyping (cutting out chromosomes from a chromosome spread and placing on a sorting card) is a good way for students to learn about chromosome abnormalities and also about possible career opportunities. Several different abnormal spreads can be slipped into the mix as students identify the abnormalities in the karyotypes. For some students this can be a very relaxing activity. However, if cutting seems tedious there are several kits that can be purchased where the chromosomes can be placed on cards (magnetic) and then moved around as needed. This is a good opportunity to look at how deletions, translocations, and inversions can affect the phenotype of the organism.
- Meiosis Modeling: students use traditional materials (pop bead, pipe cleaners, clay, socks, or purchased manipulative kits for meiosis/mitosis) and write summaries of how they modeled meiosis and crossing over. Swimming "noodles" can be used to "act out" what happens in meiosis and mitosis.
- References for "Making Babies"
- The Genetics of Parenthood: this is a simulation (see Vignette) that easily captures student interest and can be varied to meet different ability levels. Making the assumption that the P (parental) generation is heterozygous at all loci and that independent assortment occurs (no linkages), students flip coins to determine which allele they will pass on to the F1 generation, and draw the resulting child's face.
- Case Study: "Baby Doe vs. the Prenatal Clinic."
- From the website: "In this fictionalized story, John and Jane, whose new baby is born with Down syndrome, are suing the prenatal clinic where Jane received her care, blaming the clinic for the baby's condition. Designed for an introductory biology course, this (simulation) has students assume the role of an expert witness hired by a law firm to give evidence in the case."
- Chromosome: threadlike structure within the nucleus that contains genetic information and is passed from one generation to the next.
- Egg: the female gamete. Parent cell is the oogonium in the ovary.
- Fertilization: the process of combining two gametes (egg and sperm) to restore the full complement of homologous chromosomes and resulting in ½ coming from the egg and ½ coming from the sperm.
- Gamete: the sexual reproductive cell produced through the process of meiosis. Contains ½ the chromosomes of the parent cell (oogonium or spermatogonium).
- Independent assortment: a term which describes the fact that homologous pairs of chromosomes segregate independently of each other. Alleles that are carried on the homologs segregate together unless crossing over has taken place during prophase I of meiosis, and they now find themselves on different homologs than they started in the parent cell.
- Meiosis: sexual reproduction of cells in which the daughter cells contain ½ the genetic make-up of the parent cell. One of each homolog is found in separate gametes.
- Mitosis: asexual reproduction of cells in which the daughter cells are "clones" with the exact same genetic make-up as the parent cell.
- Mutation: a change in the DNA of a genome. May take the form of a single nucleotide change to a full change in large segments of chromosomes.
- Recombination (crossing over): the process of exchanging genetic material during prophase I of meiosis.
- Segregation: a term which describes how the homologs are divided during the process of meiosis. Each of the two homologs of any particular chromosome are in separate gametes at the end of anaphase I of meiosis.
- Sperm: the male gamete. Parent cell is the spermatogonium in the testes.
- Dragon Meiosis: Students view an animation of meiosis as they breed dragons. They see how the random selection of a sperm and egg lead to the diversity of the baby dragons. They also see how genes are randomly selected to determine the traits of the offspring. They can even try to choose the traits they want in a baby dragon.
- Variety of formats for teaching meiosis, including dancing and singing
- What A Difference an "A" Makes: looks at mutations in transcription and the possible results.
- Connect to the teaching of probability in math classes.
- Health: Human sexuality and reproduction.
Karyotype kits can be purchased from Hudson Alpha. They are "stick on" chromosomes for students to match pairs and find abnormalities. It saves tons of time from the "cut out" activities most teachers have used. It cost about $150 for a class set. Teachers can make their own chromosome sets using velcro.
Assessment of Students
Include questions designed to probe student understanding of concepts, both formative and summative.
- 18.104.22.168.3 Students interpret a pedigree and describe at the multiple levels of biological organization the variation among offspring in a particularly generation. Summative.
- 22.214.171.124.2 Use of models can inform teacher of students understanding of meiosis. Formative.
- 126.96.36.199.2 Compare meiosis and mitosis. Summative.
- 188.8.131.52.2 Explain how meiosis allows organisms to maintain their chromosome numbers from one generation to the next. Summative.
- 184.108.40.206.1 Explain the difference between segregation and independent assortment and how each contributes to genetic variability. Summative.
Assessment of Teachers
- How does meiosis and fertilization result in genetic differences between two siblings who have inherited their genes from the same mother and father? Who do you have the better chance of having more genes in common with, your parents or your sibling?
ANSWER: Each parent contributes one of each of his/her homologs, which contains one of each of his/her alleles. In the process of crossing over, homologs may not segregate together but only one is passed to each gamete. Therefore, all genes are resorted in the new gametes. A child has 50 percent of their genes from each parent. However, depending on probability, they may have a much high percent of their genes in common with a sibling or also depending on the laws of probability, they may have a much lower percent in common.
- Would meiosis work if an organism had an odd number of chromosomes? What would this mean for speciation (example: mule)?
ANSWER: When a horse mates with a donkey, the resulting progeny (a mule) has an odd number of chromosomes. Although viable, this animal cannot perform meiosis as the chromosomes cannot pair in prophase I. Therefore, the mule is sterile.
- Sometimes, when parents are unable to conceive or when miscarriages are frequent, it is because one of them carries a balanced translocation. All genetic material is present in the parent so the parent is unaffected. What are the chances of having abnormal offspring?
ANSWER: When chromosomes pair in a balanced translocation, they generally do so according to the origin of their centromeres. If the p or q arm of the chromosome isactually from a different chromosome, a variety of abnormal situations may ensue. The most common one is that the gamete receives an unequal complement of genes. They could have the normal chromosomes, 25 percent will carry the balanced translocation but 50 percent of the zygotes will be both missing and having extra chromosomal segments, resulting in abnormal embryos.
Struggling and At-Risk
The human outcome of reproduction and the things that can go wrong are interesting to students. The mechanics of cellular reproduction may not have the same impact. The trick is to keep the topic relevant through ties to human experiences and problems such as birth defects while learning that the underlying causes often involve cellular reproduction.
Using a hand model will help students understand the steps of mitosis/meiosis while also decreasing the amount of vocabulary in these concepts. See this site.
Students research different genetic diseases caused by chromosomal abnormalities and pretending they are a genetic counselor, they write a letter to the parents of an unborn child with the disorder these abnormalities caused.
The cultural implications of reproduction are numerous. Teachers must be sensitive to these issues and read the students' reactions as they introduce the subject. When discussing reproduction and sexuality, use discretion and be mindful of cultural practices.
This can be a particularly abstract concept for student with learning disabilities. The more concrete the topic is, the easier it is to comprehend. Beads, paper models, and hand on activities are most effective here. One example is to make chromosomes out of pairs of socks. This is a good laboratory for demonstrating mitosis and meiosis and how the pairs of socks separate in mitosis and meiosis.
Administrators observing a lesson on this standard might expect to see:
- A marriage ceremony and baby announcements!
- Students demonstrating the process of mitosis and meiosis using strings of beads or pipe cleaners. The beads are particularly useful as translocations and deletions can also be simulated.
Students can make a family tree based on illness/diseases that may have been inherited.