Meiosis and Fertilization Understanding How Genes Are Inherited 1 How does a child inherit one copy of each gene from each parent? Compare what you already know with this flowchart. 1. Fill in each blank in the flowchart with the appropriate term from this list: Chromosomes (contain genes in DNA) Fertilization (a sperm unites with an egg to produce a zygote = a fertilized egg) Meiosis (a special type of cell division that produces eggs and sperm) Repeated Mitosis (the type of cell division that produces most of our body s cells) The zygote contains all the chromosomes with all the genes that were in the egg and in the sperm that fertilized the egg. Repeated cycles of DNA replication and mitosis ensure that each cell in a child s body has the same genes as the zygote had. 3a. Mitosis is part of the cell cycle. In this flowchart, show how the G 1, S and G 2 phases complete the cell cycle. 3b. Meiosis is part of the human lifecycle. Meiosis produces the egg and sperm that unite to make a zygote which develops into a baby. To complete the lifecycle shown above, draw arrows to represent the child becoming an adult. 1 by Drs. Ingrid Waldron, Jennifer Doherty, R. Scott Poethig, and Lori Spindler, Department of Biology, University of Pennsylvania, 2018; Word files for this Student Handout and for a shorter Student Handout, together with Teacher Preparation Notes with background information and instructional suggestions are available at http://serendipstudio.org/exchange/waldron/mitosis.
Almost all of the cells in your body are diploid. A diploid cell has pairs of homologous chromosomes. 4. What is a pair of homologous chromosomes? A haploid cell has only one chromosome from each pair of homologous chromosomes. 5. In this flowchart, label each type of cell as diploid or haploid. 6. Match each item in the top list with the best match from the bottom list. Diploid cell haploid cell Haploid cells diploid cell Diploid cell diploid cell a. Fertilization b. Meiosis c. Mitosis 7a. Eggs and sperm are called gametes. What problem would result if gametes were diploid? 7b. Explain why gametes cannot be made by mitosis. 8. Use the information you have learned thus far to explain how a child gets one copy of each gene from his/her mother and another copy of each gene from his/her father. A complete answer will include the following terms: haploid, gametes, gene, pair of homologous chromosomes, meiosis, egg, sperm, fertilizes or fertilization, zygote, diploid (Cross off each of these terms after you have included it in your answer.) 2
How Meiosis Makes Haploid Gametes How does a diploid cell divide into haploid gametes? This flowchart shows the basic steps. Notice that meiosis includes two cell divisions, meiosis I and meiosis II. 9. Put the letter for each item in the following description next to the matching part of the flowchart. A. Before meiosis, a diploid cell makes a copy of the DNA in each chromosome. Both copies have identical alleles for each gene. B. The two copies of the DNA in each chromosome are condensed into sister chromatids. C. At the beginning of meiosis I, the two homologous chromosomes line up next to each other. D. Meiosis I separates the homologous chromosomes into two daughter cells. These daughter cells are haploid since they have one chromosome from each pair of homologous chromosomes. E. During meiosis II, the sister chromatids of each chromosome are separated. Meiosis II produces four haploid daughter cells. 10. In the flowchart, sister chromatids look similar, but the two homologous chromosomes look different from each other. Why do you think they were drawn this way? 11. To produce haploid gametes, DNA is replicated time(s) and then there are cell division(s). (0/1/2/3) (0/1/2/3) 12. To describe the characteristics of meiosis I, meiosis II, and mitosis, put a check for each characteristic that applies. Separates pairs of homologous chromosomes Separates sister chromatids Produces diploid cells Produces haploid cells Meiosis I Meiosis II Mitosis 3
How Meiosis Makes Genetically Diverse Gametes In this section, you will learn how meiosis produces gametes that have different combinations of alleles. To begin, you will model meiosis using a pair of model chromosomes with the alleles shown here. Use your pair of model chromosomes to model each step of meiosis. Use your arms as spindle fibers to move the chromosomes, and use string to represent the cell membranes at each stage. 13a. Show the results of your modeling in this flowchart. Draw and label the chromosomes in each cell that is produced by meiosis I and by meiosis II. 13b. Which two combinations of alleles do you observe in the gametes? as When a pair of homologous chromosomes is lined up next to each other at the beginning of meiosis I, the two homologous chromosomes can exchange parts of a chromatid. This is called crossing over. 14. The bottom row of this figure shows the chromosomes after they have separated during meiosis I. On each chromatid of these chromosomes, label the alleles for the Aa and Ss genes. The sister chromatids will separate during meiosis II. This will produce gametes with four different combinations of the alleles for these two genes. 15. The combinations of alleles in the different gametes will be: 4
Next, you will model meiosis using these two pairs of homologous chromosomes. At the beginning of meiosis I each pair of homologous chromosomes lines up independently of how the other pair of homologous chromosomes has lined up; the as chromosome can be lined up on the same side as either the l chromosome or the L chromosome. This is called independent assortment. Use your two pairs of homologous chromosomes to model meiosis I and meiosis II for both of the possible ways of lining up the model chromosomes at the beginning of meiosis I. 16. Record the results of your modeling in this chart. Human cells have 23 pairs of homologous chromosomes, so independent assortment can produce more than 8 million different combinations of chromosomes in the gametes produced by one person! Each chromosome has many genes, so crossing over can result in many more combinations of alleles in the gametes produced by one person. 17. Explain how a person with the chromosomes shown at the top of this page could produce a gamete with the AsL alleles. 5
Comparing Meiosis and Mitosis 18a. In this figure, label the column that shows meiosis and the column that shows mitosis. 18b. What are some similarities between cell division by mitosis and cell division by meiosis? DNA replicated and chromosomes condensed DNA replicated and chromosomes condensed The dotted lines represent cytokinesis. 19. Complete this table to describe some important differences between mitosis and meiosis. Characteristic Mitosis Meiosis Type of cells produced # of daughter cells Are daughter cells genetically identical or different? # of cell divisions 20. Draw chromosomes in this figure to show how a pair of homologous chromosomes is lined up in a cell at the beginning of mitosis vs. the beginning of meiosis I. 6
Genes are inherited via meiosis and fertilization. To learn how meiosis and fertilization determine the genetic makeup of a child, you will analyze inheritance for two parents who both have the Aa genotype. This flowchart shows one possible outcome of meiosis and fertilization for these parents. 21a. Fill in the blanks in this flowchart to show when meiosis and fertilization occurred. 21b. Label the alleles in the child s cells. Explain how you know what these alleles are. 21c. Do you think that this is the only possible outcome of meiosis and fertilization for these two Aa parents? Explain why or why not. To investigate the possible outcomes of meiosis and fertilization for these Aa parents, you will model meiosis and fertilization with chromosomes that look like these. To prepare the mother s model chromosomes, use a pair of as and AS model chromosomes. Tape blank strips of paper to cover the s and S alleles. To prepare the father s model chromosomes, use a different color pair of the l and L model chromosomes. First, cover the l and L alleles with blank strips of paper. Then, use strips with the a allele or the A allele to finish preparing the father s model chromosomes. 7
This chart will guide you as you model meiosis and fertilization. Outline the rectangles of this chart on your lab table, using chalk, dry erase marker or tape. Each rectangle should be big enough for a model chromosome. Use one pair of model homologous chromosomes to demonstrate how meiosis produces eggs. Put a model chromosome for each type of egg in the top boxes in your chart on your lab table. Use the other color pair of model homologous chromosomes to demonstrate how meiosis produces sperm. Put a model chromosome for each type of sperm in the boxes on the left. 22. Write the allele for each type of egg and sperm in the appropriate white boxes in the above chart. Model fertilization by moving the chromosome from one of the eggs and the chromosome from one of the sperm to produce a zygote with one chromosome from the egg and one from the sperm. Repeat, using each type of sperm to fertilize each type of egg. 23. Write the genetic makeup of each type of zygote in the appropriate gray box in the chart. 24a. Use the information in this table to determine the phenotypic characteristic (albinism or normal skin and hair color) of the mother, the father, and the child who would develop from each zygote. Write these phenotypes in the above chart. Genotype Phenotype (characteristics) AA or Aa Normal skin and hair color aa Albino (very pale skin and hair color) 24b. Circle the genotypes of each zygote that would develop into a person with the same phenotypic characteristic as the parents. Use an * to mark the zygote that would develop into a person who would have a different phenotypic characteristic that neither parent has. 25a. Explain why children often have the same phenotypic characteristics as their parents. 25b. Explain how a child can have a different phenotypic characteristic that neither parent has. 8
Each person has thousands of genes in 23 pairs of homologous chromosomes, so crossing over and independent assortment can produce millions of different combinations of alleles in his/her gametes. If each different type of egg from one mother could be fertilized by each different type of sperm from one father, this would produce zygotes with trillions of different combinations of alleles. 26. Explain why no two siblings inherit exactly the same combination of alleles from their parents (except for identical twins who both developed from the same zygote). A complete answer will include the following terms: chromosomes, genes, alleles, meiosis, crossing over, independent assortment, gametes, egg, sperm, fertilization, zygote. How Mistakes in Meiosis Can Result in Down Syndrome or Death of an Embryo Occasionally, there is a mistake in meiosis, so the chromosomes are not divided equally between the daughter cells. 27a. This flowchart shows how a mistake in meiosis can result in a gamete with two copies of the same chromosome. Circle the part of the flowchart where this mistake occurred. 27b. Suppose that a mistake in meiosis has produced an egg with two copies of a human chromosome called chromosome 21. How many copies of each chromosome are in a normal sperm? If the egg with two copies of chromosome 21 were fertilized by a normal sperm, how many copies of chromosome 21 would there be in the zygote? How many copies of chromosome 21 would there be in each cell in the embryo that developed from this zygote? 9
If the cells in an embryo have three copies of a chromosome, this results in abnormal cell function and abnormal embryonic development. The following points explain why. Each chromosome has specific genes with the instructions for making specific proteins. An extra copy of all the genes in one chromosome results in too many copies of the proteins coded for by the genes in that chromosome. Normal cell function depends on the correct balance of the different types of proteins in a cell. Abnormal cell function affects development. For example, a child with an extra copy of chromosome 21 in each cell has Down syndrome, with cognitive delays, a broad flat face, and often heart defects. 28. Suppose that each cell in an embryo had only one copy of chromosome 21. Do you think that this embryo would develop normally? Explain why or why not. This figure shows a karyotype for a normal boy. A karyotype is a picture of a magnified view of the chromosomes from a human cell, with pairs of homologous chromosomes arranged next to each other and numbered. 29. Label the sister chromatids and centromere in chromosome 3. 30a. A karyotype is prepared from a cell that is a. in the G 1 phase. b. at the beginning of mitosis. c. at the end of mitosis. 30b. How do you know? 31. Draw an extra chromosome 21 to show the karyotype of a boy with Down syndrome. If each cell in an embryo has an extra copy of one of the chromosomes in the top row of the karyotype, the resulting abnormalities are so severe that the embryo always dies; this causes a miscarriage. Therefore, no babies are born with three copies of chromosome 1, 2, 3, 4 or 5 in their cells. In contrast, an extra copy of chromosome 21 results in less severe abnormalities so the embryo can often survive to be born as a baby with Down syndrome. 32. What do you think is the reason why an extra copy of chromosome 1, 2, 3, 4 or 5 results in more severe abnormalities than an extra copy of chromosome 21? 10