Meiosis and Fertilization – Understanding How Genes Are Inherited1

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 G1, S and G2 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 longer 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. Meiosisc. 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.)

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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.

Meiosis I

Meiosis II

Mitosis

Separates pairs of homologous chromosomesSeparates sister chromatids

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Produces diploid cellsProduces haploid cells

Meiosis Makes Genetically Diverse Gametes

In this section, you will learn some ways that 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__ _____

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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:

______ ______ ______ ______

Comparing Meiosis and Mitosis

16a. In this figure, label the column that shows meiosis and the column that shows mitosis.

16b. What are some similarities between cell division by mitosis and cell division by meiosis?

DNA replicated and DNA replicated and chromosomes chromosomes condensed condensed

The dotted lines represent cytokinesis.

17. Complete this table to describe some important differences between mitosis and meiosis.

Characteristic Mitosis Meiosis

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Type of cells produced

# of daughter cells

Are daughter cells genetically identical or different?

# of cell divisions

18. 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.

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.

This chart will guide you as you model meiosis and fertilization. Say 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.

20. 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.

21. Write the genetic makeup of each type of zygote in the appropriate gray box in the chart.

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22a. 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)

22b. 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.

23a. Explain why children often have the same phenotypic characteristics as their parents.

23b. Explain how a child can have a different phenotypic characteristic that neither parent has.

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