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 theegg. 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.
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 ___
  1. Fertilization
  2. Meiosis
  3. 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.)

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 chromosomes
Separates sister chromatids
Produces diploid cells
Produces 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 allelesdo you observe in the gametes?
__as______
/
When a pair of homologous chromosomes is lined up next to each other at the beginning ofmeiosis 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 theAa and Ss genes.
The sister chromatids will separate during meiosis II. Thiswill 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
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 inheritancefor two parents who both have the Aa genotype. This flowchart shows one possible outcome of meiosis and fertilization for these parents.

19a. Fill in the blanks in this flowchart to show when meiosis and fertilization occurred.

19b. Label the alleles in the child’s cells.Explain how you know what these alleles are.

19c.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 preparethese model chromosomes, use two different colored pairs of your as and AS model chromosomes. Tape blank strips of paper to cover the s and S alleles. /

This chart will guide you as you model meiosis and fertilization. SayOutline the rectangles of this chart on your lab table, usingchalk, 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.

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 eggsand 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.

22a. Use the information in this tableto 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.

Each person has thousands of genes in 23 pairs of homologous chromosomes, so meiosis 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 spermfrom one father, this would produce zygotes with trillions of different combinations of alleles.

24.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, gametes, egg, sperm, fertilization, zygote.

Asexual vs. Sexual Reproduction

Some animals and plants use a combination of mitosis and splitting off to reproduce. For example, a hydra can reproduce by budding. The bud is formed of cells made by many repetitions of mitosis. Then the bud breaks off to form a daughter hydra. /
A hydra is an animal that lives in the water and uses its
tentacles to catch food.

25. Will there be any genetic differences between the mother hydra and the daughter hydra? Explain your reasoning.

Budding is an example of asexual reproduction. Asexual reproduction involves only mitosis, without meiosis and fertilization. Many types of plants have asexual reproduction.

26a. What would be the advantage of asexual reproduction for a plant that lives in a stable environment that does not change?

26b. What would be the advantage of sexual reproduction for a plant that grows in a variable environment that often changes?

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[1]by Drs. Ingrid Waldron, Jennifer Doherty, R. Scott Poethig, and Lori Spindler, Department of Biology, University of Pennsylvania, © 2018; Word filesfor this Student Handout and for a longerStudent Handout, together with Teacher Preparation Notes with background information and instructional suggestions are available at