SECTION

6.1CHROMOSOMES AND MEIOSIS

Study Guide

KEY CONCEPT

Gametes have halfthe number ofchromosomes thatbody cells have.

VOCABULARYsomatic cell autosome fertilizationgamete sex chromosome diploid

haploidhomologous chromosome sexual reproductionmeiosis

MAIN IDEA: You have body cells and gametes.

1. What are the two major groups of cell types in the human body?

2. Where are gametes located?

3. How many chromosomes are in a typical human body cell?

MAIN IDEA: Your cells have autosomes and sex chromosomes.Fill in the concept map below to summarize what you know about chromosomes.

46 chromosomes in human body cells

22 homologous pairs

7. 8.

4.

5.

6.

inc lude

hal f come from

inc lude inc lude

consist of

inc lude

sex chromosomes

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9. What is the sex of a person with two X chromosomes?

10. Which chromosome carries the fewest number of genes?

MAIN IDEA: Body cells are diploid; gametes are haploid.

11. What happens to the nuclei of the egg and sperm during fertilization?

12. What type of cells are haploid?

13. What is the haploid chromosome number in humans?

14. How many autosomes are present in each human gamete? How many sex chromosomes?

15. Complete the following table to summarize the differences between mitosis and meiosis.Use Figure 6.2 to help you.

Mitosis Meiosis

Makes diploid cells

Makes genetically unique cells

Happens throughout lifetime

Involved in sexual reproduction

Vocabulary Check16. What are homologous chromosomes?

17. The word soma means “body.” How does this relate to the meanings of autosome andsomatic cell?

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SECTION

6.2PROCESS OF MEIOSIS

Study Guide

KEY CONCEPT

During meiosis, diploid cells undergo two cell divisions thatresult in haploid cells.

VOCABULARYgametogenesis eggsperm polar body

MAIN IDEA: Cells go through two rounds of division in meiosis.

1. After a chromosome is replicated, each half is called a .

2. Two chromosomes that are very similar and carry the same genes are called

.

In the space below, sketch the phases of meiosis I and II and write the name of each phasebelow it. Use Figure 6.5 to help you.

3. 4. 5. 6.

7. 8. 9. 10.

Meiosis I

Meiosis I I

11. During which phase do homologous chromosomes separate?

12. During which phase do sister chromatids separate?

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MAIN IDEA: Haploid cells develop into mature gametes.

13. What does a sperm cell contribute to an embryo?

14. What does an egg contribute to an embryo?

15. Where are polar bodies made, in the male or in the female?

Complete the diagram of gametogenesis in the boxes below. Use Figure 6.6 to help you.

Sperm Formation Egg Formation

Vocabulary Check16. Genesis comes from a Greek word that means “to be born.” How does this relate to

the meaning of gametogenesis?

17. What is a polar body?

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SECTION

6.3MENDEL AND HEREDITY

Study Guide

KEY CONCEPT

Mendel’s research showed that traitsare inherited as discrete units.

VOCABULARYtrait purebredgenetics cross

law of segregation

MAIN IDEA: Mendel laid the groundwork for genetics.

1. What is genetics?

2. Whose early work is the basis for much of our current understanding of genetics?

3. How did Mendel’s views on inheritance differ from the views of many scientists ofhis time?

MAIN IDEA: Mendel’s data revealed patterns of inheritance.In designing his experiments, Mendel made three important choices that helped him seepatterns of inheritance. In the table below, list Mendel’s three choices and write an exampleof how he put each of these choices into action.

Mendel’s Choices Example

4.

5.

6.

7. Why did Mendel use pea plants?

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8. Fill in the sequence diagram below to summarize Mendel’s experimental process.

Resulted in F2 generation with both dominant and recessive phenotypes.

Bred flowers resulting in F1 generation with dominant phenotype.

9. Mendel concluded that traits are inherited as “discrete units.” What do we call thesediscrete units today?

10. What two conclusions make up Mendel’s law of segregation?

Vocabulary Check11. Segregation means “separation.” What is “segregated” in Mendel’s law of segregation?

12. What does “purebred” mean?

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Traits, Genes, and AllelesKEY CONCEPT Genes encode proteins that produce a diverse range of traits.

The same gene can have many versions.As you learned, the units of inheritance that Mendel studied are now called genes. You can think of a gene as a piece of DNA that stores instructions to make a certain protein. Each gene is located at a particular place on a chromosome called a locus. Just like a house has an address on a street, a gene has a locus on a chromosome.

Many things come in different forms. For example, bread can be wheat, white, or rye. Most genes have many forms, too. An allele (uh-LEEL) is any of the different forms of a gene. The gene for pea shape, for example, has two alleles—one for round peas and another for wrinkled peas.

Your cells, like the pea plant’s cells, have two alleles for each gene—one on each chromosome of a homologous pair. The term homozygous (HOH-moh-ZY-guhs) means the two alleles of a gene are the same—for example, both alleles are for round peas. The term heterozygous (HEHT-uhr-uh-ZY-guhs) means the two alleles are different—for example, one allele is for wrinkled peas and one is for round peas.

Draw a circle around each of the alleles shown in the Visual Vocab to the right.

Genes influence the development of traits.

For Mendel’s peas, if a plant was heterozygous for pea shape, the pea shape would be round. This is because the allele for round peas is dominant, or expressed when two different alleles are present. A recessive allele is expressed only when there are two copies of the recessive allele. A dominant allele is not better or stronger or more common; it is simply the allele that is expressed when there are two different alleles. Mendel studied traits that had just two alleles, one that was dominant and one that was recessive. Most traits involve much more complicated patterns of inheritance.

Alleles are represented with letters—capital letters for dominant alleles and lowercase letters for recessive alleles. For example, the dominant allele for round pea shape can be

SECTION

6.4Student text pages 180–182

Homozygous alleles are identical to each other.

VISUAL VOCAB

Heterozygous alleles are different from each other.

homozygousalleles

heterozygousalleles

wrinkled

wrinkled round

wrinkled

The drawing on p. 93 shows a homolo-gous pair of duplicated chromosomes. Notice that here the chromosomes are drawn unduplicated. These are two homologous pairs of unduplicated chromosomes.

A dominant allele is expressed when two different alleles are present.

A recessive allele is expressed only when two copies are present.

VISUAL VOCAB

genotype phenotype

wrinkled rounddominantrecessive

genotype phenotype

recessive recessivewrinkled wrinkled

R

r

Interactive Reader 99

B.5.2

Draw a circle around each of the alleles shown in the Visual Vocab to the right.

bhsir_c06.indd 99 5/6/10 10:12:20 AM

written as R, for round. The recessive allele, for wrinkled pea shape, can be represented with the same letter, but lowercase—r.

A genotype is the set of alleles an organism has for a trait. For exam-ple, a genotype could be homozygous dominant (RR), heterozygous (Rr), or homozygous recessive (rr). A phenotype is what the resulting trait looks like—for example, round or wrinkled. A genome is all of an organism’s genetic material—all of the genes on all of the chromosomes.

What is the difference between a genotype and a phenotype?

gene recessive

allele genotype

homozygous phenotype

heterozygous genome

dominant

1. What is the difference between a gene and an allele?

2. What is the difference between a dominant allele and a recessive allele?

3. Fill in the blanks in the chart below regarding pea shape.

GENOTYPE PHENOTYPE HOMOZYGOUS OR HETEROZYGOUS

RR homozygous dominant

Rr round peas

rr homozygous recessive

4. Which of the alleles in the chart above is dominant?

6.4 Vocabulary Check

Go back and highlight each sentence that has a vocabulary word in bold.

Mark It Up

6.4 The Big Picture

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SECTION

6.5TRAITS AND PROBABILITY

Study Guide

KEY CONCEPT

The inheritance of traitsfollows the rules ofprobability.

VOCABULARYPunnett square testcross law of independent

assortmentmonohybrid cross dihybrid cross probability

MAIN IDEA: Punnett squares illustrate genetic crosses.Identify what each of the numbered parts represents in the Punnett square below. Then drawlines from each of the parents’ alleles to the corresponding alleles in the offspring.

AA

Aa

A

a

Aa A a

aa 3.1.

2.

4. Why does each parent contribute only one allele to the offspring?

MAIN IDEA: A monohybrid cross involves one trait.

5. You know a ratio is a comparison that tells how two or more things relate. What is agenotypic ratio? a phenotypic ratio?

6. What is the genotypic ratio of the offspring in Figure 6.15?

7. What is the phenotypic ratio of the offspring in Figure 6.15?

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MAIN IDEA: A dihybrid cross involves two traits.

8. What is a dihybrid cross?

9. Why does each parent organism in the F1 generation have four alleles listed in Figure6.17?

10. Suppose an organism had the genotype AABb. What two types of gametes could resultfrom this allele combination?

11. What is the phenotypic ratio that results from a dihybrid cross between two organismsthat are heterozygous for both traits? See Figure 6.17 for help.

MAIN IDEA: Heredity patterns can be calculated with probability.

12. Probability predicts the number of occurrences, not the

number of occurrences.

13. To calculate the probability that two independent events will happen together,

the probability of each individual event.

14. In Figure 6.18, the probability of getting one coin that is heads up and one coin that is

tails up is .

Vocabulary Check15. What is a testcross?

16. What is independent in the law of independent assortment?

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SECTION

6.6MEIOSIS AND GENETIC VARIATION

Study Guide

KEY CONCEPT

Independent assortment and crossing over during meiosisresult in genetic diversity.

VOCABULARYcrossing over genetic linkage

MAIN IDEA: Sexual reproduction creates unique gene combinations.

1. What are two ways that sexual reproduction helps create and maintain genetic diversity?

2. Which does sexual reproduction create, new alleles or new combinations of alleles?

3. How is the production of unique genetic combinations an advantage to organisms andspecies?

MAIN IDEA: Crossing over during meiosis increases genetic diversity.

4. Are chromosomes in a duplicated or an unduplicated state when crossing over occurs?

Use sketches to illustrate how crossing over contributes to genetic diversity. Use Figure 6.20for reference. 1. Draw a cell with four chromosomes in the first box. Make one pair ofchromosomes large and the other pair small. Color in one large chromosome and one smallchromosome. Leave the other two chromosomes white. 2. In the next box, draw the cell inprophase I. Have each pair of homologous chromosomes line up together—large with large,small with small. 3. In the third box, show crossing over between each pair of homologouschromosomes. 4. In the last box, show what the chromosomes look like as a result of crossingover. You will use this sketch in the next exercise.

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Refer to your cell sketch in the last box on the previous page. Also refer to Figure 6.5 ifnecessary. 1. In the first box below, show what your cell would look like at the end of meiosisI. Remember, the result will be two cells that have one duplicated chromosome from eachhomologous pair. 2. In the second box, show what your cell would look like at the end ofmeiosis II. Remember, the result will be four cells that have one (unduplicated) chromosomefrom each homologous pair.

5. If genes A and B are located on separate, nonhomologous chromosomes, will theyfollow Mendel’s law of independent assortment? Explain.

6. If genes A and B are located at opposite ends on the same chromosome, are they likelyto follow Mendel’s law of independent assortment? Explain.

7. If genes A and B are located very close together on the same chromosome, are theylikely to follow Mendel’s law of independent assortment? Explain.

Vocabulary Check8. The exchange of chromosome segments between homologous chromosomes is called

.

9. The tendency for two genes that are located close together on a chromosome to be

inherited together is called .

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