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# 8.1: Human Chromosomes and Genes

Difficulty Level: At Grade Created by: CK-12

## Lesson 8.1: True or False

Name___________________ Class______________ Date________

Write true if the statement is true or false if the statement is false.

_____ 1. The human genome consists of about 3 million base pairs.

_____ 2. By 2003, scientists had sequenced all of the base pairs of a sample of human DNA.

_____ 3. In humans, chromosome 22 is the largest chromosome, and chromosome 1 is the smallest chromosome.

_____ 4. None of the genes on the Y-chromosome is essential to survival.

_____ 5. The role of the majority of the 3 billion base pairs in the human genome is not known.

_____ 6. Humans have 22 pairs of autosomes.

_____ 7. Genes that are located on the different chromosomes are linked genes.

_____ 8. Linkage is related to crossing-over during meiosis.

_____ 9. Females have two X chromosomes, and males have an X and a Y chromosome.

_____ 10. Genes that assort independently during meiosis will always be in different gametes.

_____ 11. The hemophilia A gene is on the X chromosome.

_____ 12. Only the X chromosome contains genes that determine sex.

_____ 13. The female is the “default” sex of the human species.

_____ 14. Most sex-linked genes are on the Y chromosome.

_____ 15. Most human cells have 23 chromosomes.

Name___________________ Class______________ Date________

Read these passages from the text and answer the questions that follow.

Chromosomes and Genes

Each species has a characteristic number of chromosomes. The human species is characterized by 23 pairs of chromosomes, as shown in the FlexBook.

Autosomes

Of the 23 pairs of human chromosomes, 22 pairs are autosomes. Autosomes are chromosomes that contain genes for characteristics that are unrelated to sex. These chromosomes are the same in males and females. The great majority of human genes are located on autosomes.

Sex Chromosomes

The remaining pair of human chromosomes consists of the sex chromosomes, X and Y. Females have two X chromosomes, and males have one X and one Y chromosome. In females, one of the X chromosomes in each cell is inactivated and known as a Barr body. This ensures that females, like males, have only one functioning copy of the X chromosome in each cell.

The X chromosome is much larger than the Y chromosome. The X chromosome has about 2,000 genes, whereas the Y chromosome has fewer than 100, none of which are essential to survival. Virtually all of the X chromosome genes are unrelated to sex. Only the Y chromosome contains genes that determine sex. A single Y chromosome gene, called SRY (which stands for sex-determining region Y gene), triggers an embryo to develop into a male. Without a Y chromosome, an individual develops into a female, so you can think of female as the default sex of the human species. Can you think of a reason why the Y chromosome is so much smaller than the X chromosome?

Human Genes

Humans have an estimated 20,000 to 22,000 genes. This may sound like a lot, but it really isn’t. Far simpler species have almost as many genes as humans. However, human cells use splicing and other processes to make multiple proteins from the instructions encoded in a single gene. Of the 3 billion base pairs in the human genome, only about 25 percent make up genes and their regulatory elements. The functions of many of the other base pairs are still unclear.

The majority of human genes have two or more possible alleles. Differences in alleles account for the considerable genetic variation among people. In fact, most human genetic variation is the result of differences in individual DNA bases within alleles.

Questions

1. What are autosomes? How many do humans have?

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2. Compare the X and Y chromosome.

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3. In terms of sex chromosomes, what is the genotype of a female? a male?

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4. How do humans use their genes to produce more than 22,000 proteins?

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5. What is the importance of alleles in humans?

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## Lesson 8.1: Multiple Choice

Name___________________ Class______________ Date________

Circle the letter of the correct choice.

1. All of the DNA of the human species makes up the human
1. genes.
2. genome.
3. chromosomes.
4. DNA.
2. Humans have ____________ bases divided among ____________ chromosomes.
1. 3 million, 23
2. 3 million, 23 pairs of
3. 3 billion, 23
4. 3 billion, 23 pairs of
3. Differences between the X and Y chromosomes include which of the following? (1) The X chromosome has many more genes than the Y chromosome. (2) Virtually all of the X chromosome genes are unrelated to sex, whereas the Y chromosome contains genes that determine sex. (3) Both males and females have only one functioning copy of the X chromosome in each cell.
1. 1 only
2. 2 only
3. 1 and 2
4. 1, 2, and 3
4. The goal of the Human Genome Project was to
1. sequence all 3 billion base pairs of human DNA.
2. sequence all human DNA and identify all 22,000 proteins.
3. develop linkage maps of all 22 autosomes of chromosomes.
4. all of the above
1. are on homologous chromosomes.
2. are on the same chromosome.
3. are on sister chromatids.
4. are on non-homologous chromosomes.
6. Most of the human genome is made of
1. genes.
2. regulatory regions.
3. intergenic regions.
4. chromosomes.
7. Which of the following statements is correct?
1. The higher the frequency of crossing-over, the closer together on the same chromosome the genes are presumed to be.
2. The lower the frequency of crossing-over, the closer together on the same chromosome genes are presumed to be.
3. The lower the frequency of crossing-over, the farther apart on the same chromosome the genes are presumed to be.
4. With a high frequency of crossing-over, genes are presumed to be on different chromosomes.
8. A normal human male has
1. 22 autosomes, and one X chromosome and one Y chromosome.
2. 22 pairs of autosomes, and one X chromosome and one Y chromosome.
3. 23 autosomes, and one X chromosome and one Y chromosome.
4. 23 pairs of autosomes, and one X chromosome and one Y chromosome.

## Lesson 8.1: Vocabulary I

Name___________________ Class______________ Date________

Match the vocabulary word with the proper definition.

Definitions

_____ 1. 20,000 to 22,000 in humans

_____ 2. an international project to sequence the entire human genome

_____ 3. determine the sex of the person

_____ 4. genes that are located on the same chromosome

_____ 5. all of the DNA of the human species

_____ 6. genes on the X-chromosome

_____ 7. chromosomes that contain genes for characteristics that are unrelated to sex

_____ 8. female

_____ 9. male

_____ 10. 23 pairs in humans

_____ 11. shows the locations of genes on a chromosome

_____ 12. genes located on the sex chromosomes

Terms

a. autosome

b. chromosomes

c. gene

d. human genome

e. Human Genome Project

h. sex chromosomes

k. XX

l. XY

## Lesson 8.1: Vocabulary II

Name___________________ Class______________ Date________

Fill in the blank with the appropriate term.

1. The human ____________ is all of the DNA of a human.

2. The X and Y chromosomes are known as the ____________ chromosomes.

3. Genes that are located on the same chromosome are ____________ genes.

4. Differences in alleles account for the considerable ____________ variation among people.

5. Most sex-linked genes are on the ____________ chromosome.

6. Chromosomes 1 to 22 are known as ____________.

7. Chromosome ____________ is the largest chromosome.

8. A linkage ____________ shows the locations of genes on a chromosome.

9. The number of human genes is about ____________ to ____________ .

10. Genes on non-homologous chromosomes are not linked.

11. Linkage explains why certain characteristics are frequently ____________ together.

12. Humans have 23 pairs of ____________.

## Lesson 8.1: Critical Writing

Name___________________ Class______________ Date________

Thoroughly answer the question below. Use appropriate academic vocabulary and clear and complete sentences.

Being very specific, what makes you different from everyone else?

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Feb 23, 2012
Aug 16, 2016
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