1. Copyright Pearson Prentice Hall
14–2 Human Chromosomes
14-2 Human Chromosomes
Photo credit: Richard Hutchings/Photo Researchers, Inc.
Copyright Pearson Prentice Hall

2. Human Genes and Chromosomes
Chromosomes 21 and 22 are the smallest human autosomes and were the first chromosomes to have their base sequences determined.
Sequencing of these proteins showed that chromosomes have long stretches of repetitive DNA that do not code for proteins, and therefore do not contain genes.
Copyright Pearson Prentice Hall

3. Human Genes and Chromosomes
Each human chromosome contains millions of base pairs and most have hundreds of genes.
Genes located close to each other on chromosomes are linked and tend to be inherited together.
Copyright Pearson Prentice Hall

4. Copyright Pearson Prentice Hall
Sex-Linked Genes
Sex-Linked Genes
The X chromosome and the Y chromosomes determine sex.
Genes located on these chromosomes are called sex-linked genes.
More than 100 sex-linked genetic disorders have now been mapped to the X chromosome.
Copyright Pearson Prentice Hall

5. Copyright Pearson Prentice Hall
Sex-Linked Genes
X Chromosome
Duchenne muscular dystrophy
The Y chromosome is much smaller than the X chromosome and appears to contain only a few genes.
Melanoma
X-inactivation center
X-linked severe combined immunodeficiency (SCID)
Colorblindness
Hemophilia
Genes on X and Y chromosomes, such as those shown in the diagrams, are called sex-linked genes.
Y Chromosome
Testis-determining
factor
Copyright Pearson Prentice Hall

6. Copyright Pearson Prentice Hall
Sex-Linked Genes
Why are sex-linked disorders more common in males than in females?
Copyright Pearson Prentice Hall

7. Copyright Pearson Prentice Hall
Sex-Linked Genes
For a recessive allele to be expressed in females, there must be two copies of the allele, one on each of the two X chromosomes.
Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive.
Copyright Pearson Prentice Hall

8. Copyright Pearson Prentice Hall
Sex-Linked Genes
Colorblindness
Three human genes associated with color vision are located on the X chromosome.
In males, a defective version of any one of these genes produces colorblindness.
Copyright Pearson Prentice Hall

9. Copyright Pearson Prentice Hall
Sex-Linked Genes
Possible Inheritance of Colorblindness Allele
X-linked alleles are always expressed in males, because males have only one X chromosome. Males who receive the recessive Xc allele all have colorblindness. Females, however, will have colorblindness only if they receive two Xc alleles.
Copyright Pearson Prentice Hall

10. Copyright Pearson Prentice Hall

11. List the possible genotypes of the following hemophilia pedigree chart below. Remember hemophilia is a sex linked trait that is caused by a recessive allele, therefore you must denote the individuals sex chromosomes (XNXn and XnY or XNXn and XnY) as well as the hemophilia allele (n).

12. Examine the following pedigree chart of color-blindness
Examine the following pedigree chart of color-blindness. In humans, color blindness is caused by a recessive sex-linked allele. On the diagram, label the genotypes of the individuals 1-16.

13. Copyright Pearson Prentice Hall
Sex-Linked Genes
Hemophilia
The X chromosome also carries genes that help control blood clotting. A recessive allele in either of these two genes may produce hemophilia.
In hemophilia, a protein necessary for normal blood clotting is missing.
Hemophiliacs can bleed to death from cuts and may suffer internal bleeding if bruised.
Copyright Pearson Prentice Hall

14. Copyright Pearson Prentice Hall
Sex-Linked Genes
Duchenne Muscular Dystrophy
Duchenne muscular dystrophy is a sex-linked disorder that results in the weakening and loss of skeletal muscle.
It is caused by a defective version of the gene that codes for a muscle protein.
Copyright Pearson Prentice Hall

15. X-Chromosome Inactivation
British geneticist Mary Lyon discovered that in female cells, one X chromosome is randomly switched off.
This chromosome forms a dense region in the nucleus known as a Barr body.
Barr bodies are generally not found in males because their single X chromosome is still active.
Copyright Pearson Prentice Hall

16. Chromosomal Disorders
What problems does nondisjunction cause?
Copyright Pearson Prentice Hall

17. Chromosomal Disorders
The most common error in meiosis occurs when homologous chromosomes fail to separate.
This is known as nondisjunction, which means, “not coming apart.”
Copyright Pearson Prentice Hall

18. Chromosomal Disorders
If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result.
Copyright Pearson Prentice Hall

19. Chromosomal Disorders
Nondisjunction
Homologous chromosomes fail to separate.
Meiosis I:
Nondisjunction
Nondisjunction causes gametes to have abnormal numbers of chromosomes. The result of nondisjunction may be a chromosome disorder such as Down syndrome.
Meiosis II
Copyright Pearson Prentice Hall

20. Chromosomal Disorders
Down Syndrome
If two copies of an autosomal chromosome fail to separate during meiosis, an individual may be born with three copies of a chromosome.
Down syndrome involves three copies of chromosome 21.
Copyright Pearson Prentice Hall

21. Chromosomal Disorders
Down Syndrome Karyotype
Down syndrome produces mild to severe mental retardation.
It is characterized by:
increased susceptibility to many diseases
higher frequency of some birth defects
This karyotype is from a person with Down syndrome. Down syndrome causes mental retardation and various physical problems. People with Down syndrome can, however, lead active, happy lives. Photo credit: ©Dr. Dennis Kunkel/CNRI/Phototake
Copyright Pearson Prentice Hall

22. Chromosomal Disorders
Sex Chromosome Disorders
In females, nondisjunction can lead to Turner’s syndrome.
A female with Turner’s syndrome usually inherits only one X chromosome (karyotype 45,X).
Women with Turner’s syndrome are sterile.
Copyright Pearson Prentice Hall

23. Chromosomal Disorders
In males, nondisjunction causes Klinefelter’s syndrome (karyotype 47,XXY).
The extra X chromosome interferes with meiosis and usually prevents these individuals from reproducing.
Copyright Pearson Prentice Hall

24. Copyright Pearson Prentice Hall
14–2
Copyright Pearson Prentice Hall

25. Copyright Pearson Prentice Hall
14–2
The average human gene consists of how many base pairs of DNA?
3000
300 20
30,000
Copyright Pearson Prentice Hall

26. Copyright Pearson Prentice Hall
14–2
Which of the following genotypes indicates an individual who is a carrier for colorblindness?
XCX
XCXc
XcY
XCY
Copyright Pearson Prentice Hall

27. Copyright Pearson Prentice Hall
14–2
Colorblindness is much more common in males than in females because
the recessive gene on the male’s single X chromosome is expressed.
genes on the Y chromosome make genes on the X chromosome more active.
females cannot be colorblind.
colorblindness is dominant in males and recessive in females.
Copyright Pearson Prentice Hall

28. Copyright Pearson Prentice Hall
14–2
The presence of a dense region in the nucleus of a cell can be used to determine the
sex of an individual.
blood type of an individual.
chromosome number of an individual.
genotype of an individual.
Copyright Pearson Prentice Hall

29. Copyright Pearson Prentice Hall
14–2
Nondisjunction occurs during
meiosis I.
mitosis.
meiosis II.
between meiosis I and II.
Copyright Pearson Prentice Hall

30. END OF SECTION