1. Name 2 Genetic Diseases.
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2. 14–1 Human Heredity 14-1 Human Heredity
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Photo credit: Richard Hutchings/Photo Researchers, Inc.

3. Human Chromosomes
Cell biologists analyze chromosomes by looking at karyotypes: a picture of chromosomes.
A karyotype is made by:
1. Photographing cells during mitosis.
2. Cutting out chromosomes
3. Grouping them together in pairs.
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4. Copyright Pearson Prentice Hall
These human chromosomes have been cut out of a photograph and arranged to form a karyotype. Photo credit: ©CNRI/Science Photo Library/Photo Researchers, Inc.

5. Human Chromosomes
Humans have 46 chromosomes. 2 Sex chromosomes determine an individual's sex (X and Y). X= Female Y = Male 44 chromosomes autosomal chromosomes.
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6. Human Chromosomes
Males and females are born in a roughly 50 : 50 ratio because of the way in which sex chromosomes segregate during meiosis.
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In humans, egg cells contain a single X chromosome. Sperm cells contain either one X chromosome or one Y chromosome. In a population, approximately half of the zygotes are XX (female) and half are XY (male).

7. Pedigree Charts A pedigree chart shows the relationships within a family. Genetic counselors analyze pedigree charts to infer the genotypes of family members.
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8. A vertical line and a bracket connect the parents to their children.
Human Traits
A horizontal line connecting a male and a female represents a marriage.
A circle represents
a female.
A square represents
a male.
A vertical line and a bracket connect the parents to their children.
A shaded circle or square indicates that a person expresses the trait.
A circle or square that is not shaded indicates that a person does not express the trait.
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This drawing shows what the symbols in a pedigree represent.

9. Genes and the Environment
Most human traits are impossible to associate with 1 gene.
Traits, such as the shape of your eyes or ears, are polygenic.
Personal traits are only partly governed by genetics. The rest is environment.
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10. Human Genes
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11. Human Genes Copyright Pearson Prentice Hall
Some disorders like Cyctic Fibrosis and sickle cell are caused by a single gene mutation or deletion.

12. Sickle Cell is found in 1 in 500 African Americans.
Human Genes
Sickle Cell is found in 1 in 500 African Americans.
People who are heterozygous for the sickle cell allele are generally healthy and resistant to malaria.
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13. Regions where malaria is common
Malaria and the Sickle Cell Allele
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The map on the left shows where malaria is common. The map on the right shows regions where people have the sickle cell allele.
Regions where malaria is common
Regions where the sickle cell allele is common

14. 14-2 Human Chromosomes 14–2 Human Chromosomes
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Photo credit: Richard Hutchings/Photo Researchers, Inc.

15. Sex-Linked Genes
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.
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16. 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
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Genes on X and Y chromosomes, such as those shown in the diagrams, are called sex-linked genes.
Y Chromosome
Testis-determining
factor

17. 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.
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18. Sex-Linked Genes An example: 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.
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19. Sex-Linked Genes Possible Inheritance of Colorblindness Allele
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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.

20. Sex-Linked Genes An example: 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.
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21. Sex-Linked Genes An Example: 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.
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22. 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.
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23. Chromosomal Disorders
Nondisjunction: the most common error in meiosis occurs when homologous chromosomes fail to separate.
If nondisjunction occurs, abnormal numbers of chromosomes find their way into gametes, and a disorder of chromosome numbers may result.
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24. Chromosomal Disorders
Nondisjunction
Homologous chromosomes fail to separate.
Meiosis I:
Nondisjunction
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Nondisjunction causes gametes to have abnormal numbers of chromosomes. The result of nondisjunction may be a chromosome disorder such as Down syndrome.
Meiosis II

25. Chromosomal Disorders
An Example: Down Syndrome
If two copies of a chromosome fail to separate during meiosis, an individual may be born with 3 copies of a chromosome.
Down syndrome involves three copies of chromosome 21.
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26. Chromosomal Disorders
Down syndrome produces mild to severe mental retardation.
It is characterized by:
increased susceptibility to many diseases
higher frequency of some birth defects
Down Syndrome Karyotype
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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

27. Chromosomal Disorders
An Example: 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.
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28. 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.
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