Post on 14-Jan-2016
Name 2 Genetic Diseases.
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14-1 Human Heredity
14–1 Human Heredity
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Human ChromosomesCell 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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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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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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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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Human Traits
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A circle representsa female.
A horizontal line connecting a male and a female represents a marriage.
A shaded circle or square indicates that a person expresses the trait.
A square representsa male.
A vertical line and a bracket connect the parents to their children.
A circle or square that is not shaded indicates that a person does not express the trait.
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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Human Genes
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Human Genes
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Human Genes
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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.
Malaria and the Sickle Cell Allele
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Regions where malaria is common
Regions where the sickle cell allele is common
14-2 Human Chromosomes
14–2 Human Chromosomes
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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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Sex-Linked Genes• The Y chromosome is much smaller than the X chromosome and appears to contain only a few genes.
Duchenne muscular dystrophy
X Chromosome
Melanoma
X-inactivation center
X-linked severe combined immunodeficiency (SCID)
Colorblindness
Hemophilia
Y Chromosome
Testis-determiningfactor
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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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Sex-Linked GenesAn 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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Sex-Linked Genes• Possible Inheritance of Colorblindness Allele
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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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Sex-Linked GenesAn 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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X-Chromosome Inactivation
• 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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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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Chromosomal Disorders• Nondisjunction Homologous
chromosomes fail to
separate.
Meiosis I:Nondisjunction
Meiosis II
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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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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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Chromosomal DisordersAn 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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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.