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Chapter 13: Chromosomes and Human Genetics

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and Human Genetics. Key Concepts: A gene is a region of DNA within the DNA of a chromosome. ... One approach to the study of human genetics is to analyze pedigrees. ... – PowerPoint PPT presentation

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Title: Chapter 13: Chromosomes and Human Genetics


1
Chapter 13 Chromosomesand Human Genetics
2
Key Concepts
  • A gene is a region of DNA within the DNA of a
    chromosome. Each gene has a specific location on
    the chromosome.
  • In humans, males have one X and one Y chromosome,
    and females have two X chromosomes. A specific
    gene on the Y chromosome is required for human
    embryos to develop as males.
  • Unless they are located far from each other,
    genes on the same chromosome tend to be inherited
    together, or linked. Genes on different
    chromosomes are not linked.
  • The homologous chromosomes that pair during
    meiosis can exchange genes in a process called
    crossing-over.
  • The genotypes of offspring can be different from
    that of either parent as a result of
    crossing-over, the random distribution of
    maternal and paternal chromosomes into gametes,
    and fertilization.
  • Many inherited genetic disorders in humans are
    caused by mutations of single genes. A far
    smaller number of human genetic disorders are
    caused by abnormalities in chromosome number or
    structure.

3
The Role of Chromosomes in Inheritance
  • Mendel did not know what the physical properties
    of his particles were when he proposed his laws
    of inheritance.
  • August Weismann suggested that chromosomes
    (discovered in 1882) were the location of
    hereditary material.
  • Genes are located on chromosomes
  • The idea that genes are located on chromosomes is
    known as the chromosome theory of inheritance.
  • Chromosomes are composed of a single DNA molecule
    and many proteins.
  • The physical location of a gene on a chromosome
    is called a locus.
  • Chromosomes that pair during meiosis and contain
    the same gene loci and structure are called
    homologous chromosomes.
  • See Figure 13.1 for a diagram of homologous
    chromosomes.

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5
Autosomes and Sex Chromosomes
  • Chromosomes that determine gender are called sex
    chromosomes all other chromosomes are called
    autosomes.
  • Autosomes are homologous pairs.
  • Sex chromosomes can be homologous or
    non-homologous pairs (see Figure 13.2).
  • Sex determination in humans
  • Human females have two X chromosomes, and all
    their gametes contain one X chromosome.
  • Human males have one X and one Y chromosome half
    their gametes contain an X chromosome, and the
    other half contain a Y chromosome.
  • The chromosome carried by the sperm determines
    sex in humans.
  • Genetic anomalies such as XY females and XX males
    can occur because of chromosomal mutations.

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7
Linkage and Crossing-Over
  • Exceptions to the law of independent assortment
  • Thomas Hunt Morgan discovered some genes that
    were inherited together in his research on fruit
    flies (see Figure 13.3).
  • Genes that are located on the same chromosome and
    do not assort independently are said to be
    genetically linked

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9
Linkage and Crossing-Over
  • Crossing-over disrupts genetic linkage
  • If the linkage between two genes on a chromosome
    were complete, all offspring would be of a
    parental type.
  • Morgans experiments showed that complete linkage
    was not occurring as a result of the presence of
    nonparental genotypes (see Figure 13.3).
  • To explain the appearance of nonparental
    genotypes in linked genes, Morgan proposed that
    genes are physically exchanged between homologous
    chromosomes during meiosis.
  • The exchange of genes between homologous
    chromosomes is called crossing-over (see Figure
    13.4).
  • Genes that are far from each other on a
    chromosome are more likely to be separated by
    crossing-over than are genes that are close to
    each other.
  • Genes that are very distant from one another on a
    chromosome will assort independently.

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11
Origins of Genetic Differences among Individuals
  • Offspring are never exact replicas of their
    parents.
  • New alleles arise from mutation once formed, the
    alleles can be rearranged by crossing-over,
    independent assortment, and fertilization.
  • Crossing over during meiosis produces new
    chromosomes that contain some alleles from each
    parent.
  • Independent assortment of chromosomes occurs
    because the orientation of the maternal and
    paternal chromosomes is random when they line up
    at the metaphase plate during meiosis.
  • Because of independent assortment, there are more
    than 8 million ways of arranging the chromosomes
    in human gametes.
  • Fertilization has the potential to add genetic
    variation to offspring, because one sperm
    fertilizes one egg.
  • Each gamete goes through independent assortment,
    so there are more than 64 trillion (8 million
    sperm multiplied by 8 million eggs) different
    genotypes possible for a human zygote.

12
Human Genetic Disorders
  • The study of human genetic disorders poses some
    problems to geneticists.
  • These problems include long generation times,
    self-selection of mates, and small families.
  • One approach to the study of human genetics is to
    analyze pedigrees.
  • A pedigree is a chart that shows genetic
    relationships among family members over two or
    more generations of a familys history (see
    Figure 13.5).
  • Genetic disorders can be caused by new mutations
    that occur during an individuals lifetime (most
    cancers).
  • Inherited genetic disorders are caused by
    mutations of a single gene others are caused by
    abnormalities in chromosome number or structure.

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14
Autosomal Inheritance of Single-Gene Mutations
  • Several thousand human genetic disorders are
    inherited as recessive characters, most of which
    are caused by recessive mutations of genes on
    autosomes (Figure 13.6).
  • For genetic disorders caused by a recessive
    allele (a), only homozygous (aa) individuals get
    the disease.
  • Heterozygous individuals (Aa) are called
    carriers because they have one copy of the
    disease-causing allele but do not get the
    disease.
  • If two carriers of a recessive genetic disorder
    (Aa) mate, there is a 25 percent chance that
    their offspring will get the disease (see Figure
    13.7).
  • If a genetic disorder is dominant (A), then AA
    and Aa individuals are symptomatic for the
    disease.

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16
Sex-Linked Inheritance of Single-Gene Mutations
  • Genes located on the X or Y chromosome are called
    sex-linked.
  • Genes on the X chromosome are called X-linked.
  • Males inherit one X chromosome from their
    mothers therefore, genes on sex chromosomes have
    different patterns of inheritance than genes on
    autosomes (see Figure 13.8).
  • Males are more likely than females to have
    recessive X-linked genetic disorders, because
    they need to inherit only one copy of the
    disease-causing allele to be affected.

17
Inherited Chromosomal Abnormalities
  • Few human genetic disorders are caused by
    inherited chromosomal abnormalities.
  • Cri du chat is a disease caused by the truncation
    of chromosome 5 (see Figure 13.9).
  • Trisomy 21 (Downs syndrome) is the only case in
    which a person who inherits the wrong number of
    autosomes reaches adulthood.
  • Changes in the number of sex chromosomes can have
    relatively minor effects.

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19
Highlight Uncovering the Genetics of
Huntingtons Disease
  • In 1983, the gene for Huntingtons disease (HD)
    was determined to be on chromosome 4.
  • Further research isolated the HD gene.
  • The protein coded for by the HD gene was
    determined to be abnormal.
  • The abnormal protein forms aggregates in the
    brains of afflicted individuals that are likely
    to be related to the etiology of the disease.
  • A genetic test has been developed to determine
    whether a person will develop HD.

20
The Scientific Process Tracing the Inheritance
of a Disease Gene
  • The inheritance pattern of congenital generalized
    hypertrichosis (CGH) is illustrated with a
    pedigree diagram.
  • CGH is an X-linked dominant genetic disorder.
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