Chapter 13: Chromosomes and Human Genetics

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Chapter 13 Chromosomesand Human Genetics
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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.

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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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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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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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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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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.

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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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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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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.

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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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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.

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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.